Thermomechanical properties and equation of state for the γ-polymorph of hexahydro-1,3,5-trinitro-1,3,5-triazine

The thermomechanical properties of the gamma-polymorph of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) are predicted using a non-reactive fully flexible Smith and Bharadwaj molecular potential using molecular dynamics simulations. The elastic modulus tensor, coefficients of thermal expansion, and lattice constants are calculated as functions of pressure (4-11 GPa) and temperature (200-550 K). Much like alpha-RDX, the components of the elastic modulus tensor of gamma-RDX are found to increase monotonically with pressure, but soften only mildly with increasing temperature. Another interesting observation is that the b-lattice parameter is insensitive to temperature changes and is almost unchanged at higher pressures. A third order Birch-Murnaghan equation of state and the coefficients of thermal expansion are obtained from the pressure-volume-temperature (pVT) data. Around the alpha-gamma phase transition pressure, the coefficients of thermal expansion for gamma-RDX are greater than those of alpha-RDX. In contrast to alpha-RDX, the coefficients of thermal expansion for gamma-RDX exhibit negligible variation with pressure. The predicted values of the thermomechanical properties and crystal parameters agree reasonably well with experimental results and other molecular simulations reported in the literature.