Combined first-principles and EXAFS study of structural instability in BaZrO_3

摘要

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_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 (σ_1~2 ～ 0.015 A~2) revealed in the experiment is associated with the predicted structural instability. The average amplitude of the thermal rotations estimated from the measured σ_1~2 value is ～ 4° 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 formation of a quenched rotational disorder in BaZrO3 at low temperature, which can explain the discrepancy between the calculations and experiment.