Untangling the Raman spectra of cubic and tetragonal BaZrO3

Abstract

Raman spectroscopy is a widely used experimental technique to study the vibrational properties of solids. Atomic scale simulations can be used to predict such spectra, but reliable studies at finite temperatures are challenging, mainly due to the requirement of accurate and computationally efficient models for the dielectric susceptibility. Here, we have used molecular dynamics simulations together with a density functional theory-based model for the dielectric susceptibility to determine the Raman spectrum of barium zirconate, BaZrO3 (BZO), a well-studied oxide perovskite. At ambient conditions, where the system is cubic, we find excellent agreement with experimentally measured Raman spectra. Our study establishes that the relatively sharp spectra seen experimentally are due to second-order scattering. At higher pressures, where BZO is tetragonal, all first-order Raman active modes are identified. Additionally, slightly below the phase transition, in the cubic phase, a broad central Raman peak appears. The origin of this type of peak is controversial and extensively debated in connection with the dynamics of the halide perovskites. Here, we show that it is also present in a hard oxide perovskite, and it originates from the highly overdamped R-tilt mode in the cubic structure.