Competing polar and antipolar phases in n=2 Ruddlesden-Popper niobates and tantalates from first principles

The Li-based layered Li₂AB₂O₇ perovskites (A=Ca, Sr; B= Nb, Ta) have a crystal structure closely related to the n=2 Ruddlesden-Popper structure, where perovskite slabs interleave with lithium oxide layers. These materials host competing ferroelectric and antiferroelectric states which arise from coupled octahedral rotation distortions and (anti)polar instabilities. We combine density functional theory calculations with group theoretic analysis to elucidate the mechanism underlying the competition between these states. We also identify transition paths between the competing polar and antipolar states with very low energy barriers (less than 3 meV per formula unit) and show that stacking domain walls can facilitate an antipolar-polar transition. Additionally, we show that epitaxial strain tunes the energy balance between the polar and antipolar phases.