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Institute
Chemische Transportreaktionen weisen ein gemeinsames Merkmal auf: In Gegenwart eines gasförmigen Reaktionspartners, des Transportmittels, wird eine feste oder flüssige Komponente verflüchtigt. An anderer Stelle scheidet sie sich meist in Form gut ausgebildeter Kristalle wieder ab. So ist der Chemische Transport z. B. für den Festkörperchemiker ein unentbehrliches Verfahren zur Herstellung reiner, gut kristallisierter Feststoffe. Als umfassendes Handbuch behandelt dieses Werk die vielseitigen Aspekte von chemischen Transportreaktionen: Von der Grundlagenforschung bis hin zur praktischen Bedeutung, beispielsweise für die Funktionsweise von Halogenlampen.
Since their first recognition in mineral forming processes
some 150 years ago chemical vapor transport reactions (CVTR) have attracted continuous scientific interest. Due to the pioneering work of Harald Schäfer quantitative understanding and exploitation of transport reactions for crystal growth, synthesis, investigation of high-temperature gas species, and thermodynamic studies have become possible. Renewed interest in CVT is triggered by the demand of material sciences for novel compounds with tailor-made physical properties and by the need for efficient recycling strategies for various metals from industrial waste.
Chemical Vapour Transport of Bismuth and Antimony Chalcogenides M2Q3 (M = Sb, Bi, Q = Se, Te)
(2010)
Thermodynamic modelling of the ternary systems M/Q/I (M = Sb, Bi, Q = Se, Te) indicated solid-gas equilibria suitable for chemical vapour transport of bismuth and antimony chalcogenides. The predictions of the modelling were confirmed by transport experiments on a transport balance. The optimum transport conditions using iodine as transport agent were determined for all systems to: ϑsource = 500 °C and ϑsink = 450 °C. For ΔT > 50 K the sequential transport of chalcogenide iodides MQI followed by M2Q3 occurs. Thermodynamic standard data of the gas species SbI(g) were concluded from equilibrium calculations: ΔH0f,298(SbI(g)) = 106 ± 3 kJ·mol–1; S0298(SbI(g)) = 255 ± 3 J·mol–1·K–1; Cp(SbI(g)) = 37 ± 1 J·mol–1·K–1
The term chemical vapor transport (CVT) summarizes a variety of reactions that show one common feature: a condensed phase, typically a metallic or salt like solid, is volatilized in the presence of a gaseous reactant, the so-called transport agent, and deposits elsewhere, usually in the form of crystals. The deposition will take place if the site of volatilization and the site of crystallization have different temperatures. In many cases, chemical vapor transport is associated with a purification effect. CVT-reactions of elements, oxides, sulfides, selenides, tellurides, phosphates, sulfates, halides, oxide halides, phosphides, arsenides, intermetallics, and so on, are known.
A variety of processes of crystal growth proceeds via the gas phase. If the initial solid material is volatilized in a heterogeneous reaction under presence of a gaseous reactant, the transport agent, the term Chemical Vapor Transport Reaction (CVT) is applied. Crystallization processes by CVT are known for both elements, intermetallics, binary and complex oxides, halides, chalcogenides, and pnictides. Even if the formation of volatile halides is a common feature of almost all vapor transport reactions, significant differences are there concerning the choice of a suitable transport agent depending on the nature of the initial solid phase. Actually, the appropriateness of transport agents for the respective transport reaction can be described in a thermodynamic way. Besides some basic principles for systematic evaluation more practical recommendations for suitable experimental conditions are given for CVT of different classes of inorganic materials.
Chemische Transportreaktionen weisen ein gemeinsames Merkmal auf: In Gegenwart eines gasförmigen Reaktionspartners, des Transportmittels, wird eine feste oder flüssige Komponente verflüchtigt. An anderer Stelle scheidet sie sich meist in Form gut ausgebildeter Kristalle wieder ab. So ist der Chemische Transport z. B. für den Festkörperchemiker ein unentbehrliches Verfahren zur Herstellung reiner, gut kristallisierter Feststoffe.
Als umfassendes Handbuch behandelt dieses Werk die vielseitigen Aspekte von Chemischen Transportreaktionen: Von der Grundlagenforschung bis hin zur praktischen Bedeutung, beispielsweise für die Funktionsweise von Halogenlampen.
This comprehensive handbook covers the diverse aspects of chemical vapor transport reactions from basic research to important practical applications. The book begins with an overview of models for chemical vapor transport reactions and then proceeds to treat the specific chemical transport reactions for the elements, halides, oxides, sulfides, selenides, tellurides, pnictides, among others. Aspects of transport from intermetallic phases, the stability of gas particles, thermodynamic data, modeling software and laboratory techniques are also covered. Selected experiments using chemical vapor transport reactions round out the work, making this book a useful reference for researchers and instructors in solid state and inorganic chemistry.
This comprehensive handbook covers the diverse aspects of chemical vapor transport reactions from basic research to important practical applications. The book begins with an overview of models for chemical vapor transport reactions and then proceeds to treat the specific chemical transport reactions for the elements, halides, oxides, sulfides, selenides, tellurides, pnictides, among others. Aspects of transport from intermetallic phases, the stability of gas particles, thermodynamic data, modeling software and laboratory techniques are also covered. Selected experiments using chemical vapor transport reactions round out the work, making this book a useful reference for researchers and instructors in solid state and inorganic chemistry.
Fibrous phosphorus is one of the known crystalline allotropes under standard pressure conditions. It has been predicted prior to its successful synthesis and structural characterization. The allotrope consists of parallel, tubular double strands of phosphorus, in contrast to the violet form of Hittorf's phosphorus, where the same strands are arranged in a perpendicular orientation towards each other. This structural similarity results in an almost identical energetic stability leading to a somehow problematic realization of single-phase materials. We herein report on the successful synthesis route to single phase fibrous phosphorus and the in situ characterization of its formation via the gas phase applying CuCl2 as a mineralizer. The sublimation pressure of fibrous phosphorus is slightly lower than the one of the black allotrope. This behavior indicates thermodynamic stability of fibrous phosphorus.
[Cd3Cu]CuP10 is the first representative of a class of compounds featuring polyphosphidic adamantine-analogous [P10] units, which tend to be fragmented and polymerized to [P6] rings and tubular [P26] units in the solid state. A new polyphosphide with nominal composition Cd15Cu10P46 results, featuring isolated and polymerized polyanions. Thermoanalytic experiments and phase analytic measurements substantiated the consecutive loss of P4 and Cd of the starting material via different intermediate steps. After P4 loss, the new polyphosphide occurred followed by various binary copper phosphides to the final product Cu3P.
The 1-butyl-3-methylimidazolium halide ionic liquids are stable up to temperatures of 246(1) °C ([BMIm]Cl), 260(1) °C ([BMIm]Br), and 238(1) °C ([BMIm]I). The thermal decomposition proceeds in thermogravimetric measurements with a total mass loss of 100%. Using evolved gas analysis (EGA) a complete degradation of [BMIm]X ionic liquids under formation of characteristic fragments CH3+, NHn+, C4Hn+, and CH3X+ (X = Cl, Br, I) has been observed. [BMIm]Cl shows enantiotropic polymorphism with a phase transition temperature at 30(1) °C, and melts at 74(1) °C (ΔHfus = 18 ± 0.5 kJ mol−1). Spontaneous e-crystallization and reversible phase transition have been found for cooling of the substance.[BMIm]Br melts at 78(1) °C (ΔHfus = 29 ± 0.5 kJ mol−1). The re-crystallization fails and thus a glassy solid is formed. The glass transition temperature is about −65 °C, the cold crystallization occurs between −30 and −20 °C. The application of both homogeneous and heterogeneous nucleation agents does not interfere the glassy state.
[BMIm]I undergo solidification without crystallization. The melting effect for the amorphous substance arise at −70(5) °C with ΔHfus = 0.4 ± 0.2 kJ mol−1.
Development of salt hydrate eutectics as latent heat storage for air conditioning and cooling
(2014)
Sustainable air conditioning systems require heat reservoirs that operate between 4 and 20 ◦C. A systematic search for binary and ternary eutectics of inorganic salts and salt hydrates with melting temperatures in this temperature regime and with high enthalpies of fusion has been performed by means of differential scanning calorimetry (DSC). Promising results were obtained for the pseudo-ternary system Zn(NO3)2·6H2O, Mn(NO3)2·4H2O, and KNO3 with the melting temperature range 18–21 ◦C and the enthalpy of fusion of about 110 kJ kg−1. Suitable nucleating and thickening agents have been found and tested to prevent the mixture from supercooling and phase separation.
