The Fermi model in describing phase transitions initiated by pressure variation

Having analyzed the pressure dependence of volume V(σ), we demonstrate that the mechanism of the phase transition in HfO₂ corresponds to the Fermi model. This indicates that at σ = 10 GPa, the ground state of Hf ions changes. Within the Fermi model, the stiffness moduli in the phases stable at σ < 10 GPa and σ > 10 GPa are calculated. It is shown that the results obtained are in better agreement with the well-known experiment than the results obtained in the framework of the quantum chemistry model (ab initio calculations) and the Birch-Murnaghan equation of state.