On Predicting Mössbauer Parameters of Iron-Containing Molecules with Density-Functional Theory

The performance of six frequently used density functional theory (DFT) methods (RPBE, OLYP, TPSS, B3LYP, B3LYP*, and TPSSh) in the prediction of Mössbauer isomer shifts(δ) and quadrupole splittings (ΔEQ) is studied for an extended and diverse set of Fe complexes. In addition to the influence of the applied density functional and the type of the basis set, the effect of the environment of the molecule, approximated with the conducting-like screening solvation model (COSMO) on the computed Mössbauer parameters, is also investigated. For the isomer shifts the COSMO-B3LYP method is found to provide accurate δ values for all 66 investigated complexes, with a mean absolute error (MAE) of 0.05 mm s–1 and a maximum deviation of 0.12 mm s–1. Obtaining accurate ΔEQ values presents a bigger challenge; however, with the selection of an appropriate DFT method, a reasonable agreement can be achieved between experiment and theory. Identifying the various chemical classes of compounds that need different treatment allowed us to construct a recipe for ΔEQ calculations; the application of this approach yields a MAE of 0.12 mm s–1 (7% error) and a maximum deviation of 0.55 mm s–1 (17% error). This accuracy should be sufficient for most chemical problems that concern Fe complexes. Furthermore, the reliability of the DFT approach is verified by extending the investigation to chemically relevant case studies which include geometric isomerism, phase transitions induced by variations of the electronic structure (e.g., spin crossover and inversion of the orbital ground state), and the description of electronically degenerate triplet and quintet states. Finally, the immense and often unexploited potential of utilizing the sign of the ΔEQ in characterizing distortions or in identifying the appropriate electronic state at the assignment of the spectral lines is also shown

Polarization reversal electroluminescence at low frequencies in barium titanate crystals

Audiofrequency-induced electroluminescence in BaTiO<SUB>3</SUB> single crystals is studied as a function of the strength and the frequency of the applied field, the temperature of the crystal, the nature of the electrode material, and the biasing field. It is shown that this electroluminescence differs in characteristics and origin from the previously reported radiofrequency-induced electroluminescence and is intimately associated with polarization reversal

Surface layers on ferroelectric BaTiO<SUB>3</SUB> crystals

The rf induced electroluminescence emitted from a BaTiO<SUB>3</SUB> crystal surface is studied in detail as a function of temperature, voltage, frequency of the rf field, electrode material, etc. It is shown that electroluminescence is essentially produced in the surface layer which exists on BaTiO<SUB>3</SUB> crystals and is not associated with dipole reversal or domain rearrangement, although the dielectric properties of the bulk alter the light output considerably. The characteristics of the surface layer, which is shown to be a space charge layer, have been determined and the mechanism of electroluminescence elucidated

Development of pure and doped gamma ferric oxide

Optimum conditions and experimental details for the formation of v-Fe203 from goethite have been worked out. In another method, a cheap complexing medium of starch was employed for precipitating acicular ferrous oxalate, which on decomposition in nitrogen and subsequent oxidation yielded acicular y-Fe203. On the basis of thermal decomposition in dry and moist nitrogen, DTA, XRD, GC and thermodynamic arguments, the mechanism of decomposition was elucidated. New materials obtained by doping ~'-Fe203 with 1-16 atomic percent magnesium, cobalt, nickel and copper, were synthesised and characterizedCUSATBull. Mater. Sci., Vol. 6, No. 1, February 1984, pp. 59--64