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- Mössbauer (3)
- Thermal analysis (3)
- Al2O3 (2)
- ESR (2)
- ESR spectroscopy (2)
- A. Inorganic compounds (1)
- Activation/deactivation processes (1)
- Antiferromagnetic coupling (1)
- Boehmite (1)
- C. Mössbauer spectroscopy (1)
The precursor [FeIII(L)Cl] (LH2 = N,N'-bis(2'-hydroxy-3'-X-benzyliden)-1,6-diamino-3-N-hexane) is a high-spin (S = 5/2) complex (with X = -CH3, -O-CH3). This precursor is combined with the bridging unit [SnIV(CN)4] to yield star-shaped pentanuclear clusters, [(L-X-FeIII)4Sn(CN)4]Cl4 57Fe-Mössbauer, 119mSn-Mössbauer, and ESR spectroscopy are used to study our samples. For X = -CH3 the 57Fe-Mössbauer data show a multiple spin transition between iron(III) in the high-spin and low-spin state. Changing the functional group from X = -CH3 to X = -O-CH3 turns the switchability off.
Radical species like CO-2,CO-3, SO-2, and SO-3 can be created by exposing natural and synthetic calcium carbonates to sunlight or to the light of a Hg(Xe) lamp. This poses as a risk for ESR dating of these materials, because the radicals formed by light exposure cannot be distinguished from those generated by radioactivity. Furthermore, paramagnetic centers like SO-2, electrons trapped near Zn2+ or Cd2+ ions, surface defects, and radicals with g?=2.0040, can be bleached in ?-irradiated samples by light and show conversion effects.
Radiation defects in alkaline earth silicate glasses are studied by means of ESR and optical spectroscopies. Previously undiscovered relations between findings of both methods are deduced using four different experimental tools: (i) variation of chemical glass composition; (ii) isochronal annealing of ggr-irradiated samples; (iii) optical bleaching of ggr-irradiated samples; (iv) variation of the ggr-dose. Oxygen hole centres are mainly responsible for the radiation induced absorption in the visible part of the optical spectrum. Previously undescribed electron centres with broad and asymmetric ESR resonances (gprime=1·955) contribute to the optical spectra (2002000 nm) in a small way. It is suggested they are electrons trapped on small aggregates of alkaline earth ions. The radiation induced recharging of incorporated Fe3+ ions is also evidenced by optical and ESR spectroscopies. Fe3+ ions react as electron traps forming not only Fe2+ but also {Fe3+}- species which might be responsible for an absorption band at 300 nm. A correlation was found between the changes of intensity with those of the ESR spectroscopically detected Fe3+ content of the samples.
The 57Fe-Moessbauer spectrum of (dmpipzH2)2[FeIII(H2O)2Cl4][FeIII Cl4]Cl2 (1) consists of two doublets caused by a strongly distorted octahedral and a non-distorted tetrahedral component. Both (dmpipzH2)[FeIIIBr4]2 (2) and (trienH2)[FeIIIBr4]Br (3) show only one doublet indicating a more strongly distorted iron(III) species for 2 and a less-distorted one in the case of 3. For all compounds investigated, the ESR spectra reveal the existence of ground states with predominant antiferromagnetic coupling. The Néel temperatures amount to <4.2 K for 1, 4 K for 3 and 48 K for 2. In the case of 2, both the Moessbauer and the ESR spectra reveal the formation of exchange-coupled {FeBr4} units which has been attributed to a significant cation influence. The fine structure of residual paramagnetic [FeBr4]- subunits remaining in the crystalline matrix could be directly determined. Temperature dependent AC and DC susceptibility measurements of 2 and 3 confirmed the results.
Direct coupling of an excimer laser to the cavity of an X-band ESR spectrometer enabled in situ observation of ultraviolet irradiation (248 nm) induced effects in aluminosilicate glasses. Under these conditions the radiation was absorbed by iron traces present in the glasses and finally electron/hole pairs were formed. The trapping and detrapping of electrons and holes, e.g. generation and decay of paramagnetic defects, could be followed in dependence on repetition rate of the laser (120 Hz), temperature during irradiations (4·2300 K) and composition of the glasses. The holes were trapped at SiO and AlOSi sites. Zn2+ and Cd2+ ions (likely as parts of ZnO or CdO aggregates) as well as Fe3+ ions served as effective electron traps. Additionally, changes of the dielectric properties of the samples during and after ultraviolet irradiations could be monitored directly. These are large if relatively mobile ions (e.g. Na+) are present in the sample. Comparison of ultraviolet to -irradiated samples shows that the type of traps present in the samples mainly depends on the glass composition. However, the relative amounts of stabilised holes and electrons essentially depend on the conditions and parameters of irradiation.
Electron paramagnetic resonance (EPR) of Fe3+ ions in Al2O3 is studied in powder samples prepared by different routes and/or modified by thermal or mechanical treatments, with different doping levels and grain sizes. The measurements are performed in various frequency bands (S, X, K, Q and W) and with bimodal detection in X-band. Simulations of the spectra are achieved with a code designed for computing EPR powder spectra described by any spin Hamiltonian including second-, fourth-and sixth-order ZFS terms (S ? 7/2). The linewidths, intensities and lineshapes are accounted for. The lineshape is Gaussian at low Fe3+ concentration whereas it is Lorentzian for higher concentration. The linewidths are interpreted as the superimposition of three main contributions: intrinsic linewidth, dipolar broadening and broadening due to lattice imperfections. The latter is tentatively interpreted in terms of quadrupolar spin Hamiltonian parameter distributions treated using first-order perturbation theory. Whatever the sample, only the b22 spin Hamiltonian parameter is found to be distributed around a mean zero value which corresponds to rhombic distortions. Angle and bond length distributions are tentatively extracted from the b22 distributions which gives some insight into the local order around the spin probe in relation to the preparation and treatment of the samples.
The transformation of doped or seeded pseudoboehmite to corundum was studied by combining thermal analysis, X-ray diffraction, transmission electron microscopy, and electron paramagnetic resonance spectroscopy. The temperature of phase transformation to corundum was lowered by about 130 °C when Fe3+ or corundum seeds were added to the sols. Action of Fe3+ ions depends on the actual degree of thermally induced transformation of pseudoboehmite via transition aluminas to corundum and the ability of these alumina phases to incorporate Fe3+ ions. These ions tend to aggregate with increasing iron concentration of the alumina phases and can work as nucleation centers. Small (not, vert, similar20 nm) corundum particles act as active nucleation sites whereas larger grains (200400 nm) also present in the samples are less effective. For the first time trapping and stabilization of NO2 molecules in transition aluminas formed by a solgel route was shown.