Abstract
The ground-stale structure of BaZrO3 is experimentally known to be cubic down to absolute zero. However, there exist several measured properties and experimental characterizations that earlier computational works have failed to accurately describe and explain within this cubic symmetry. Among these properties and observations are the dielectric constant and the parallel mean-squared relative displacement value that tracks the fluctuations in distance for Ba-O atom pairs. Previous density-functional theory (DFT) studies have resolved the issue by assuming that BaZrO(3 )undergoes a phase transition from cubic to tetragonal I4/mcm symmetry, possibly while forming a glasslike state that reflects cubic symmetry on average. In this paper, we show that the set of experimental results can indeed be satisfactorily explained by DFT entirely within the cubic symmetry. We find that past theory limitations arose from the choice of exchange-correlation-functional approximations and that the inclusion of Fock exchange in hybrids significantly improves the DFT performance. We also find that the inclusion of nonlocal correlation effects is beneficial. We conclude by making a prediction for the phase-transition pressure for the transition from cubic to tetragonal symmetry at zero kelvin.
Highlights
Perovskites comprise a large family of materials with the general formula ABO3, few limitations on A and B, and a large variety in crystal structure
From the family of perovskites, BaZrO3 stands out as a material which experimentally does not show any indication to a phase transition away from the cubic Pm3 ̄m, at least as far down as 2 K [2,3,4,5,6]
In that previous study [11], we found that the PBE version of the generalized gradient approximation (GGA) predicts the cubic phase down to T = 0, the R25 mode is significantly softer in PBE
Summary
Perovskites comprise a large family of materials with the general formula ABO3, few limitations on A and B, and a large variety in crystal structure. When the temperature is lowered, most perovskites are prone to some kind of distortion away from the high-symmetry cubic structure Often, this distortion can be described either as a displacement of the B cations causing a ferroelectric transition or a (rigid) rotation of the oxygen octahedron. Several theoretical studies, based on the local density approximation (LDA) to density-functional theory (DFT), report antiferrodistortive (AFD) instabilities in BaZrO3 [6,8,9,10] They find other symmetries with lower ground-state energies than the cubic in LDA [9,10]. We documented experimentally that the R25 mode, sometimes discussed as a soft mode, exhibits no softening with temperature This is what is predicted by the two hybrid functionals that we used, namely, HSE [15], and vdW-DF-cx0p [16]. We hope to inspire low-temperature experiments on the pressure-induced phase transition in BaZrO3 for independent checks on our modeling
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