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

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.