A Unified Theoretical Description of the Thermodynamical Properties of Spin Crossover with Magnetic Interactions

After the discovery of the phenomena of light-induced excited spin state trapping (LIESST), the functional properties of metal complexes have been studied intensively. Among them, cooperative phenomena involving low spin-high spin (spin-crossover) transition and magnetic ordering have attracted interests, and it has become necessary to formulate a unified description of both phenomena. In this work, we propose a model in which they can be treated simultaneously by extending the Wajnflasz-Pick model including a magnetic interaction. We found that this new model is equivalent to Blume-Emery-Griffiths (BEG) Hamiltonian with degenerate levels. This model provides a unified description of the thermodynamic properties associated with various types of systems, such as spin-crossover (SC) solids and Prussian blue analogues (PBA). Here, the high spin fraction and the magnetization are the order parameters describing the cooperative phenomena of the model. We present several typical temperature dependences of the order parameters and we determine the phase diagram of the system using the mean-field theory and Monte Carlo simulations. We found that the magnetic interaction drives the SC transition leading to re-entrant magnetic and first-order SC transitions.Comment: 30pages, 11figure

ÉTUDE DES PROPRIÉTÉS CRISTALLOGRAPHIQUES, MAGNÉTIQUES ET ÉLASTIQUES DE RbCoF3

Une étude aux rayons X a montré qu'en dessous de 101 °K, RbCoF3 devient faiblement quadratique (c/a = 0,997 1 à 4,2 °K). Des mesures par diffraction de neutrons ont confirmé qu'au-dessous de 98 ± 5 °K, RbCoF3 devenait antiferromagnétique, les moments des atomes de cobalt formant un arrangement de type G. µco = 3,0 ± 0,2 µB à 4,2 °K. La susceptibilité magnétique suit entre 300 et 600 °K une loi de Curie-Weiss (θp = 180 °K, neff = 5,53). La température de Néel est au voisinage de 100 °K. Les vitesses de propagation des ultra-sons ont été mesurées en fonction de la température pour différentes directions de propagation.X-rays have shown that below 101 °K, RbCoF3 becomes slightly tetragonal (c/a = 0.997 1 at 4.2 °K). Neutron diffraction measurements confirmed that below 98 ± 5 °K RbCoF3 is antiferromagnetic. The magnetic configuration is of G-type. µco = 3.0 ± 0.2 µB at 4.2 °K. The magnetic susceptibility obeys a Curie-Weiss law between 300 and 600 °K (θp = 180 °K, neff = 5.53). The Néel temperature is in the neighbourhood of 100 °K. Velocities at which ultrasonic waves propagate have been measured as a function of temperature for different directions