Spin-crossover in molecular crystals: An electron-coupled local vibrations model

A model of eg electrons at octahedral sites coupled to local vibrational “breathing” modes interacting with its near neighbors is proposed to describe spin-crossover (SCO) in molecular crystals. Decoupling vibrations leads to an effective electron Hamiltonian with renormalized site energies, where ferromagnetic and antiferromagnetic, short range and long range electron-electron interactions arise in a natural way. An exact analytic expression for the free energy functional is derived. A phase diagram for homogenous phases, describing the basic phenomenology of SCO is obtained and the transition temperatures are expressed in terms of model parameters. Under appropriate conditions, two-step transitions are found to take place for systems with both, short range and long range interactions. Stability with respect to spatial fluctuations is discussed.