Multiple structural transitions driven by spin-phonon couplings in a perovskite oxide.

Spin-phonon interactions are central to many interesting phenomena, ranging from superconductivity to magnetoelectric effects. However, they are believed to have a negligible influence on the structural behavior of most materials. For example, magnetic perovskite oxides often undergo structural transitions accompanied by magnetic signatures whose minuteness suggests that the underlying spin-phonon couplings are largely irrelevant. We present an exception to this rule, showing that novel effects can occur as a consequence. Our first-principles calculations reveal that spin-phonon interactions are essential to reproduce the experimental observations on the phase diagram of magnetoelectric multiferroic BiCoO. Moreover, we predict that, under compression, these couplings lead to an unprecedented temperature-driven double-reentrant sequence of ferroelectric transitions. We propose how to modify BiCoO via chemical doping to reproduce such marked effects under ambient conditions, thereby yielding useful multifunctionality.