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

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