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.

中文翻译：

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自旋平方算子的平均值作为自旋相变的阶数参数，而不会自发降低对称性

从低自旋态到高自旋态的相变是一种独特的物理现象，不会降低对称性。与将相量或物理量张量用作阶跃参数的磁相变相反，我们已经表明，对于自旋相变，阶跃参数是标量-自旋平方算子的热力学平均值，以前没有使用在相变理论中根本没有。确定该阶跃参数的函数形式的自由能，并构建自旋跃迁的相图。还分析了压力对自旋转变的影响。结果表明，具有该阶数参数的自旋哈密顿量使人们可以获得化合物中所有可能的自旋变换。同时，