Anharmonicity of the antiferrodistortive soft mode in barium zirconate BaZrO3

BaZrO${}₃$ is one of the very few perovskites that is claimed to remain cubic down to 0 $K$, but its tendency towards an antiferrodistortive (AFD) instability has fueled a lot of debates. Here, the AFD phonon mode is studied using inelastic neutron scattering (INS) and inelastic x-ray scattering (IXS), combined with a computational approach including anharmonicity. Both classical molecular dynamics (MD) and a self-consistent phonon approach are used, the latter including quantum fluctuations of the atomic motion. This robust description of the AFD mode clarifies that BaZrO${}₃$ stays cubic down to 0 $K$.