Total free energy of a spin-crossover molecular system

Abstract

The free energy of spin-crossover molecular systems studied so far deal with the inner degrees of freedom of the spin-crossover molecules and a variety of interaction schemes between the molecules in the high spin (HS) and low spin (LS) states. Different types of transition curves, gradual, abrupt, hysteresis, and also two step transitions have been simulated or even satisfactorily fitted to experimental data. However, in the last decade spin transition curves were measured, especially under pressure, which could not be explained within these theoretical models. In this contribution the total free energy of an anharmonic lattice incorporating spin-crossover molecules which have a certain misfit to the lattice and interact elastically by their change in volume and shape has been constructed for a finite spherical crystal treated as a homogeneous isotropic elastic medium. The simulations demonstrate that already the knowledge of average properties of the crystal, as elastic constants and the anharmonicity of the potential of the lattice, and relative effective sizes of the molecules and their misfit to lattice is sufficient to interpret spin transition behavior. Almost all known anomalous spin transitions behaviors have been reproduced within reasonable limits of such parameters.