General Landau theory of non-symmetry-breaking and symmetry-breaking spin transition materials

Abstract

Spin-transition materials, including the families of spin-crossover and charge-transfer systems, and more generally molecular-based materials exhibiting electronic and/or structural bistability, may undergo various types of phase transitions. The change of electronic state is stabilized by molecular reorganizations and both phenomena, which are usually non-symmetry breaking, can be described through the evolution of an order parameter q. Due to symmetry, q linearly couples to volume change. It is known that such elastic interactions are responsible for cooperative phenomena in non-symmetry-breaking spin-transitions. However, spin-transition materials may also exhibit symmetry-breaking phenomena related to various types of orders such as structural order as well as spin-state concentration waves. The universal framework of the Landau theory of phase transition is relevant for describing such ordering processes through the evolution of a symmetry-breaking order parameter η. The simultaneous or sequential occurrence of spin-transition and symmetry-breaking phenomena are reported for numerous spin-transition materials, and the coupling between these two types of instabilities is responsible for the emergence of various types of functions. In this work, we use the Landau approach to describe both symmetry-breaking phenomena and non-symmetry-breaking spin transition. We discuss how their coupling can generate sequences of phase transitions, from simple spin-crossover to spin-transition, continuous or discontinuous symmetry breaking, including ferroelasticity or stepwise spin transitions.