First-Principles Study of the Three Polymorphs of Crystalline

The electronic structure, optical spectra, and thermodynamic properties of the three FOX-7 polymorphs (α, β, and γ) have been studied systematically using density functional theory. The LDA (CA-PZ) and generalized gradient approximation (GGA) (PW91) functions were used to relax the three FOX-7 phases without any constraint. Their density of states and partial density of states were calculated and analyzed. The band gaps for the three phases were calculated and the sequence of their sensitivity was presented. Their absorption coefficients were computed and compared. The thermodynamic functions including enthalpy (H), entropy (S), free energy (G), and heat capacity (Cp) for the three phases were evaluated.