Phase formation, stability, crystal and electronic structures of IrPTe are investigated from experiment and quantum chemical calculations. The phase formation is studied from thermodynamic data and CalPhaD modeling. Applying a high-temperature gas-balance a formation pathway for IrPTe is studied from the elements and the binary parent compounds IrTe2 and IrP2. The obtained paracostibite (CoSbS) type structure contains rarely occurring heteroatomic P-Te dumbbells. The stability of IrPTe and the found structure is studied from DFT calculations with respect to the elements, IrTe2 and IrP2, and possible polymorphs. Probable metastable modifications with XY dumbbells (as known for isoelectronic compounds like CoAsS) are obtained from systematic DFT modelling. Phase transitions are predicted form the equation of states (EOS). According to its electronic band structure IrPTe is predicted as small gap (Eg = 0.5 eV) semiconductor.
The thermochemical behavior of 1-ethyl-3-methylimidazolium [EMIm] halides (Cl, Br and I) has been investigated for their crystalline and liquid states in the temperature range from −90 °C to 600 °C using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The temperatures and enthalpies of phase transitions have been determined: Tfus = 86(1) °C, ΔHfus = 14.2(0.7) kJ mol−1 ([EMIm]Cl); Tfus = 67(1) °C, ΔHfus = 19.3(0.7) kJ mol−1 ([EMIm]Br); and Tfus = 74(1) °C, ΔHfus = 16.9(0.6) kJ mol−1 ([EMIm]I). The decomposition temperatures, determined by onset of DTG at 1 K min−1 are 233(5) °C ([EMIm]Cl), 246(5) °C ([EMIm]Br), and 249(5) °C ([EMIm]I). The maximum operation temperature (MOT) has been estimated based on dynamic TGA for an operation time of 24 h: 132 °C ([EMIm]Cl), 149 °C ([EMIm]Br), 139 °C ([EMIm]I) and 8000 h: 76 °C ([EMIm]Cl), 90 °C ([EMIm]Br), 77 °C ([EMIm]I). The decomposition products of the investigated ionic liquids (ILs) after heating experiments were identified by means of TGA complemented with mass spectrometry (MS), for establishment of the mechanism of thermal decomposition of the ILs. Complete degradation of [EMIm]X ionic liquids occurs under formation of characteristic molecule fragments CH3+, NH+, and X+, CH3X+, C2H5X+ (X = Cl, Br, I).
The reaction of antimony and selenium in the Lewis-acidic ionic liquid 1-butyl-3-methyl-imidazolium tetrachloridoaluminate, [BMIm]Cl·4.7AlCl3, yielded dark-red crystals of [Sb2Se2]AlCl4. The formation starts above 160 °C; at about 190 °C, irreversible decomposition takes place. The compound crystallizes in the triclinic space group Pequation image with a = 919.39(2) pm, b = 1137.92(3) pm, c = 1152.30(3) pm, α = 68.047(1)°, β = 78.115(1)°, γ = 72.530(1)°, and Z = 4. The structure is similar to that of [Sb2Te2]AlCl4 but has only half the number of crystallographically independent atoms. Polycationic chains 1∞[Sb2Se2]+ form a pseudo-hexagonal arrangement along [01-1], which is interlaced by tetrahedral AlCl4– groups. The catena-heteropolycation 1∞[Sb2Se2]+ is a sequence of three different four-membered [Sb2Se2] rings. The chemical bonding scheme, established from the topological analysis of the real-space bonding indicator ELI-D, includes significantly polar covalent bonding in four-member rings within the polycation. The rings are connected into an infinite chain by homonuclear non-polar Sb–Sb bonds and highly polar Sb–Se bonds. Half of the selenium atoms are three-bonded.
The synthesis of the phosphide telluride Zr2+xPTe2 was accomplished by a solid-state reaction from the elements. Le Bail refinements of the as synthesized crystalline powders as well as the thermal decomposition of Zr2PTe2 along the homogeneity range Zr2+xPTe2 under the release of P4(g) and Te2(g) evidences a maximum zirconium content according to the composition Zr2.5PTe2. The thermal decomposition product of Zr2.5PTe2 undergoes a phase
transformation to “Zr2PTe” adopting the structure motive of the binary phases ZrTe and ZrTe2. The new phase “Zr2PTe” has a wide homogeneity range Zr2–xP1–yTe1+y tolerating a deficit in the cation position and a mixed occupation of the anions. The composition of the crystalline decomposition product was determined to Zr1.95P0.84Te1.16 by Rietveld refinement and by analysing the elemental composition with ICP-OES. Zr1.95P0.84Te1.16 crystallizes in hexagonal space group P63/mmc (no. 194) with lattice constants: a = 3.8726(1) Å and c = 13.008(1) Å.
Selected allotropes of phosphorus are investigated
by different levels of density functional theory
(DFT) calculations to evaluate the relative stability orders with a special focus on the role of van der Waals interactions.Phosphorus is an excellent reference system with a large number of allotropes. Starting from low-dimensional molecular (0D, white P) and polymer structures (1D, P nanorods) to layered (2D, black P) and tubular structures (2D and 3D, crystalline forms of red P), covalent structure motifs are interconnected by van der Waals interactions. They are a key factor for the correct energetic description of all P allotropes. A comparative study is carried out within the local density approximation (LDA) and the generalized gradient approximation (GGA), with and without implementation of a dispersion correction by Grimme (GGA-D2). Our intention is to achieve a reasonable agreement of our calculations with experimental data, the plausibility of energy values, and the treatment of long-range interactions. The effect of van der Waals interactions is exemplified for the interlayer distances of black phosphorous and its electronic structure.
Phosphorus displays fascinating structural diversity and the discovery of new modifications continues to attract attention. In this work, a complete stability range of known and novel crystalline allotropes of phosphorus is described for the first time. This includes recently discovered tubular modifications and the prediction of not-yet-known crystal structures of [P12] nanorods and not-yet-isolated [P14] nanorods. Despite significant structural differences, all P allotropes consist of covalent substructures, which are held together by van der Waals interactions. Their correct reproduction by ab initio calculations is a core issue of current research. While some predictions with the established DFT functionals GGA and LDA differ significantly from experimental data in the description of the P allotropes, consistently excellent agreement with the GGA-D2 approach is used to predict the solid structures of the P nanorods.
Cd4Cu7As, the first representative of a fully ordered, orthorhombically distorted MgCu2 Laves phase
(2012)
The ternary Laves phase Cd4Cu7As is the first intermetallic compound in the system Cu–Cd–As and a representative of a new substitution variant for Laves phases. It crystallizes orthorhombically in the space group Pnnm (No. 58) with lattice parameters a = 9.8833(7) Å; b = 7.1251(3) Å; c = 5.0895(4) Å. All sites are fully occupied within the standard deviations. The structure can be described as typical Laves phase, where Cu and As are forming vertex-linked tetrahedra and Cd adopts the structure motive of a distorted diamond network. Cd4Cu7As was prepared from stoichiometric mixtures of the elements in a solid state reaction at 1000 °C. Magnetic measurements are showing a Pauli paramagnetic behavior. During our systematical investigations within the ternary phase triangle Cd–Cu–As the cubic C15-type Laves phase Cd4Cu6.9(1)As1.1(1) was structurally characterized. It crystallizes cubic in the space group Fd3m̅ with lattice parameter a = 7.0779(8) Å. Typically for quasi-binary Laves phases Cu and As are both occupying the 16c site. Chemical bonding, charge transfer and atomic properties of Cd4Cu7As were analyzed by band structure, ELF, and AIM calculations. On the basis of the general formula for Laves phases AB2, Cd is slightly positively charged forming the A substructure, whereas Cu and As represent the negatively charged B substructure in both cases. The crystal structure distortion is thus related to local effects caused by Arsenic that exhibits a larger atomic volume (18 Å3 compared to 13 Å3 for Cu) and higher ionicity in bonding.
Phase barograms – phased diagrams of vapour pressure: Eutectoid Phase Formation in Binary Systems
(2012)
Phase barograms are specific representations of conventional (x, T) phase diagrams considering the vapor pressure as additional thermodynamic parameter. Thus, the determination of the vapor pressure is complementary to conventional DTA or DSC measurements. A stringent relationship between the phase diagram and the corresponding phase barogram is derived by thermodynamic considerations of phase relations for complex heterogeneous equilibria. For that purpose, a derivative of the three-dimensional Clausius–Clapeyron equation is utilized. As a result, a validation of DTA/DSC measurements becomes feasible by the transformation of characteristic effects of vapor pressure measurements into thermal effects in conventional phase diagrams.
