The first-principle studies of the crystal phase transitions: Fd3m-MgAl2O4→F4-3m-MgAl2O4

Magnesium aluminum spinel (MgAl2O4) is a major constituent of the shallow upper mantle. It is of great geophysical importance to explore its physical properties under high pressure and temperature. The first-principle density functional theory (DFT) with the plane wave along with pseudopotential was employed to obtain the total energy for both Fd3m-MgAl2O4 and F4-3m-MgAl2O4, which was used to generate the Gibbs free energy as a function of temperature and pressure with the quasi-harmonic Debye model. It is found that the phase transition temperature from Fd3m-MgAl2O4 to F4-3m-MgAl2O4 is beyond 452.6 K in the pressure regime studied, which is consistent with the experiment. The phase transition temperature is related to pressure by a linear function, i.e. T=8.05P+452.6, which is the first equation of this kind to describe the phase transition Fd3m→F4-3m. The elastic constants, equation of states and thermodynamic properties of Fd3m-MgAl2O4 are also reported in this paper to make a complete study.