Abstract
A microscopic model of low-spin∕high-spin transitions in solids containing transition metal ions with ${d}^{6}$ configuration is described. The underlying physics of the transition is related to the large totally symmetric relaxation of ligands accompanying the two electron promotion from the ${t}_{2}$ to $e$ orbitals. The electron distribution function, the electronic entropy, and the heat capacity are determined from the analysis of the free energy. The qualitative estimation of the relaxation is in agreement with x-ray determination. The electronic and lattice entropies are comparable because of the large lattice relaxation. The stepwise low-spin∕high-spin transition of ${\mathrm{Fe}}^{2+}$ with temperature is demonstrated and, hence, the temperature induced abrupt changes in the properties of solids. The weak long-range intermolecular interaction is not excluded but it is not necessary for the explanation of a variety of experimental observations.
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