719 Progress of Theoretical Physics, Vol. 114, No. 4, October 2005 Structures of Metastable States in Phase Transitions with a High-Spin Low-Spin Degree of Freedom Seiji Miyashita, 1, ∗) Yusuke Konishi, 1, ∗∗) Hiroko Tokoro, 1, ∗∗∗) Masamichi Nishino, 2, †) Kamel Boukheddaden 3,††) and Fran¸ cois Varret 3, †††) 1 Department of Physics, School of Science, The University of Tokyo, Tokyo 113-8656, Japan 2 Computational Materials Science center, National Institute for Materials Science, Tsukuba 305-0047, Japan 3 Laboratoire de Magnetisme et d’Optique, Universite de Versailles Saint-Quentin-en-Yvelines, 45 Avenue des Etats Unis 78035 Versailles, France (Received April 23, 2005) The difference in the degeneracy of low-spin (LS) and high-spin (HS) states causes inter- esting entropy effects on spin-crossover phase transitions and charge transfer phase transi- tions 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 transi- tion 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 transforma- tion 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 system- atically 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. §1. Introduction In so-called spin-crossover atoms, e.g., Fe and Co, spin states of the atoms can change between the high-spin (HS) state and the low-spin (LS) state as a result of small perturbations, such as changes of temperature or pressure, photo-irradiation, etc. This change of the spin state is called a spin-crossover (SC) transition. 1) – 38) The HS state has a large degeneracy, and it is favorable at high temperatures, while the LS state has a low energy, which is favorable at low temperatures. The competition ∗) E-mail: miyashita@phys.s.u-tokyo.ac.jp ∗∗) E-mail: konishi@spin.phys.s.u-tokyo.ac.jp ∗∗∗) E-mail: tokoro@spin.phys.s.u-tokyo.ac.jp †) E-mail: NISHINO.Masamichi@nims.go.jp ††) E-mail: kbo@physique.uvsq.fr †††) E-mail: fvarret@physique.uvsq.fr Downloaded from https://academic.oup.com/ptp/article/114/4/719/1835930 by guest on 31 December 2023