Pressure-Dependent Structure of BaZrO3 Crystals as Determined by Raman Spectroscopy

Abstract

The structure of dielectric perovskite BaZrO3, long known to be cubic at room temperature without any structural phase transition with variation in temperature, has been recently disputed to have different ground state structures with lower symmetries involving octahedra rotation. Pressure-dependent Raman scattering measurements can identify the hierarchy of energetically-adjacent polymorphs, helping in turn to understand its ground state structure at atmospheric pressure. Here, the Raman scattering spectra of high-quality BaZrO3 single crystals grown by the optical floating zone method are investigated in a pressure range from 1 atm to 42 GPa. First, based on the analyses of the infrared and Raman spectra measured at atmospheric pressure, it was found that all the observed vibrational modes could be assigned according to the cubic Pm3¯m structure. In addition, by applying pressure, two structural phase transitions were found at 8.4 and 19.2 GPa, one from the cubic to the rhombohedral R3¯c phase and the other from the rhombohedral to the tetragonal I4/mcm phase. Based on the two pressure-induced structural phase transitions, the true ground state structure of BaZrO3 at room temperature and ambient pressure was corroborated to be cubic while the rhombohedral phase was the closest second.

Stability of the tetragonal phase of BaZrO3 under high pressure

“…Our single crystal spectra are overall in agreement with the single crystal spectra from Ref. [31] and the powder spectra from Ref. [14], but also differ in some aspects.…”

“…[14], but with significantly less intensity. The very same mode does appear very clearly in the single crystal study by Gim et al [31] This difference in intensity between different experiments presumably arises from orientation effects due to selection rules, as will be discussed in the following.…”

“…The same reasoning is made in Ref. [31], but the second high-pressure phase is identified as tetragonal instead. As stated in our previous study, identifying the high pressure structure from Raman spectra may not be as easy an anticipated.…”

“…In BaZrO 3 , splitting of Raman peaks was also observed and invoked to propose a transition to an orthorhombic phase, also inspired from phases found in the phase diagram of the (Ba,Ce)ZrO 3 system. [14], and a recent Raman study concluded for a P m 3m → R 3c → I4/mcm phase sequence [31]. Nevertheless, this was not confirmed by the high-pressure x-ray study in Ref.…”

“…Experimentally, it was indeed proposed in several instances that a second phase transition occurs both in SrTiO 3 and BaZrO 3 [11,13,14,31], but the experimental results remain controversial. In SrTiO 3 , the existence of a second phase transition has been hypothesized based on the observation of a Raman peak splitting [11], or changes in the Ti pre-edge features in x-ray absorption measurements [32,33].…”