Structural instability in BaZrO3 crystals: Calculations and experiment
The phonon spectrum of cubic barium zirconate is calculated from first principles using the density functional theory. An unstable R25 phonon mode observed in the phonon spectrum indicates an instability of the BaZrO3 structure with respect to the oxygen octahedra rotations. It is shown that the symmetry of the ground-state structure of the crystal is I4/mcm. The local structure of BaZrO3 is studied by the EXAFS spectroscopy at the BaLIII absorption edge at 300 K to search for the instability predicted by calculations. Anomalously high values of the Debye-Waller factor for the Ba-O atomic pairs (σ12 ∼ 0.015 Å2) are attributed to the appearance of this structural instability. The average amplitude of the octahedra rotations caused by thermal vibrations is estimated from the measured σ12 value to be ∼4° at 300 K. The closeness of the calculated energies of various distorted phases resulting from the condensation of the R25 mode suggests a possible formation of the structural glass state in BaZrO3 as the temperature is lowered. It explains the origin of the disagreement between the results of calculations and diffraction experiments.
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