Structures of metastable states in phase transitions with a high-spin low-spin degree of freedom

The difference in the degeneracy of low-spin (LS) and high-spin (HS) states causes interesting entropy effects on spin-crossover phase transitions and charge transfer phase transitions in materials composed of spin-crossover atoms. Mechanisms of the spin-crossover (SC) phase transitions have been studied using the Wajnflasz model, where the degeneracy of the spin states (HS or LS) is taken into account, and the cooperative nature of the spin-crossover phase transitions has been well described. Recently, a charge transfer (CT) phase transition due to electron hopping between LS and HS sites has been studied using a generalized Wajnflasz model. Systems with SC and CT both have a high temperature structure (HT) and a low temperature structure (LT), and the transition between them can be a smooth crossover or a discontinuous first-order phase transition, depending on the parameter values of the systems. Although, apparently, the standard SC system and the CT system are very different, it has been shown that the two models are equivalent under a certain transformation of variables. In both systems, the structure of the metastable state at low temperatures is a matter of interest. We study the temperature dependence of the fraction of HT systematically in a unified model and find several structures of equilibrium and metastable states of the model as functions of the system parameters. In particular, we find a reentrant-type metastable branch of HT in a low temperature region, which sould play an important role in the study of the photo-irradiated processes of related materials.