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 cu...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 300K.An enhanced value of the Debye–Waller factor for the Ba-O atomic pair(σ^(2)_(1)-0:015A^(2))revealed in the experiment is associated with the predicted structural instability.The average amplitude of the thermal rotations estimated from the measured σ^(2)_(1) value is-4°at 300K.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 BaZrO_(3) at low temperature,which can explain the discrepancy between the calculations and experiment.展开更多
基金This work was partially supported by the Russian Foundation for Basic Research grant No.13-02-00724.
文摘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 300K.An enhanced value of the Debye–Waller factor for the Ba-O atomic pair(σ^(2)_(1)-0:015A^(2))revealed in the experiment is associated with the predicted structural instability.The average amplitude of the thermal rotations estimated from the measured σ^(2)_(1) value is-4°at 300K.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 BaZrO_(3) at low temperature,which can explain the discrepancy between the calculations and experiment.
