Microscopic picture of paraelectric perovskites from structural prototypes

Abstract

We show with first-principles molecular dynamics the persistence of intrinsic $⟨111⟩$ Ti off-centerings for BaTiO${}_3$ in its cubic paraelectric phase. Intriguingly, these are inconsistent with the Pm$\bar{3}$m space group often used to atomistically model this phase using density functional theory or similar methods. Therefore we deploy a systematic symmetry analysis to construct representative structural models in the form of supercells that satisfy a desired point symmetry but are built from the combination of lower-symmetry primitive cells. We define as structural prototypes the smallest of these that are both energetically and dynamically stable. Remarkably, two 40-atom prototypes can be identified for paraelectric BaTiO${}_3$; these are also common to many other $AB$O${}_3$ perovskites. These prototypes can offer structural models of paraelectric phases that can be used for the computational engineering of functional materials. Last, we show that the emergence of B-cation off-centerings and the primitive-cell phonon instabilities are controlled by the equilibrium volume, in turn dictated by the filler A cation.

18 pages, 4 figures in main text, 14 figures in appendix