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

摘要

Phonon spectrum of cubic barium zirconate is calculated from first principlesusing the density functional theory. Unstable phonon mode with the $R_{25}$symmetry in the phonon spectrum indicates an instability of the cubic structurewith respect to rotations of the oxygen octahedra. It is shown that theground-state structure of the crystal is $I4/mcm$. In order to find themanifestations 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 associatedwith the predicted structural instability. The average amplitude of the thermaloctahedra rotation estimated from the measured $\sigma^2_1$ value is $\sim$4degrees at 300 K. The closeness of the calculated energies of differentdistorted phases resulting from the condensation of the $R_{25}$ mode suggestsa possible structural glass formation in BaZrO$_3$ when lowering temperature,which explains the cause of the discrepancy between the calculations andexperiment.