Eect of hydrostatic pressure on phase transitions in spin-crossover 1D systems Sophia Klokishner a,b , Jorge Linares a, * , Franc ßois Varret a a Laboratoire de Magn etisme et d'Optique, CNRS-UMR 8634, Universit e de Versailles St. Quentin en Yvelines, 78035 Versailles cedex, France b State University of Moldova, Mateevich Str. 60, Kishnev 2028, Moldova Received 6 October 1999; in ®nal form 13 March 2000 Abstract A model is developed to elucidate the role of hydrostatic pressure on the spin-crossover phase transitions in one-dimensional systems. The model involves long-range electron-deformational and short-range interactions. The conditions when pressure suppresses the thermal hysteresis are discussed. The pressure-induced systemÕs bistability is predicted. The qualitative explanation of experimental data is given. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Spin crossover; Ising 1D; Pressure eects; Hysteresis; Elastic interactions 1. Introduction Experimental and theoretical studies of spin crossover have been intensively carried out in the past years. The synthesis of new spin-crossover compounds was mainly aimed at the creation of low-dimensional systems with high temperature of the spin-crossover transition. On the one hand, such systems give the opportunity of investigation of an ensemble with a limited number of mole- cules. On the other hand, new problems appear in the theory and the most important of these is the nature of interactions stipulating the spin trans- formation in 1D and 2D systems. Recently, it has been shown [1] that for 1D systems the experi- mental data on hysteresis curves of a large width [2] can be interpreted only by the consideration of common action of long- and short-range interac- tions. It is evident that information about these two types of interactions as well as about the re- lation between them can be obtained when ap- plying external ®elds. Hydrostatic pressure aects appreciably the thermodynamic characteristics of spin-crossover compounds and can provide this information. The spin transition behaviour under hydrostatic pressure has been studied for a series of mononuclear and polynuclear metal complexes [3±10]. In Ref. [11], a model was developed where it was stated that the behaviour of the hysteresis and the transition temperature in mononuclear and polynuclear compounds can be described by the mean ®eld theory, if the dependence of the bulk modulus on pressure is introduced. Here, we suggest a model that can be used to analyse the in¯uence of pressure on the spin crossover in 1D crystals. The model takes into account short- and long-range interactions, whereas the long-range interactions are assumed to originate from the Chemical Physics 255 (2000) 317±323 www.elsevier.nl/locate/chemphys * Corresponding author. Fax: +33-139-254652. E-mail address: jlinares@physique.uvsq.fr (J. Linares). 0301-0104/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 ( 0 0 ) 0 0 0 8 1 - 1