The system CsBr/SeO2 is presented as a case study of systems with a eutectoid phase formation. CsSeO2Br is formed from the binaries at ϑf = 180(5) °C; the compound decomposes peritectically at ϑp = 470(10) °C.
The potential of industrial porous concrete for using as a template for the synthesis of porous carbon materials has been investigated. Carbon replicas of porous concrete have been prepared by carbonization of sucrose. The pores of the resulting carbon materials range from the macropore to the micropore region, pointing to a hierarchy.
Several methods are established in thermal analysis to investigate phase formation, phase transition, and decomposition reactions. The analysis of phase equilibria with volatile components is particularly feasible by using standard method of thermogravimetry. Hardly any investigations of phase formation reactions are possible to realize if one of the components is lost by vaporization. By using the “High-Temperature Gas-Balance” (HTGB), the vapor phase is enclosed in a silica ampoule and thus forms an equilibrium gas phase in permanent contact with the solid phase. The measurement signal Δmmeas is caused by change of the leverage of the horizontal balance support during evaporation and condensation. The application of the HTGB allows the analysis of solid–gas equilibria in the working range from 0.01 till 15 bar at temperatures up to 1,100 °C. The first comparison of evaporation reactions determined by standard thermogravimetric analyses and by measurements using the HTGB is given for the inorganic systems: P, As, SeO2, PtI2, and Hg/I.
Back in black: All metastable and stable phases can be identified for the solid solution arsenic/phosphorus by a combination of quantum-chemical calculations and investigations of the phase formation. Reaction paths for phase formations and transitions in situ were also evaluated. The results show that orthorhombic black arsenic (o-As) is metastable in pure form and has only been previously obtained by stabilizing impurities.
Stoichiometric end member galloalunites of the general formula AGa3(SO4)2(OH)6, with A = Na+, K+, Rb+, H3O+, and NH4+ have been synthesized under hydrothermal conditions. These galloalunites were characterized by chemical methods, thermal analysis (DSC, TG coupled with mass spectroscopy), and powder X-ray diffraction (XRD).
The stages of thermal decomposition of sodium, potassium and rubidium galloalunite show a common decomposition mechanism forming β-Ga2O3 and A2SO4 (A = Na+, K+, and Rb+) while ammonium and oxonium galloalunite decompose under formation of pure β-Ga2O3. The thermogravimetric results confirmed the analytical results on the galloalunites and thereby verified the stoichiometry of these synthetic products.
Galloalunites with different monovalent cations in A site (i.e. Na+, K+, Rb+, H3O+ and NH4+) crystallize in the rhombohedral space group R-3m (#166). The effects of substitution on the unit cell parameters are rationalized in terms of the structural arrangements in galloalunites. The unit cell parameter c increases with increasing effective ionic radii of the cation in the A site, whereas the parameter a changes to a much lesser degree.
Zinc oxide is a promising wide band gap semiconductor for future optoelectronic devices. Today ZnO bulk single crystals are grown by three different techniques: hydrothermally, from the melt, and by chemical vapor transport. For our studies, the authors employed in addition a simple and low cost vapor phase method which gives us good quality crystals and flexibility in crystal doping. The as-grown needle-shaped single crystals were characterized by resistivity measurements, scanning electron microscopy, electron backscatter diffraction, and low temperature photoluminescence spectroscopy.
The phosphide tellurides Zr2+δPTe2 (0 ≤ δ ≤ 1) can be synthesized from the elements in a solid state reaction or by thermal decomposition of Z. Zr2PTe2 decomposes under release of Te2(g) + P4(g) forming the homogeneity range Zr2+δPTe2. The growth of single crystals of Zr2+δPTe2 succeeded by chemical vapour transport using iodine as transport agent from 830 °C in direction of higher temperatures up to 900 °C. Zr2+δPTe2 crystallizes in the rhombohedral space group R-3m (no. 166) with lattice parameters a = 383(1)…386(1) pm and c = 2935(4)…2970(4) pm for δ = 0…1, respectively. Single crystal data have been determined for Zr2.40(2)PTe2 with lattice parameters a = 385.24(4) pm and c = 2967.8(4) pm.
The electronic structure and chemical bonding in Zr2+δPTe2 was investigated by the linear muffin–tin orbital (LMTO) method. Both Zr2PTe2 and Zr3PTe2 show non-vanishing DOS values at the Fermi level (EF) indicating metallic character. According to COHP bonding analyses, mainly the heteroatomic Zr–P and Zr–Te bonds are responsible for the structural stability of Zr3PTe2. The new Zr2–Te bond, which is not present in Zr2PTe2, is stronger than Zr1–Te and is thought to be responsible for the stability of phases having Zr in excess.
The chemical and physical properties of [AgO2C(CH2OCH2)3H] (3) and its use as an aqueous, ligand-free MOD ink (MOD = metal−organic decomposition) for piezo inkjet printing is discussed. The printed, thermal, or photochemical sintered silver features are electrically conductive on glass (2.7 × 107 S m−1) and PET (PET = polyethylene terephthalate) substrates (1.1 × 107 S m−1) corresponding to 43% and 18% of the bulk silver conductivity. Conducted tape tests show the suitability of the ink for particularly polymer substrates. TG-MS studies demonstrate a two-step decomposition for the conversion of 3 to elemental silver. The structure of 3 in the solid state was determined by single X-ray structure determination.
Gadolinium disulfide was prepared by high-pressure synthesis at 8 GPa and 1173 K. It crystallizes in the monoclinic space group P121/a1 (No. 14) with lattice parameters a = 7.879(1) Å; b = 3.936(1) Å, c = 7.926(1) Å and β = 90.08(1)°. The crystal structure is a twofold superstructure of the aristotype ZrSSi and consists of puckered cationic [GdS]+ double slabs that are sandwiched by planar sulfur sheets containing S22– dumbbells. The thermal decomposition of GdS2 proceeds via the sulfur-deficient polysulfides GdS1.9, GdS1.85 and GdS1.77 and eventually results in the sesquisulfide Gd2S3. GdS2 is a paramagnetic semiconductor which orders antiferromagnetically at TN = 7.7(1) K. A metamagnetic transition is observed in the magnetically ordered state.
The use of silver(I)-2-[2-(2-methoxyethoxy)ethoxy]acetate, [AgO2C(CH2OCH2)3H], and its application as an aqueous metal-organic decomposition (MOD) inkjet ink is reported. The chemical and physical properties of the silver carboxylate and the ink formulated thereof are discussed. The ink meets all requirements of piezo driven inkjet printing. The printed features were converted into electrically conducting silver patterns by thermal or photo-thermal treatment. The conversion of [AgO2C(CH2OCH2)3H] to elemental silver follows a two-step decomposition as demonstrated by thermogravimetry–mass spectrometry (TG–MS) measurements. The measured conductivities of the printed features on glass and polyethylene-terephthalate (PET) are 2.7 × 107 S m−1 and 1.1 × 107 S m−1, respectively, which correspond to 43% (glass) and 18% (PET) of the bulk silver conductivity.
Phase pure polycrystalline PbP7 was prepared from the elements via a lead flux. Crystalline pieces with edge-lengths up to 1 mm were obtained. The assignment of the previously published 31P solid state NMR spectrum to the seven distinct crystallographic sites was accomplished by radio-frequency driven dipolar recoupling (RFDR) experiments. As commonly found in other solid polyphosphides there is no obvious correlation between the 31P chemical shift and structural parameters. PbP7 decomposes incongruently under release of phosphorus forming liquid lead as remainder. The thermal decomposition starts at T>550 K with a vapor pressure almost similar to that of red phosphorus. Electronic structure calculations reveal PbP7 as a semiconductor according to the Zintl description and clearly shows the stereo-active Pb-6s2 lone pairs in the electron localization function ELF.
Chemical vapor transport (CVT[1]) is a suitable method for production of single-crystals of high purity. In order to find proper strategies for CVT various tools of thermodynamic calculations and analysis of solid-gas equilibria are available. Those applications are demonstrated for vapor transport of zinc oxide under addition of phosphorous and water.
SnIP is the first atomic-scale double helical semiconductor featuring a 1.86 eV bandgap, high structural and mechanical flexibility, and reasonable thermal stability up to 600 K. It is accessible on a gram scale and consists of a racemic mixture of right- and left-handed double helices composed by [SnI] and [P] helices. SnIP nanorods <20 nm in diameter can be accessed mechanically and chemically within minutes.
