A. Kalinko

First-Principles LCAO Calculations on 5d Transition Metal Oxides: Electronic and Phonon Properties

ABSTRACT First-principles quantum chemistry calculations within the periodic linear combination of atomic orbitals (LCAO) formalism have been used to probe electronic and phonon properties of crystalline 5d transition metal oxides ReO3, ZnWO4 and CaWO4. The obtained equilibrium crystal structure is in good agreement with known crystallographic data. Rhenium trioxide is correctly predicted to be a metal, whereas both tungstates—a wide gap insulating compounds. The phonon frequencies, calculated by the frozen phonon method, agree rather well with those obtained by infrared and Raman spectroscopies.

Interpretation of EXAFS in ReO3 using molecular dynamics simulations

Temperature dependent Re L3-edge EXAFS signals from perovskite-type cubic ReO3 have been successfully interpreted using a combination of classical NVT molecular dynamics (MD) and ab initio multiple-scattering approach. The force field model, required for MD simulations, has been determined by fitting the Re-O and O-O pairwise interatomic potentials to a set of experimental data (lattice parameter, elastic constants and bulk modulus) and phonon frequencies, theoretically calculated from the first principles at high symmetry points of the Brillouin zone. The MD simulations reproduce well the anisotropy of thermal vibration for oxygen atoms and confirm a deviation of the mean Re-O-Re angle from 180°. The atomic configurations from the MD simulations have been used to calculate the configuration-averaged EXAFS spectra at several temperatures. The use of the MD results allows a straightforward treatment of thermal disorder in the multiple-scattering contributions to the total EXAFS signal. Good agreement between calculated and experimental EXAFS signals has been found, that additionally supports the accuracy of our force field model.

Extended x-ray absorption fine structure spectroscopy and first-principles study of SnWO4

The local atomic structure in - and -SnWO4 was studied by synchrotron radiation W L3-edge x-ray absorption spectroscopy at 10 and 300K. Strongly distorted WO6 octahedra were found in -SnWO4, whereas nearly regular WO4 tetrahedra were observed in -SnWO4, confirming previous results. The structural results obtained were supported by the first-principles calculations, suggesting that the second-order Jahn‐Teller effect is responsible for octahedral distortion.

High-pressure x-ray absorption spectroscopy study of tin tungstates

Room-temperature pressure-dependent (0-25 GPa) x-ray absorption spectroscopy at the W -edges of α-SnWO4 and β-SnWO4 was performed using a dispersive setup and a high-pressure nanodiamond anvil cell. The detailed analysis of experimental x-ray absorption near-edge structure and extended x-ray absorption fine structure data suggests that upon increasing pressure, a displacement of tungsten atoms by about 0.2 A toward the center of the WO6 octahedra occurs in α-SnWO4, whereas the coordination of tungsten atoms changes from tetrahedral to distorted octahedral in β-SnWO4.