Possible experimental separation of the mechanisms of phase transitions under pressure

Two phenomenological models of phase transitions occurring under isotropic pressure have been considered. The models are based on different assumptions regarding the transition mechanism: the Mott model (in which the nonequilibrium Helmholtz thermodynamic potential is a smooth continuous function of the unit cell volume and is characterized by two inflection points) and the Fermi model (according to which the ground state of the atoms of the material changes with an increase in pressure due to the change in the relative positions of two lower energy levels, and, therefore, the thermodynamic potential of the unit cell is a piecewise continuous function of the unit cell volume). A detailed analysis has been made of the consequences of both models, and the criteria for application of these models to the description of phase transitions in specific materials have been obtained. The applicability of the derived criteria and the chosen model has demonstrated using the example of the phase transition observed in selenium.