Theoretical investigations on an axial next nearest neighbour Ising-like model for spincrossover solids: one- and two-step spin transitions

Abstract

We investigate the thermodynamical properties of an adapted ANNNI-like (axial nextnearest neighbour Ising-like) model for 1D spin crossover solids. We performed an exacttreatment within the framework of the well known transfer matrix method and derived thethermal evolution of the high spin fraction, the correlation function and the heat capacity.We show that a model of competition of the interactions between similar spin state ionsreproduces qualitatively the main features of experimental data: gradual, abrupt andtwo-step spin crossover transitions. In addition, we found that the intermediate region ofthe two-step transition exhibits two types of spatial organization, according to the sign ofthe interactions. Indeed, antiferro-like and ‘antiphase’ (called the phase in the ANNNI model) structures are obtained. We also found that these twoconfigurations can be distinguished through the thermal evolution of their correspondingcorrelation functions.The existence of the phase , predicted by the present model, is in good agreement with recent observations on a spin crossover sampleof [{Fe(CNBH3)(4phpy)}2(μ-bpypz)2] for which this phase was evidenced for the first time, in these materials.