Combined first-principles and EXAFS study of structural instability in BaZrO3

Abstract

Phonon spectrum of cubic barium zirconate is calculated from first principles using the density functional theory. Unstable phonon mode with the $R_{25}$ symmetry in the phonon spectrum indicates an instability of the cubic structure with respect to rotations of the oxygen octahedra. It is shown that the ground-state structure of the crystal is $I4/mcm$. In order to find the manifestations of the predicted instability, EXAFS measurements at the Ba $L_{\rm III}$-edge are used to study the local structure of BaZrO$_3$ at 300 K. An enhanced value of the Debye-Waller factor for the Ba-O atomic pair ($\sigma^2_1 \sim 0.015$ {\AA}$^2$) revealed in the experiment is associated with the predicted structural instability. The average amplitude of the thermal octahedra rotation estimated from the measured $\sigma^2_1$ value is $\sim$4 degrees at 300 K. The closeness of the calculated energies of different distorted phases resulting from the condensation of the $R_{25}$ mode suggests a possible structural glass formation in BaZrO$_3$ when lowering temperature, which explains the cause of the discrepancy between the calculations and experiment.