The application of the EnPhaSyn (theoretical Energy diagrams, experimental Phase formation, Synthesis and characterisation) concept is reviewed with respect to prediction of structures and stability of element allotropes and compound polymorphs, their phase formation and transition processes, and their directed synthesis, respectively. Therein, the relative energetical stability (En) of target compounds and possible decomposition are determined from quantum chemical DFT calculations. Phase formation and transition (Pha) is probed by a gas balance method, developed as high temperature gas balance concept. It helped to study the synthesis and stability range of several compounds experimentally. Applications of the concept and synthesis principles (Syn) of non-equilibrium phases are presented for allotropes of P, As, P1-xAsx, as well as binary and ternary compounds including the Zintl and Laves like phases IrPTe, NiP2, CoSbS, NiBiSe, Li0.2CdP2, Cu3CdCuP10, and Cd4Cu7As.
Structural engineering of topological bulk materials is systematically explored with regard to the incorporation of the buckled bismuth layer [Bi₂], which is a 2D topological insulator per se, into the layered BiTeI host structure. The previously known bismuth telluride iodides, BiTeI and Bi₂TeI, offer physical properties relevant for spintronics. Herewith a new cousin, Bi₃TeI (sp.gr. R3m, a = 440.12(2) pm, c = 3223.1(2) pm), joins the ranks and expands this structural family. Bi₃TeI = [Bi₂][BiTeI] represents a stack with strictly alternating building blocks. Conditions for reproducible synthesis and crystal-growth of Bi₂TeI and Bi₃TeI are ascertained, thus yielding platelet-like crystals on the millimeter size scale and enabling direct measurements. The crystal structures of Bi₂TeI and Bi₃TeI are examined by X-ray diffraction and electron microscopy. DFT calculations predict metallic properties of Bi₃TeI and an unconventional surface state residing on various surface terminations. This state emerges as a result of complex hybridization of atomic states due to their strong intermixing. Our study does not support the existence of new stacking variants BiₓTeI with x > 3; instead, it indicates a possible homogeneity range of Bi₃TeI. The series BiTeI–Bi₂TeI–Bi₃TeI illustrates the influence of structural modifications on topological properties.
Synthesizing crystals of ternary solid solutions M2Q3 (M=Bi, Q=Se, Te) with definite composition requires advanced knowledge on the phase relations in the system. By chemical vapor transport [1] the request of homogeneous crystallization can be fulfilled. The experimental scope gets supported by modeling. A phase diagram of the pseudobinary system generated with FactSage [2] (Fig. 1) provides the thermodynamic parameters of the miscibility gap respectively the solid solution. Additionally, these parameters are used to optimize the processes while chemical vapor transport with iodine [3]. The optimum transport conditions of ϑsource = 500 °C and ϑsink = 450 °C has been applied as for the synthesis of the binary compounds [4]. Both by modeling and experiments the congruent transport can be demonstrated, Fig. 2.
Calculation of electromotive series of solids allows easy prediction of reaction pathways[1]. Using the electromotive series of systems As-P-O and As-P-X (X = F, Cl, Br, I) the formation of element allotropes via the oxide and halide compounds in thermite type reactions can be deduced: phosphorous will reduce the arsenic oxides to elemental arsenic, forming grey or black allotrope, depending on reaction mixture composition.
The crystallization mechanism of uranium phosphide telluride UPTe by chemical vapor transport reaction [1] is presented based on thermodynamic modelling using TRAGMIN program. Evaluation of standard data of all expected species was performed by modeling of the binary phase diagrams U/Te and U/P. Data of UPTe were deduced from known phosphide tellurides[2,3]. Identifying the viable composition area within the quaternary system U/P/Te/I the temperature depending transport equilibria could be obtained.
The High-temperature Gas-balance (HTGB[1]) has been established as a new method for investigations of heterogeneous phase equilibria with volatile components [1,2]. This equipment is notably well suited to analyze reversible heterogeneous equilibria which occur during chemical vapor transport reactions (CVT[3]).
Experimental proof has been furnished by vapor transports of germanium and germanium(II) telluride using iodine as a transport addition. Initially the sublimation of iodine has been observed (section a – Fig. 1). The formation of GeI4(g) as the actual transport agent follows (b). Finally, the transport reaction occurs under reversible formation GeI2(g) (c).
The concept of Electromotive Series of Solids presents an easy tool for prediction and interpretation of red-ox processes of inorganic solid materials. Thus, a rational synthesis design succeeds with low effort compared to complex thermodynamic considerations. The principle has been applied and validated for interpretation of known chemical systems as well for the development of new systems and processes.
The thermochemical behavior of two groups of ionic liquids (ILs), 1-ethyl-3-methylimidazolium [EtMeIm] and 1-butyl-3-methylimidazolium [BuMeIm] halides (Cl, Br and I) was investigated for their crystalline and liquid states in the temperature range from −100 °C to 600 °C using DSC and thermogravimetric analysis (TGA). Some investigated ILs exhibit significant subcooling effect and the glass state formation [1]. The decomposition mechanism has been identified by means of TGA with coupled mass spectrometry (MS). All investigated ILs decompose with a total mass loss of about 100 % in the temperature range of Tonset from 230 to 290 °C (Fig. 1). By complete degradation of IL, dominant fragments CnH2n+1+, CnH2n+, CnHn+, NHn+, and the respective alkyl halides (X = Cl, Br, I) are formed.
Der Einsatz von thermischen Speichern erlaubt eine bessere Lastanpassung von Erzeugeranlagen zur Gebäudeklimatisierung sowie eine Optimierung des Betriebsregimes unter energetischen Gesichtspunkten. Für solche Anwendungsfälle können Latentwärmespeicher zum Einsatz kommen, bei denen üblicherweise der Schmelz- und Erstarrungsvorgang sogenannter Phasenwechselmaterialien (Phase Change Materials, PCM) ausgenutzt wird. Für die Anwendungstemperaturbereiche der Klimatisierung von etwa 4 °C bis 15 °C und für die Kühlung bis etwa 25 °C ist die Auswahl an Reinstoffen mit einer passenden Schmelztemperatur sehr begrenzt. Durch die Bildung von eutektischen Gemischen aus zwei oder mehr Komponenten – die ähnlich wie Reinstoffe einen scharfen Schmelzpunkt aufweisen – kann die Bandbreite an potentiellen PCM’s für diesen Temperaturbereich vergrößert werden. Für die genannten Temperaturbereiche werden vorzugsweise anorganische Salzhydrate als potentielle Speichermedien betrachtet, da diese gegenüber organischen Substanzen häufig größere Schmelzenthalpien und geringere Kosten
aufweisen.
Based on the experience of thermo-chemical studies on a High-Temperature Gas-Balance a time dependent analysis has been developed for the investigations of the kinetics of solid-gas reactions. The course of solid-gas reactions can be examined unambiguously by separation of the initial solids. By this setup the visualization of different stages of solid-gas reaction (vaporization, phase formation) becomes feasible. Time dependent analysis of the formation of FeAs results in an assignment of the underlying principles of kinetics derived from the conversion factor α.
Phosphorus, first found in the seventeenth century, played an important role in the definition of the element term by Lavoisier and thus shaped the beginning of the era of modern chemistry. It was discovered for the first time in the most unstable crystalline modification—the white phosphorus. Today, a variety of experimentally proven allotropes are known. The most common allotropes, such as black, violet, and fibrous phosphorus, are described here with respect to their synthesis, crystal structures, thermal, and thermodynamic properties. Besides, more than 50 crystalline allotropes have been predicted, and their stabilities have been estimated using quantum‐chemical methods. This way, phosphorus becomes one of the most structurally variable elements of the periodic table. In this article, some of the most reasonable and sophisticated calculations are presented.
The applications of elemental phosphorus are mainly connected with its semiconducting properties. Thus, the development of current applications is strongly related to new synthesis methods for direct preparation of individual, phase pure allotropic forms of phosphorus. The past decade supplied basic results on the formation of black phosphorus and other modifications, primarily using the mineralizer concept.
Related to graphene and other two‐dimensional, layered structures, phosphorene is of drastically rising interest. The pertinent modifications are characterized by corrugated arrangement of six‐membered P‐rings, where both the boat conformation and the chair conformation are known. The application of phosphorene is in a jumping evolution. Currently, phosphorene is already in use in manifold ways, including as a sensor, optical device, transistor, energy‐conversion material, and supercapacitor material.
