Universal Polar Instability in Highly Orthorhombic Perovskites

The design of novel multiferroic ABO₃ perovskites is complicated by the presence of necessary magnetic cations and ubiquitous antiferrodistortive modes, both of which suppress polar distortions. Using first-principles simulations, we observe that the existence of quadlinear and trilinear invariants in the free energy, coupling tilts, and antipolar motions of A and B sites to the polar mode drives an avalanche-like transition to a non-centrosymmetric Pna2₁ symmetry in a wide range of magnetic perovskites with small tolerance factors, overcoming the above restrictions. We find that the Pna2₁ phase is especially favored with tensile epitaxial strain, leading to an unexpected but technologically useful out-of-plane polarization. We use this mechanism to predict various novel multiferroics, displaying interesting magnetoelectric properties with small polarization switching barriers.