Landau theory for spin transition and ordering phenomena in Fe(II) compounds

We analyze spin transitions in molecular crystals with the help of Landau theory. Experimental observations show that a pure spin transition may be described as an isostructural transition with a totally symmetric order parameter. For long-range ordering of different spin states, a model is developed in which the spin transition is coupled with a structural transformation. The resulting phase diagrams are in good qualitative agreement with experiments.