Ionic liquids are often considered as green alternatives of volatile organic solvents. The thermal behavior of the ionic liquids is relevant for a number of emerging large-scale applications at elevated temperature. Knowledge about the degradation products is indispensable for treatment and recycling of the used ionic liquids. The objective of this paper was an investigation of the short- and long-term stability of several 1-alkyl-3-methylimidazolium halides, determination of the degradation products, and the elucidation of their decomposition patterns and structure–stability relations. Short-term stability and mechanism of thermal degradation were investigated by a self-developed, innovative thermal analysis single-photon ionization time-of-flight mass spectrometry device with Skimmer coupling. The applied technology provides real-time monitoring of the forming species and allows tracing their change during the course of the decomposition. Therein, the almost fragment-free soft ionization with vacuum ultraviolet photons plays a crucial role. We have detected unfragmented molecules whose formation was only assumed by electron ionization. Nevertheless, the main decomposition products of the selected ionic liquids were alkyl imidazoles, alkenes, alkyl halides, and hydrogen halides. From the decomposition products, we have deduced the fragmentation patterns and discussed their interrelation with the length of the alkyl chain and the type of the halide anion. Our results did not suggest the evaporation of the investigated ionic liquids prior to their decomposition under atmospheric conditions. Long-term thermal stability and applicability were determined based on thermogravimetric analysis evaluated with a kinetic model. Thus, the time-dependent maximum operation temperature (MOT) for the respective ionic liquids has been calculated. As a rule, the short-term stability overestimates the long-term decomposition temperatures; the calculated MOT are significantly lower (at least 100 K) than the standardly obtained decomposition temperatures.
Inorganic solid-state synthesis with phosphorus and tellurium requires a careful control of the reaction parameters because of the high volatility of the components. This initial disadvantage can be used as a benefit for the investigation of phase formation mechanisms by analyzing the individual vapor pressure behavior. The high-temperature gas-balance is introduced as a device for detection of heterogeneous solid-gas equilibria in closed reaction systems. The experimentally challenging synthesis of the phosphide telluride Zr₂PTe₂ is examined as a model system: optimized synthesis runs at lower temperatures (ϑ = 650 °C) in a faster time, while the quantity as well as the crystalline powder quality is increased. A stepwise solid-solid reaction of zirconium and tellurium according to Ostwald's rule of stages and the shrinking core model is revealed while phosphorus sublimes and subsequently condenses to react to the ternary compound. Additional phenomena such as melting, expansion, and mechanical instabilities can be observed that broaden the possibilities of the gas- balance.
Layered α-TiCl₃: Microsheets on YSZ Substrates for Ethylene Polymerization with Enhanced Activity
(2019)
The transition metal halide α-titanium(III) chloride (α-TiCl₃) is a layered two-dimensional compound and a well-established Ziegler–Natta catalyst for the polymerization of ethylene. A new synthesis technique is used to obtain thin sheets of α-TiCl₃ that show exceptional physical properties in contrast to their bulk counterparts, due to an enlarged surface-to-volume ratio. Chemical vapor transport (CVT) of α-titanium(III) chloride directly on substrates results in microsheets that exhibit an improved catalytic effect. For rational planning of synthesis conditions, thermodynamic simulations of occurring gas-phase equilibria were performed. Based on these calculation results, crystal growth was achieved by adding GaCl₃via pure short term CVT in a temperature gradient of 700–600 K on yttrium-stabilized zirconia (YSZ) substrates. Phase pure, as-grown, single crystal sheets with high crystallinity and a thickness of around 4 μm were obtained. These thin sheets of α-TiCl₃ on YSZ substrates were implemented as catalysts for the ethylene polymerization reaction. A significant improvement of catalytic activity of 16% was achieved, probably due to an increased surface-to-volume ratio. As a highlight, the as-grown microsheets were exfoliated subsequently to a thickness lower than 200 nm and an even higher catalytic activity of up to 24% was confirmed experimentally due to delamination effects. This is the first time that an improved catalytic effect of α-TiCl₃ is observed as a result of downscaling from bulk to microsheets by CVT.
Allgemeine Chemie
(2019)
Impacts of TGA furnace parameters for prediction of long-term thermal stability of ionic liquids
(2021)
The concept of maximum operation temperature is established for the prediction of the time dependent thermal stability of ionic liquids based on kinetic evaluation of thermogravimetric analysis. The influence of the furnace control parameters on the maximum operation temperature (MOT) is shown using the example of 1-methyl-3-propylimidazolium iodide ([C3C1im]I) with respect to three different parameter sets of a programmed proportional integral derivative (PID) controller of the TGA. Kinetics of thermal decomposition of [C3C1im]I have been investigated with the implementation of an improved kinetic model. The activation energy obtained using the Kissinger-Akahira-Sunose equation showed variations apparently due to the decomposition degree. The model compound is decomposed by a one-step kinetics, which results from pseudo zero order relationship of the activation energy to the conversion rate. The activation energy, pre-exponential factor, and the activation energy are strongly dependent on the parameters of TGA furnace controller.
Nanoscale phase separation was induced in the K-doped RuCl3 van der Waals material by annealing, and studied with the goal to find a natural design strategy for the formation of two-dimensional architectures as an alternative to the costly and time-consuming experimental artificial growth methods. Phase conversion was traced by means of thermogravimetric analysis combined with mass spectrometry. The local crystal structure of co-existing K3Ru2Cl9 domains with the sizes of about 100 nm was solved by 3D electron diffraction.
Ein wichtiges Ziel der Bundesregierung ist der Ausbau des Anteils erneuerbarer Energien an der gesamten Stromerzeugung. Die Entwicklung optimierter Wärme- und Kältespeicher stellt einen wichtigen Baustein bei der Erreichung dieser Zielstellung dar. Elektrisch betriebene Wärmepumpen und Kompressionskältemaschinen sowie dezentrale Klein-BHKW bieten in Verbindung mit entsprechend dimensionierten, thermischen Speichern ein großes Potential zum Lastmanagement in Smart Grids und somit zur Netzintegration von Strom aus erneuerbaren Energien. Die im Projekt entwickelte Methodik zur Suche nach neuen Latentspeichermedien kann für ein breites Anwendungsfeld von Heiz- und Kühlanwendungen in Gebäuden bis hin zur Kraftwerkstechnik eingesetzt werden. Innerhalb der Fördermaßnahme „Anwendungsorientierte Forschung und Entwicklung zur nichtnuklearen Energieforschung“ im 6. Energieforschungsprogramm der Bundesregierung wurde durch die Projektarbeiten das Schwerpunktthema Energiespeicher (3.8) adressiert, wobei der Aspekt der Entwicklung neuer Materialien mit reduzierten Kosten für einen wirtschaftlichen Betrieb im Vordergrund stand. Speziell wurden dabei die Unterpunkte 3.8.4 Thermische Speicher zur Weiterentwicklung, Optimierung und Erprobung von PCM sowie 3.8.5 Übergeordnete Themen mit Modellen und Simulationswerkzeugen für die Auswahl und Bewertung neuer Speichermaterialien adressiert.
Die Projektergebnisse besitzen aber auch Berührungspunkte mit den Schwerpunkten 3.12 Energieeffizienz in Industrie, Gewerbe, Handel und Dienstleistungen sowie 3.4 Solarthermische Kraftwerke, da mit der entwickelten Screening-Methode für diese Anwendungsfälle neue Wärmespeichermaterialien auf der Basis wasserfreier Salze mit höheren Prozesstemperaturen gefunden werden können.
Durch den Einsatz von Methoden zur Modellierung und Simulation von Prozessen erfolgt gleichermaßen die Umsetzung einer energie- und ressourcenschonenden Forschungstätigkeit. Das im Projekt entwickelte Vorgehen hilft dabei, einen äußerst sparsamen Einsatz von Chemikalien, eine niedrige Belastung der Umwelt durch verringerten Entsorgungsaufwand der verbrauchten Stoffe sowie einen deutlich geringeren Aufwand und Energieeinsatz für die Messtechnik zu ermöglichen. Damit wurde zugleich das Schwerpunktthema 3.8.5 Modelle und Simulationswerkzeuge für die Auswahl und Bewertung neuer Speichermaterialien angesprochen.
Verbindungen mit ionischen Baueinheiten erweisen sich als geeignete Materialien zur Energie- und Ressourcennutzung. Beispiele sind Li-Elektrodenmaterialien oder neue Thermoelektrika, die aufgrund ihrer Teilstrukturen schaltbar werden. Das Design neuer funktionaler Materialien verlangt dabei nach einem grundlegenden Verständnis von Struktur-Eigenschafts-Beziehungen, metastabilen Strukturen und Umwandlungen. Dies bedarf des Einsatzes kombinierter experimenteller und theoretischer Methoden. Zur systematischen Erforschung metastabiler Strukturen wurde jüngst das Konzept der Strukturvorhersage in Energielandschaften mit ab initio Methoden auf das System P1-xAsx angewandt und mit experimentellen Messungen verknüpft. Im nächsten Schritt wurden Verbindungen mit ionischen Teilstrukturen des As, Sb, Se und Te untersucht. Modellhaft wurden nun Zintl-Systemen wie IrPTe und PtSnTe neue mögliche metastabile Strukturen identifiziert und auf Funktionalisierungen untersucht.
