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Phase transitions from the low-spin to the high-spin state are a unique physical phenomenon without lowering of symmetry. In contrast to magnetic phase transitions, for which vector or tensor of physical quantities are used as order parameters, we have shown that for spin phase transitions the order parameter is a scalar quantity—the thermodynamic mean of spin square operator, which was not previously used at all in the theory of phase transitions. The free energy in the form of a functional of this order parameter is determined, and the phase diagrams for spin transitions are constructed. The influence of the pressure on spin transition is analyzed also. It is shown that the spin Hamiltonian with this order parameter allows one to obtain all possible spin transformations in compounds. At the same time, this order parameter correctly reflects the physical nature of the spin transition phenomena.

The average value of the spin squared operator as an order parameter for spin phase transitions without spontaneous lowering of symmetry