Nanoscale-correlated octahedral rotations in BaZrO3

Abstract

${\mathrm{BaZrO}}_{3}$ is reportedly one of few perovskites that retain an ideal cubic structure down to 0 K even though its ground state as predicted by first-principles calculations has remained contentious. Here, we combine electron diffraction with total neutron scattering measured on ceramic samples at cryogenic temperatures to demonstrate that below 80 K, this compound undergoes a structural change associated with the onset of correlated out of phase rotations of ${[\mathrm{ZrO}}_{6}]$ octahedra. The change is manifested in the appearance of weak but relatively sharp superlattice reflections at $\frac{1}{2}hkl$ $(h, k, l=\mathrm{odd})$, which are readily detectable by electron diffraction. Atomistic structural refinements from the total neutron-scattering data confirmed that these reflections are associated with octahedral tilting. The observed rotations are consistent with the ${a}^{0}{a}^{0}{c}^{--}$ type and feature a coherence length of about 3 nm. The average structure of ${\mathrm{BaZrO}}_{3}$, as seen by Bragg diffraction, remains cubic. According to the electron diffraction data, Nb-doped ${\mathrm{BaZrO}}_{3}$ undergoes a similar structural transition but, in this case, the tilting could not be recovered from the neutron scattering, suggesting that the distortions are smaller compared to those in the pure compound.