Phasenwechselmaterialien (PCM) sind chemische Verbindungen oder deren Mischungen, die bei einer definierten Temperatur schmelzen bzw. erstarren. Der zyklische Einsatz von Wärmespeichern ermöglicht den Ausgleich von Überlasten für den Bedarf oder den „Abfall“ von Wärme sowie die Vergleichmäßigung von Wärmeprofilen. Bestehende anorganische Materialien weisen überwiegend Einschränkungen bezüglich der Anforderungen an eine geringe Hysterese von Aufheizung und Abkühlung auf. Das Maß der Unterkühlung der Schmelze sowie ein zyklenstabiler Wärmeaustausch kann aber durch Zusätze als Kristallisationshilfen (Keimbildner) gesteuert werden. Im vorliegenden Bericht werden die Keimbildung und Kristallisation von Phasenwechselmaterialien unter folgenden Gesichtspunkten diskutiert: Methoden der Thermischen Analyse zur Untersuchung der thermochemischen Eigenschaften von PCM, thermochemische Eigenschaften anorganischer Salze und Salzhydrate als PCM, dominierende Kristallstrukturen und Strukturmotive anorganischer Salze und Salzhydrate als PCM, dominierende Kristallstrukturen und Strukturmotive anorganischer Stoffe als homogener und heterogener Keimbildner, Änderung der thermochemischen Eigenschaften von PCM bei Zusatz von Keimbildnern, geeignete Konzentrationsbereiche von Keimbildnern, Homogenität/Segregation von Phasen.
Das Mineralisatorkonzept für Polypniktide, basierend auf PbI2 als Mineralisator, wurde erfolgreich zur Synthese der festen Lösung von ortho-rhombischem, schwarzen As1–xPx eingesetzt (x = 0.17-1). Die Zellparameter von As0.83P0.17 sind identisch mit den Literaturwerten des As(schwarz). Aus Pulver- und Einkristalldaten bei variabler Zusammensetzung ist es möglich, das Zellvolumen und die Gitterparameter von reinem, schwarzen Arsen abzuleiten. Die Analyse der Phasenbildung basiert auf der Bestimmung der Gleichgewichtsdrücke der Intermediate Au2P3 und Asgrau sowie von PbI2 im Vergleich zu As1–xPx(schwarz): Die Kondensation erfolgt unterhalb 600 °C aus einer komplexen Gasphase (u.a. AsnP4–n).
Der Lehrstoff der Allgemeinen und Anorganischen Chemie für die ersten zwei Semester des Chemiestudiums. Ausgewogene Stoffdarstellung mit vielen Einblicken in hochaktuelle Themen und Anwendungen. Mit zahlreichen neuen Inhalten und Aktualisierungen, etwa zu großtechnischen Verfahren, zur zunehmenden Bedeutung seltener Elemente wie Gallium und Indium für Hightech-Produkte, zu Rohstoffen für Zukunftstechnologien, zum Schweißen und Löten, zu Magnetwerkstoffen und Wärmespeichern. Die Nomenklatur ist an die aktuellen IUPAC-Regeln angepasst.
The two-dimensional (2D) chromium trihalides CrCl3, CrBr3, and CrI3 are most famous for their exotic magnetic properties when their crystals get downscaled to nanometer dimensions. One way to tune the properties of such materials and to further increase their potential utility, e.g., in spintronics, is the formation of solid solutions. Here, we present a detailed theoretical and experimental study on the synthesis and nanoplatelet growth of CrBrxCl3–x and CrBrxI3–x solid solutions. Phase pure powder was obtained by tempering the respective parent compounds at 600 and 550 °C, using additional iodine in the case of CrBrxI3–x. The dominant stacking order changes at x = 1.8 for CrBrxCl3–x while remaining similar to CrI3 for the whole CrBrxI3–x series. A consecutive chemical vapor transport from 575 → 525 °C for 2 h (CrBrxCl3–x) or 600 → 550 °C for 3 h with 0.1 mmol additional iodine (CrBrxI3–x) resulted in the deposition of high-quality nanoplatelets on a substrate with only several nanometers in height. The composition can be controlled by the choice of the starting material, since only small shifts in the anion ratio occur during the transport. The deposited CrBrxCl3–x nanoplatelets can then be easily delaminated by ultrasonication in ethanol to reduce the height even further to few-layer dimensions. These nanoplatelets could potentially be used to investigate the property changes (e.g., in terms of magnetic response) for the downscaling of these solid solutions. We further demonstrate the quality of the deposited material by transmission electron microscopy, selected area electron diffraction, and X-ray photoelectron spectroscopy. Raman spectroscopy of the solid solution series reveals a complex evolution of vibrational modes. Photoluminescence measurements on solid solution samples show emission peaks in the near-infrared energy range with the specific energy and intensity being composition and temperature dependent.
Black and irregularly shaped crystals of the bismuth‐rich bromide Bi5Br4 were obtained as a by‐product of the reaction of CsBr, Bi, and BiBr3. X‐ray diffraction on a single‐crystal revealed its orthorhombic structure with the space group Pmmn (no. 59) and lattice parameters a = 1800.0(2) pm, b = 1476.1(1) pm, and c = 924.5(2) pm at 296 K. The structure is composed of Bi82+ and Bi95+ polycations and bromidobismuthate(III) anions according to the structured formula Bi5Br4 = Bi20Br16 = Bi82+Bi95+[BiBr5]2–[Bi2Br11]5–. Bi5Br4 is the bismuth‐richest among the bismuth subhalides containing isolated polycations. Extensive differential scanning calorimetry studies indicate that Bi5Br4 decomposes at 262 °C, i.e. one degree below the bismuth‐rich eutectic at 263 °C. All attempts towards a rational synthesis yielded predominantly the neighboring phases BiBr and Bi6Br7.
Integrating enzymes into thermoplastic polymers is challenging due to their lack of robustness with respect to temperature and shear fields during conventional melt processing. In the present study, blown films from low-density polyethylene (LDPE) were prepared containing a technical protease from Bacillus sp. First, LDPE/protease compounds were produced followed by blown film extrusion, both processes at melt mass temperatures of 130 °C or higher. Enzyme activity was proven, both for the LDPE/protease compound and the blown film. The highest enzyme activity in the compound was determined for processing at 132 °C and a screw speed of 75 rpm. The influence of melt temperature and shear fields was studied in detail. Enzyme activities were determined for melt temperatures up to 160 °C and for screw speeds ranging from 75 to 300 rpm during compounding by twin-screw extrusion. The process was also applied for biobased and biodegradable polyesters, where similar protease activity after compounding was verified. Electron microscopy, X-ray diffraction, nuclear magnetic resonance spectroscopy and differential scanning calorimetry served to analyze components and morphology of the enzyme formulation used here. It is proposed that the porous morphology of the protease particles is beneficial for the enzyme to remain active after processing. Additionally, the polymer matrix surrounding the particles protects the protease at elevated temperatures, which can be attributed to thermal insulation. Thus, the right combination of a suited technical enzyme formulation with appropriate mild melt compounding conditions allows enzymes to be incorporated into thermoplastics and retain their activity. This opens the way to use the abundant biological functions of enzymes in thermoplastic applications.
DUT-8(Ni) metal-organic framework belongs to the family of flexible pillared layer materials. The desolvated framework can be obtained in the open pore form (op) or in the closed pore form (cp), depending on the crystal size regime. In the present work, we report on the behaviour of desolvated DUT-8(Ni) at elevated temperatures. For both, op and cp variants, heating causes a structural transition, leading to an new, crystalline compound, containing two interpenetrated networks. The state of the framework before transition (op vs. cp) influences the transition temperature: the small particles of the op phase transform at significantly lower temperature in comparison to the macroparticles of the cp phase, transforming close to the decomposition temperature. The new compound, confined closed pore phase (ccp), was characterized by powder X-ray diffraction and spectroscopic techniques, such as IR, EXAFS, and positron annihilation lifetime spectroscopy (PALS). Thermal effects of structural cp to ccp transitions were studied using differential scanning calorimetry (DSC), showing an overall exothermic effect of the process, involving bond breaking and reformation. Theoretical calculations reveal the energetics, driving the observed temperature induced phase transition.
