Spin crossover in dinuclear compounds

Within the Ising-like model the Hamiltonian of a dinuclear system formed of a symmetric pair of metallic centres is 

The introduction of the last parameter corresponds to an approach beyond the mean field approximation. The eigenvalues of the above Hamiltonian are received in the space spanned by a direct product aA 8 aB of interacting centres 

For convenience, two symmetry-adapted parameters were introduced 

The first parameter is related to the high-spin mole fraction 

The second parameter adopts a meaning of the order parameter. For n — 0 the sublattices are equivalent whereas for n 0 there is a spontaneous symmetry breaking since the two sublattices are no longer equivalent. 

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Two-step spin crossover and spin crossover in dinuclear compounds has roots in a proper combination of the inter-sublattice interaction parameter (which is ferromagnetic) and the intra-sublattice one (antiferromagnetic). 

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