Competing structural phases in layered perovskite niobates and tantalates from first principles

The layered perovskite Li₂SrNb₂O₇ has a crystal structure characterized by alternating perovskite and lithium oxide layers, which is closely related to the n=2 Ruddlesden-Popper structure. Several recent studies have reported a competition between ferroelectric and non-polar phases in this material, mediated by coupled octahedral rotation distortions, polarization, and antipolar distortions. However, the symmetry of these structural phases as well as the ferroelectric mechanism remains controversial, with several different experimental reports. With the aim of clarifying these issues, in this work we use group-theoretic analysis together with first-principles calculations to systematically explore the competing structural phases in Li₂SrNb₂O₇. By extending our study to include related Li-based layered perovskites Li₂CaNb₂O₇, Li₂SrTa₂O₇, and Li₂CaTa₂O₇, we understand the impact of chemical composition and ionic size on the energetics of the competing phases. We furthermore explore the impact of epitaxial strain on controlling the structural phase in thin films. Our results provide new insight into the microscopic mechanism underlying phase competition in Li-based niobate and tantalate layered perovskites.