First Principles Predict the Structural Properties, Electronic Properties and Phase Diagrams of RDX under High Temperature and Pressure

This paper studies the three phases of RDX using the first-principles, and calculates electronic properties of RDX. At the same time, the properties of the three phases near the phase transition pressure are calculated. Under normal pressure, the lattice parameters of \(\alpha\)-RDX are in good agreement with the experimental values. The lattice parameters of \(\gamma\)-RDX are in good agreement with the experimental values. The theoretical value of the unit cell volume of \(\beta\)-RDX is smaller than the experimental value, because \(\beta\)-RDX will expand in volume at high temperature. Under normal pressure and pressure of 2.8 GPa, there is no virtual frequency in phonon spectrum of \(\alpha\)-RDX. The appearance of virtual frequency under the pressure of 4 GPa indicates that the structure is unstable under this pressure and phase transition has occurred. Under the action of pressure, the peak values of conduction band and valence band decrease slightly, which is more obvious near the Fermi level, indicating that the electrons around the Fermi level are more sensitive to pressure. According to the calculation of Gibbs free energy, under the pressure of 2.8 GPa, \(\alpha\)-RDX will change into \(\gamma\)-RDX, which is in good agreement with the experimental value. The result is consistent with the previous phonon spectrum calculation. At the same time, under this pressure,when the temperature rises to 467 K, \(\alpha\)-RDX transforms into high-temperature phase \(\beta\)-RDX. Finally, the phase transition temperature of \(\alpha\)-RDX and \(\beta\)-RDX at 2.7 GPa was calculated. And the phase transition temperatures of \(\gamma\)-RDX and \(\beta\)-RDX at 2.9 GPa determine the phase boundary of the three phase of RDX.