The properties of ions with intermediate valence and the theory of high-pressure phase structure
Two models of phase transitions under pressure, accompanied by electronic structure rearrangement, are proposed. A distinctive feature of the proposed models is the consideration of pressure-induced isotropic deformations and phonons breaking the symmetry. A mutual solid solution of ions of the same element in different ionization states is assumed to be regular. The second model supplements the first one by taking into account the physical nature of deviation of the mutual solution from the ideal one. The violation of the Curie principle during phase transitions under isotropic pressure is explained within these models.