On the temperature-dependent local atomic geometry and Ti 1s near-edge spectrum in PbTiO3
Eric J. Cockayne, Eric L. Shirley, Bruce D. Ravel, Joseph C. Woicik
We revisit Ti 1s near-edge spectroscopy in PbTiO3 vs temperature. We use ab initio molecular dynamics (AIMD) to determine the average internal coordinates and their fluctuations at various temperatures above and below the tetragonal-to-cubic phase transition. By sampling and averaging over atomic coordinates' statistics using representation snapshots, we use core-hole Bethe-Salpeter nist-equation (BSE) calculations to predict the associated Ti 1s near-edge spectra. The proper accounting of atomic fluctuations greatly improves the agreement between theoretical and experimental spectra. The dominant contribution to changes in spectral features comes from the mean-square displacements of the Ti ion with respect to its surrounding O6 cage, which enhances the cross section for transitions to E_g-derived core-hole exciton levels. This captures the evolution of relative strengths of spectral features vs temperature and x-ray wave vector and electric field polarization vector in considerable detail.