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The model describing the effect of anharmonicity on the spin-crossover properties of Fe(II) complex is proposed. It is shown that anharmonicity can be one of the important factors controlling the magnetic transitions of the low-spin high-spin type.

Abstract:We have considered the spin crossover (SC) complex compounds containing spin-active metal ions of iron group, which can induce the spin transitions (ST) of high spin (HS) -low spin (LS) type under influence of temperature (TIST), pressure (PIST) or light (LIESST) from non-conventional point of view. We have analyzed the physical peculiarities of the studied ST systems observed in numerous papers for X ray, spectral, calorimetric and magnetic measurements. On this basis, we suppose that ST (for example, in bulk of SC compounds with TIST) can be considered as phase transitions (PT) of postcritical type (PCT).