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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.
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.
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.