Structure of Metastable States in Phase Transitions with High-Spin Low-Spin Degree of Freedom

TL;DR

This paper investigates the metastable states in spin-crossover and charge transfer phase transitions, revealing complex temperature-dependent behaviors and reentrant metastable structures using a unified theoretical model.

Contribution

It introduces a unified model that describes both spin-crossover and charge transfer phase transitions, highlighting the structure of metastable states and reentrant phenomena.

Findings

Identification of various equilibrium and metastable states as functions of system parameters.

Discovery of a reentrant metastable high-temperature structure at low temperatures.

Unified description of spin-crossover and charge transfer systems under a common framework.

Abstract

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