Structural instability in BaZrO3 crystals: Calculations and experiment

Abstract

Phonon spectrum of cubic barium zirconate is calculated from first principles using the density functional theory. Unstable phonon mode with the R25 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 LIII-edge are used to study the local structure of BaZrO3 at 300 K. An enhanced value of the Debye-Waller factor for the Ba-O atomic pair (σ 2 1 ∼ 0.015Å2 ) revealed in the experiment is associated with the predicted structural instability. The average amplitude of the thermal octahedra rotation estimated from the measured σ 2 1 value is ∼4 degrees at 300 K. The closeness of the calculated energies of different distorted phases resulting from the condensation of the R25 mode suggests a possible structural glass formation in BaZrO3 when lowering temperature, which explains the cause of the discrepancy between the calculations and experiment.

Lattice dynamics and Raman spectrum of BaZrO3 single crystals

“…Those compounds also share common experimental signatures related to local disorder of the Ti 4+ ion in the octahedral cage, with the presence of intense diffuse scattering [50], and distinctive features in XANES [51]. We therefore expect that studies of the local structure in BaZrO 3 will be insightful to reveal the details and dynamics and disorder, and we note that a previous study has already proposed anomalous Ba displace-ments, revealed by anomalously large DebyeâĂŞWaller (DW) factor in BaZrO 3 structural refinements [11].…”

Terahertz time-domain spectroscopy and Raman scattering studies of incipient ferroelectric BaZrO3

“…According to the X-ray and neutron diffraction studies, BZO was found to be cubic symmetry up to the lowest temperature 2 K and unlike most of the perovskite oxide materials, BZO does not undergo any structural phase transition [1][2][3]. Interestingly, a potential structural instability at the R-point in the phonon spectrum was observed consistently by the recent first principles calculations [4][5][6]. As a consequence of the zone-boundary instability, the crystal is expected to break the symmetry at a sufficiently low temperature and undergo the antiferrodistortive transformation [7].…”

Low-temperature thermophysical and crystallographic properties of BaZrO3 perovskite

“…Despite the evidence from low-temperature neutron diffraction measurements that show BaZrO 3 is cubic below 10 K [3,6] with no evidence for superlattice reflections or diffuse scattering at the R or M points of the Brillouin zone, ab initio calculations have shown that an instability exists at the R point where the calculated phonons exhibit negative frequencies [3,[16][17][18]. This instability is associated with a triply degenerate phonon with irreducible representation R þ 4 in which successive ZrO 6 octahedra along the cubic crystallographic axes rotate in an antiphase manner [19].…”

“…This instability is associated with a triply degenerate phonon with irreducible representation R þ 4 in which successive ZrO 6 octahedra along the cubic crystallographic axes rotate in an antiphase manner [19]. Results of first principles calculations carried out on BaZrO 3 are consistent with a single component of this mode condensing out giving rise to a lower symmetry tetragonal phase with space group I4/mcm [3,[16][17][18]. The deduction that the theoretical ground state of BaZrO 3 is tetragonal, in conflict with crystallographic experiments, is not universally accepted, and the stabilisation of the cubic crystal structure due to quantum zero-point vibrations and anharmonic effects stabilising the soft modes has recently been proposed [20].…”

“…The quoted fractional coordinates of the anion O2 are simply those of the aristotype transformed to the tetragonal unit cell and hence would not give rise to the expected superlattice reflections. Furthermore, most ab initio calculations of the phonons in BaZrO 3 do not show that negative frequencies are associated with the M point, unlike the R point [3,[16][17][18], although the whole branch between the R and the M points is soft according to Bilić and Gale [20]. The deduced space group of BaZr 1-x Y x O 3-d is therefore open to debate.…”