The Role of Binary and Many-Centre Molecular Interactions in Spin Crossover in the Solid State. Part IV. Ordering in Systems with Many-Body Interactions

Summary.

The theoretical formalism of solid-state spin crossover taking into account many-body interactions and ordering has been extended to the case of 5-centre interactions in a diamond lattice leading to the splitting of the free energy levels into quintets. Consecutive simplification of the obtained equations has been achieved for multiplets with regularly varying splittings: the variation according to the polynomial of the third order corresponds to 5-centre interactions; quadratic and linear variations yield the formalisms of quaternary and ternary interactions respectively; equidistant multiplets correspond to the model of binary interactions. These types of variations have been deduced from a simple microscopic model of the influence of the external molecule on the considered interaction. Parameters of the developed formalism have been expressed in terms of binary potentials and relative efficiencies of external molecules. The expressions obtained provided an adequate description of experimental two-step spin crossover curves. The formalism developed in this way was found to be similar, but not identical, to the phenomenological description based on the Landau theory. Odd power terms in composition (not entering the expansion of free energy derived from Landau theory) were found to be generally non-zero and vital for obtaining adequate descriptions of experimental data.