In the search for novel 2D materials with potentially valuable properties, such as a tunable band gap for optoelectronic or catalytic applications, solid solutions hold the potential to significantly expand the inventory of available 2D nanomaterials. In this study, we present for the first time the synthesis of such 2D rhodium trihalide solid solutions: RhBrxCl3−x and RhBrxI3−x. We use thermodynamic simulations and simultaneous thermal analysis to predict conditions for their rational synthesis and to investigate suitable chemical vapor transport (CVT) parameters for these solid solutions. The evolution of the lattice parameters was investigated by powder x-ray diffraction, showing an isostructural relationship of the synthesized compounds and only minor deviation from Vegard's law. The optical band gap of these materials can be tuned in an energy range from 1.5 eV (RhCl3) to 1.2 eV (RhI3) by choosing the composition of the solid solution, while the samples also exhibit photoluminescence in similar energy ranges. Ultimately, the successful deposition of bulk as well as ultrathin 2D nanocrystals of RhBrxCl3−x by CVT from 925 °C to 850 °C is shown, where the composition of the deposited crystals is precisely controlled by the choice of the starting composition and the initial amount of material. The high quality of the obtained nanocrystals is confirmed by atomic force microscopy, high resolution transmission electron microscopy and selected area electron diffraction. For RhBrxI3−x, the CVT from 900 °C to 825 °C is more difficult and has only been practically demonstrated for an exemplary case. According to the observed properties, these novel solid solutions and nanocrystals show a great potential for an application in optoelectronic devices.
Temperature-induced change in reactivity of the frequently used ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([C4C1im]BF4) is presented as a prerequisite for the rational screening of reaction courses in material synthesis. [C4C1im]BF4 becomes active with oxidic precursor compounds in reduction reaction at ϑ > 200°C, even without the addition of an external reducing agent. The reaction mechanism of forming red amorphous selenium from SeO2 is investigated as a model system and can be described similarly to the Riley oxidation. The reactive species but-1-ene, which is formed during the decomposition of [C4C1im]BF4, reacts with SeO2 and form but-3-en-2-one, water, and selenium. Elucidation of the mechanism was achieved by thermoanalytical investigations. The mono-tropic phase transition of selenium was analyzed by the differential scanning calorimetry. Beyond, the suitability of the single source oxide precursor Bi2Se3O9 for the synthesis of Bi2Se3 particles was confirmed. Identification, characterization of formed solids succeeded by using light microscopy, XRD, SEM, and EDX.
The Ionic Liquid 1-butyl-3-methylimidazolium tetrafluoroborate [C4C1im]BF4 serves as a commonly solvent in inorganic material synthesis and analytics. Nevertheless, its application is frequently associated with trial and error approaches. Thereupon, detailed knowledge on the thermal behavior is the key information for understanding the reactivity of [C4C1im]BF4. 1-butyl-3-methylimidazolium tetrafluoroborate behaves as a glass in the cold, its glass transition temperature being ϑg = −83 °C. During heating with 10 K·min−1 [C4C1im]BF4 appears to be stable above 350 °C with onset temperatures ϑonset, DSC = 375 °C, ϑonset, DTG = 422 °C, and ϑonset, TG = 437 °C. Thereby, thermal decomposition occurs in a single step reaction forming 1-methyl-1H-imidazole (CH3C3H3N2 or C4H6N2), but-1-ene (C4H8), fluoromethane (CH3F) and boron trifluoride (BF3) as main species, as determined by thermogravimetry coupled with mass spectrometry and FTIR spectroscopy. To be more specific in thermal behavior, the temperature and time dependent stability is evaluated here on the basis of the kinetic model of maximum operation temperature − MOT. Clearly, thermal stability rises with application time, thus being 193 °C for one hour, while reaching only 141 °C for one day, and 114 °C for one week. The incipient decomposition (≤ 1 %) at the calculated time dependent maximum operation temperature finally is verified by optical analysis, infrared (IR), and nuclear magnetic resonance (NMR) spectroscopy.
Despite chemical vapor transport (CVT) being a widely used method for crystal growth of inorganic substances, detailed mechanistic studies on the course of the crystallization process are rather few. In this study, an elaborated experimental screening run combined with sophisticated modeling of the respective heterogeneous equilibria is presented: Crystal growth of germanium by vapor transport with the addition of iodine has been chosen as a model system for validation of the applied method spectrum. In order to record the course and the interplay of heterogeneous equilibrium and non-equilibrium reactions in the system Ge–I, the experimental setup of high-temperature gas-balance (HTGB) is applied. Additionally, the observed evaporation processes are compared with saturation curves of corresponding volatile substances and, thus, can be assigned to individual species within the system. In this experimental screening, a phase sequence means to examine how the condensed phases undergo iodine depletion and how the gaseous phase undergoes a germanium enrichment when the temperature is increased. This phase screening combined with annealing experiments in the course of the phase sequence helps to analyze stepwise non-equilibrium products and to identify the characteristic species. Subsequently, for the evaluation of the composition of the gaseous phase, and for the deduction of the vapor transport mechanism, thermodynamic modeling by the CalPhaD method is performed. For the reference system, it is confirmed that iodine does not act as the transport agent. Instead, GeI4 is responsible for the volatilization of germanium, forming GeI2. Nevertheless, investigations clearly illustrate how GeI4 forms naturally in the phase sequence in the system Ge–I, which makes direct addition of it unnecessary. The recommended temperature range for vapor transport of germanium spans from 460 to 800 °C. Modeling shows that migration rates for germanium reaches a maximum at a mean temperature between 540 and 550 °C. Finally, vapor transport experiments were performed from 565 to 515 °C and from 690 to 590 °C. By increasing the deposition temperature, a slight decrease of the migration rate was observed, though a positive impact on the crystal’s morphology was also found.
Germanium (Ge) nanowires (NWs) were grown in-plane on nano-structured Si(001)/SiO2
substrates by molecular beam epitaxy using gold (Au) as the solvent. The site-selective NW growth was enabled by a rectangular array of gold droplets on silicon (Si) tips with an Au nuclei density
below 0.25 μm−2 on the surrounding silicon oxide (SiO2). The initial growth of Ge NWs starting from Si–Au droplets with SixGe1−x nucleation from ternary alloy is discussed from a thermodynamic point of view. The in-plane NW elongation occurred within ⟨110⟩ directions on the substrate and NWs were mainly bounded by two 55◦ inclined 111 facets and a less pronounced planar (001) top facet. Fully relaxed crystal lattices of Ge NWs were observed from two-dimensional reciprocal space maps of x-ray diffraction measurements.
Single crystals of the polytellurides RETe1.8 of gadolinium, terbium, and dysprosium were prepared by chemical vapor transport and alkali metal halide flux reactions. To determine proper synthesis conditions for the desired target composition, the binary phase diagram Gd-Te was evaluated by CalPhaD methods. The compounds are isostructural to SmTe1.8 and crystallize in space group P4/n (no. 85) with lattice parameters of a = 966.10(4), 960.00(3), and 957.33(2) pm and c = 1794.15(10), 1785.77(6), and 1779.38(5) pm for GdTe1.8, TbTe1.8 and DyTe1.8, respectively. The structures consist of puckered [RETe] double slabs and planar telluride layers composed of Te2 dumbbells and linear Te3 units in accordance with ELI-D based bonding analyses. The latter can be understood as a Te3(4–) anion. GdTe1.8 is a semiconductor with a bandgap of 0.19 eV/0.17 eV (experimental / calculated). Magnetization data confirm trivalent RE ions and indicate antiferromagnetic order at T(N) = 12 K for TbTe1.8 and T(N) = 9.8 K for DyTe1.8, whereas GdTe1.8 remains paramagnetic down to 2 K.
Ionic liquids (ILs) are currently of high interest due to their high performance physicochemical properties over a wide tempera¬ture range of existence of the liquid state. Among the ionic liquids investigated, 1-alkyl- 3-methylimidazolium halides were found generally preferred for their low melting points and ease of handling and preparation.
Ionic liquids (ILs) are currently of high interest due to their high performance physicochemical properties over a wide temperature range of existence of the liquid state. Among the ionic liquids investigated, 1-alkyl-3-methylimidazolium halides were found generally preferred for their low melting points and ease of handling and preparation.
2D layered honeycomb magnet α-ruthenium(III) chloride (α-RuCl3) is a promising candidate to realize a Kitaev spin model. As alteration of physical properties on the nanoscale is additionally intended, new synthesis approaches to obtain phase pure α-RuCl3 nanocrystals have been audited. Thermodynamic simulations of occurring gas phase equilibria were performed and optimization of synthesis conditions was achieved based on calculation results. Crystal growth succeeded via chemical vapor transport (CVT) in a temperature gradient of 973 K to 773 K on YSZ substrates. Single crystal sheets of high crystallinity with heights ≤ 30 nm were obtained via pure CVT. The crystal properties were characterized by means of optical and electron microscopy, AFM, SAED, micro-Raman and XPS proving their composition, morphology, crystallinity and phase-purity. A highlight of our study is the successful individualization of nanocrystals and the delamination of nanosheets on YSZ substrates down to the monolayer limit (≤ 1 nm) which was realized by means of substrate exfoliation and ultrasonication in a very reproducible way.
