Jason Vielma

Shell model of BaTiO3 derived from ab-initio total energy calculations

A shell model for ferroelectric perovskites fitted to results of first-principles density functional theory (DFT) calculations is strongly affected by approximations made in the exchange-correlation functional within DFT, and in general not as accurate as a shell model derived from experimental data. We have developed an isotropic shell model for BaTiO3 based on the PBEsol exchange-correlation functional [Perdew et al., Phys. Rev. Lett. 100, 136406 (2008)], which was specifically designed for crystal properties of solids. Our shell model for BaTiO3 agrees with ground state DFT properties and the experimental lattice constants at finite temperatures. The sequence of phases of BaTiO3 (rhombohedral, orthorhombic, tetragonal, cubic) is correctly reproduced but the temperature scale of the phase transitions is compressed. The temperature scale can be improved by scaling of the ab-initio energy surface.

From Ag{sub 2}Sb{sub 2}O{sub 6} to Cd{sub 2}Sb{sub 2}O{sub 7}: Investigations on an anion-deficient to ideal pyrochlore solid solution

Abstract A complete solid solution between the anion-deficient pyrochlore Ag2Sb2O6 and the ideal pyrochlore Cd2Sb2O7 has been synthesized through the standard solid state ceramic method. Each composition has been characterized by various different techniques, including powder X-ray diffraction, optical spectroscopy, electron paramagnetic resonance and 121Sb Mossbauer spectroscopy. Computational methods based on density functional theory complement this investigation. Photocatalytic activity has been studied, and transport properties have been measured on pellets densified by spark plasma sintering. The analysis of the data collected from these various techniques enables a comprehensive characterization of the complete solid solution and revealed an anomalous behavior in the Cd-rich end of the solid solution, which has been proposed to arise from a possible radical O− species in small concentrations.