Thermal expansion of energetic material TEX obtained from x-ray diffraction and first principles calculations

X-ray powder diffraction studies on the secondary explosive TEX (C6H6N4O8) in the temperature range 25–150 °C are reported. TEX has a triclinic structure (a = 6.850 Å, b = 7.665 Å, c = 8.826 Å, α = 82.38°, β = 75.0°, and γ = 79.30° at 25 °C), space group P1¯. The crystal structure remains triclinic in this temperature range. From the temperature dependence of lattice parameters we find that thermal expansion is different along different directions. The volume thermal expansion coefficient 17.4 × 10−5 K-1 is comparable to that of a related material, CL-20. Thermal expansion along the c-direction αc is 2.2 times αa and 3 times αb. Phonon frequencies of TEX crystal at different pressures are obtained from density functional theory (DFT) calculations using CASTEP codes. The volume thermal expansion coefficient calculated using Gruneisen formalism is 6.1 × 10−5 K-1. This is the first report on the experimental determination of thermal expansion of TEX, and computation by Gruneisen formalism using DFT.