Alpha-molybdenum(III) chloride (MoCl₃) belongs to layered van-der-Waals materials, which are in focus to exhibit interesting properties due to their weak chemical and magnetic interactions. Especially the structure of alpha-MoCl₃ has been discussed in terms of symmetry breaking dimerization of Mo atoms at room temperature, which might led to exotic ground states. By exploiting the 2D materials characteristics, an investigation of physical properties on the nanoscale is intended. We herein demonstrate the probably first approach to synthesize phase pure, as-grown alpha-MoCl₃ few-layer nanosheets by means of a pure short time chemical vapor transport (CVT) process. Vapor growth benefits from a one-step deposition of high crystalline alpha-MoCl₃ nanosheets without stacking faults on a substrate. Thus, mostly applied subsequent delamination, associated with the introduction of structural defects, becomes redundant. According to the CVT process thermodynamic simulations of gas phase equilibria have been performed and the synthesis conditions could be optimized based on the calculation results. By CVT the as-grown nanolayers are deposited on sapphire (Al₂O₃) substrates by applying a temperature gradient of 70 K from 743 K to 673 K. Single crystalline sheets with thicknesses ≤75 nm down to five layer (3 nm) could be obtained by using a pure CVT process. According to the deposited nanostructures we approve the desired composition, morphology, phase purity and high crystallinity by using several microscopy and spectroscopy techniques. Furthermore, we show micro-RAMAN measurements which hint at a slight increase in phonon energies for nanosheets in comparison to the corresponding bulk phase.
Synthesis of micro- and nanosheets of CrCl3–RuCl3 solid solution by chemical vapour transport
(2022)
Solid solutions of 2D transition metal trihalides are rapidly growing in interest for the search for new 2D materials with novel properties at nanoscale dimensions. In this regard, we present a synthesis method for the Cr1−xRuxCl3 solid solution and describe the behaviour of the unit cell parameters over the whole composition range, which in general follows Vegard's law in the range of a = 5.958(6)CrCl3 … 5.9731(5)RuCl3 Å, b = 10.3328(20)CrCl3 … 10.34606(21)RuCl3 Å, c = 6.110(5)CrCl3 … 6.0385(5)RuCl3 Å and β = 108.522(15)CrCl3 … 108.8314(14)RuCl3 °. The synthesized solid solution powder was subsequently used to deposit micro- and nanosheets directly on a substrate by applying chemical vapour transport in a temperature gradient of 575 °C → 525 °C for 2 h and 650 °C → 600 °C for 0.5 h as a bottom-up approach without the need for an external transport agent. The observed chromium chloride enrichment of the deposited crystals is predicted by thermodynamic simulation. The results allow for a nanostructure synthesis of this solid solution with a predictable composition down to about 30 nm in height and lateral size of several μm. When applying a quick consecutive delamination step, it is possible to obtain few- and monolayer structures, which could be used for further studies of downscaling effects for the CrCl3–RuCl3 solid solution. X-ray photoelectron spectroscopy, transmission electron microscopy and Raman spectroscopy were used to confirm the purity and quality of the synthesized crystals.
Direct Deposition of (BixSb1–x)2Te3 Nanosheets on Si/SiO2 Substrates by Chemical Vapor Transport
(2022)
The tellurides of bismuth and antimony (Bi2Te3 and Sb2Te3) are prominent members of the V2VI3 material family that exhibit promising topological properties. We provide a method for the rational synthesis of mixed crystals of these materials ((BixSb1–x)2Te3 with x = 0.1, ..., 0.9) by means of a bottom-up chemical vapor transport (CVT) approach. Thermodynamic calculations showed the synthesis to be possible in the temperature range of 390–560 °C without significant enrichment of either component and without adding a transport agent. The starting materials were synthesized and verified by X-ray diffraction (XRD). Optimization experiments showed the ideal conditions for nanosheet synthesis to be T2 = 560 °C, T1 = 390 °C with a reaction time of t = 36 h. Crystals with heights of down to 12 nm (12 quintuple layers) were synthesized and analyzed by means of scanning electron microscopy, energy-dispersive X-ray spectrometry, and atomic force microscopy. High-resolution transmission electron microscopy confirmed the R3̅m crystal structure, high crystallinity, and overall quality of the synthesized (BixSb1–x)2Te3 nanosheets. Magnetotransport measurements revealed that such ternary compounds can have a significantly reduced carrier density compared to the binary parent compounds.
Tungsten telluride WTe2 is the sole candidate of a group of two-dimensional layered transition metal dichalcogenides (TMDCs) MX2 with a thermodynamically stable 1T′-structure at room temperature. The binary system W/Te was audited with respect to a rational approach of planning and realization of a bottom-up synthesis of WTe2 nanostructures. Thus, the parameters of the synthesis via chemical vapor transports (CVT) were derived by thermodynamic simulations of the reaction pathway according to the Calphad method. Reflecting on the peritectic melting behavior at 1020 °C, the values of ΔfHm° (298 K) = −26.5 kJ·mol−1 and Sm° (298 K) = 132 J·mol−1 ·K−1 have been obtained.
According to modeling, crystal growth by short time vapor transport is reasonable under the addition of bromine or TeBr4 in the temperature range between 650 and 750 °C. Experimental implementation of crystal growth of WTe2 nanosheets succeeded in a temperature gradient from 725 to 675 °C on yttria-stabilized zirconia (YSZ) (111) substrates, observing the deposition of single crystal sheets of high crystallinity with thicknesses of 15−20 nm (∼20−30 layers). The high crystallinity, pristine morphology, and overall quality of the deposited nanosheets is shown by means of atomic resolution transmission electron microscopy, selected area electron diffraction (SAED), and atomic force microscopy as well as profound double-polarized Raman spectroscopy.
The experimental observation of intrinsic ferromagnetism in single layered chromium trihalides CrX₃ (X = Cl, Br, I) has gained outstanding attention recently due to their possible implementation in spintronic devices. However, the reproducible preparation of highly crystalline chromium(III) halide nanolayers without stacking faults is still an experimental challenge. As chromium trihalides consist of adjacent layers with weak interlayer coupling, the preparation of ultrathin CrX₃ nanosheets directly on substrates via vapor transport proves as an advantageous synthesis technique. It is demonstrated that vapor growth of ultrathin highly crystalline CrX₃ micro‐ and nanosheets succeeds directly on yttrium stabilized zirconia substrates in a one‐step process via chemical vapor transport (CVT) in temperature gradients of 100 K (600°C → 500°C for CrCl₃ and 650°C → 550°C for CrBr₃ or CrI₃) without a need for subsequent delamination. Due to simulation results, optimization of synthesis conditions is realized and phase pure CrX₃ nanosheets with thicknesses ≤25 nm are obtained via short term CVT. The nanosheets morphology, crystallinity, and phase purity are analyzed by several techniques, including microscopy, diffraction, and spectroscopy. Furthermore, a potential subsequent delamination technique is demonstrated to give fast access to CrX₃ monolayers using the example of CrCl₃ .
Structural and Magnetic Transitions Caused by Dimer Formation in the CrCl3–MoCl3 Solid Solution
(2024)
In this work, we apply the concept of solid solutions to the two 2D transition metal trihalides CrCl3 and MoCl3. While CrCl3 belongs to the magnetically active CrX3 family, the magnetism in MoCl3 is intrinsically suppressed by the formation of aligned Mo–Mo
dimers, which also distort the regular honeycomb lattice that is typical for the 2D transition metal trihalides. We report suitable synthesis conditions for the gapless solid solution and crystal growth by chemical vapor transport. The CrCl3–MoCl3 solid solution was initially synthesized at 650 °C for 100 h under addition of MoCl5 as mineralizer, and bulk crystals were subsequently grown by vapor transport in a temperature gradient from
600 °C → 550 °C for 60 h. The obtained solid solution exhibits multiple composition-dependent phase transitions at room temperature, as confirmed by powder X-ray diffraction measurements. The possible presence of Mo–Mo dimers in the solid solution
was further investigated by infrared, Raman, electron energy loss, and nuclear quadrupole resonance spectroscopies. The combined results indicate that the Mo–Mo dimers are present over a wide range of compositions. Their orientation changes from parallel alignment for compositions from 70% ≤ c(Mo3+) ≤ 100% to random orientation for samples with c(Mo3+) < 70% content, which significantly affects the course of lattice parameters. Finally, the magnetic properties of the powder samples show a correlation between the Mo3+ content and the transition temperature into the low-temperature phase.