%0 Book Section %1 statphys23_0343 %A Konishi, Y. %A Tokoro, H. %A Nishino, M. %A Miyashita, S. %B Abstract Book of the XXIII IUPAP International Conference on Statistical Physics %C Genova, Italy %D 2007 %E Pietronero, Luciano %E Loreto, Vittorio %E Zapperi, Stefano %K charge-transfer crossover hysteresis loop metastable pressure-induced spin state statphys23 system topic-2 transition %T Models of spin-crossover materials and their stability %U http://st23.statphys23.org/webservices/abstract/preview_pop.php?ID_PAPER=343 %X Recently, detailed studies have been done on the spin-crossover (SC) transition between a high-spin (HS) state and a low-spin (LS) state.1 As related phenomena, some of Prussian blue analogues (PBAs) show thermal-induced hysteresis loops and photomagnetic properties. The two-way photo-switching of Co-Fe PBA (i.e., magnetic $łeftrightarrow$ nonmagnetic)2 has attracted much attention. In this work, we investigated two types of SC models to explain these phenomena. First, we studied an extended Wajnflasz-Pick (WP) model 3-5, focusing on stable and metastable states in this model. WP model is an Ising model, in which $s=-1/2$ represents the LS state, and $s=1/2$ represents the HS state.3 The Hamiltonian of this model is given by $H=-J\sum_i,j \rangles_i s_j+\sum_i (\Delta-12k_BTg)s_i$. Here, $J$, $\Delta$, and $g$ represent the intermolecular interaction, the energy difference between LS and HS, and the ratio of the degeneracy of HS to the one of LS, respectively. We added magnetic interactions to this model to study the magnetic property of Co-Fe PBA, and found systematic changes in the structures of stable and metastable states as functions of system parameters using mean field theory and Monte Carlo (MC) simulations.5 Second, we studied a model in which the HS and LS states interact via elastic interactions (Fig.~1).6 Using this model, we can take the effect of the volume change into account. We investigated this model by NPT-MC method, and found that the temperature dependence of the HS fraction changes from a gradual transition to a first-order phase transition with the increase of intermolecular interactions. In particular, we study the pressure dependence of the HS fraction. We will show various intrinsic properties of the phase transition as a function of the temperature and pressure; e.g., a pressure-induced hysteresis loop appears with strong intermolecular interactions.\\ 1) P.~Gutlich, A.~Hauser, and H.~Spiering, Angew. Chem. Int. Ed.\par $\quad$~33 (1994) 2024.\par 2) O.Sato, Y. Einaga, T.Iyoda, A. Fujishima, and K. Hashimoto,\par $\quad$~J. Electrochem. Soc. 144 (1997) L11.\par 3) J.~Wajnflasz, J. Phys. Status Solidi 40 (1970) 537.\par 4) S.~Miyashita, Y.~Konishi, H.~Tokoro, M.~Nishino, K.~Boukheddaden,\par $\quad$~and F.~Varret, Prog. Theor. Phys. 114 (2005) 719.\par 5) Y.~Konishi, H.~Tokoro, M.~Nishino, and S.~Miyashita,\par $\quad$~J. Phys. Soc. Jpn 75 (2006) 114603.\par 6) Y.~Konishi, H.~Tokoro, M.~Nishino, and S.~Miyashita, in preparation.

@incollection{statphys23_0343, abstract = {Recently, detailed studies have been done on the spin-crossover (SC) transition between a high-spin (HS) state and a low-spin (LS) state.[1] As related phenomena, some of Prussian blue analogues (PBAs) show thermal-induced hysteresis loops and photomagnetic properties. The two-way photo-switching of Co-Fe PBA (i.e., magnetic $\leftrightarrow$ nonmagnetic)[2] has attracted much attention. In this work, we investigated two types of SC models to explain these phenomena. First, we studied an extended Wajnflasz-Pick (WP) model [3-5], focusing on stable and metastable states in this model. WP model is an Ising model, in which $s=-1/2$ represents the LS state, and $s=1/2$ represents the HS state.[3] The Hamiltonian of this model is given by $H=-J\sum_{\langle i,j \rangle}s_i s_j+\sum_i (\Delta-\frac{1}{2}k_{\rm B}T\ln{g})s_i$. Here, $J$, $\Delta$, and $g$ represent the intermolecular interaction, the energy difference between LS and HS, and the ratio of the degeneracy of HS to the one of LS, respectively. We added magnetic interactions to this model to study the magnetic property of Co-Fe PBA, and found systematic changes in the structures of stable and metastable states as functions of system parameters using mean field theory and Monte Carlo (MC) simulations.[5] Second, we studied a model in which the HS and LS states interact via elastic interactions (Fig.~1).[6] Using this model, we can take the effect of the volume change into account. We investigated this model by NPT-MC method, and found that the temperature dependence of the HS fraction changes from a gradual transition to a first-order phase transition with the increase of intermolecular interactions. In particular, we study the pressure dependence of the HS fraction. We will show various intrinsic properties of the phase transition as a function of the temperature and pressure; e.g., a pressure-induced hysteresis loop appears with strong intermolecular interactions.\\ 1) P.~Gutlich, A.~Hauser, and H.~Spiering, Angew. Chem. Int. Ed.\par $\quad$~\textbf{33} (1994) 2024.\par 2) O.Sato, Y. Einaga, T.Iyoda, A. Fujishima, and K. Hashimoto,\par $\quad$~J. Electrochem. Soc. \textbf{144} (1997) L11.\par 3) J.~Wajnflasz, J. Phys. Status Solidi \textbf{40} (1970) 537.\par 4) S.~Miyashita, Y.~Konishi, H.~Tokoro, M.~Nishino, K.~Boukheddaden,\par $\quad$~and F.~Varret, Prog. Theor. Phys. \textbf{114} (2005) 719.\par 5) Y.~Konishi, H.~Tokoro, M.~Nishino, and S.~Miyashita,\par $\quad$~J. Phys. Soc. Jpn \textbf{75} (2006) 114603.\par 6) Y.~Konishi, H.~Tokoro, M.~Nishino, and S.~Miyashita, in preparation.}, added-at = {2007-06-20T10:16:09.000+0200}, address = {Genova, Italy}, author = {Konishi, Y. and Tokoro, H. and Nishino, M. and Miyashita, S.}, biburl = {https://www.bibsonomy.org/bibtex/283c12667fe33659f82d7073bf9362a1f/statphys23}, booktitle = {Abstract Book of the XXIII IUPAP International Conference on Statistical Physics}, editor = {Pietronero, Luciano and Loreto, Vittorio and Zapperi, Stefano}, interhash = {6f351905e895493f688cfd328ed0c090}, intrahash = {83c12667fe33659f82d7073bf9362a1f}, keywords = {charge-transfer crossover hysteresis loop metastable pressure-induced spin state statphys23 system topic-2 transition}, month = {9-13 July}, timestamp = {2007-06-20T10:16:17.000+0200}, title = {Models of spin-crossover materials and their stability}, url = {http://st23.statphys23.org/webservices/abstract/preview_pop.php?ID_PAPER=343}, year = 2007 }