Equation of state and thermodynamic Grüneisen parameter of monoclinic 1,1-diamino-2,2-dinitroethylene

Abstract

In situ synchrotron x-ray diffraction experiments were conducted on 1,1-diamino-2,2-dinitroethylene (FOX-7) at pressures up to 6.8 GPa and temperatures up to 485 K. Within the resolution of the present diffraction data, our results do not reveal evidence for a pressure-induced structural phase transition near 2 GPa, previously observed in several vibrational spectroscopy experiments. Based on unit-cell volume measurements, the least-squares fit using the third-order Birch–Murnaghan equation of state (EOS) yields K0 = 12.6 ± 1.4 GPa and = 11.3 ± 2.1 for the α-phase of FOX-7, which are in good agreement with recently reported values for the deuterated sample, indicating that the effect of hydrogen–deuterium substitution on the compressibility of FOX-7 is negligibly small. A thermal EOS is also obtained for the α-phase of FOX-7, including pressure dependence of thermal expansivity, (∂α/∂P)T = −7.0 ± 2.0 × 10−5 K−1 GPa−1, and temperature derivative of the bulk modulus, (∂KT/∂T)P = −1.1 × 10−2 GPa K−1. From these EOS parameters, we calculate heat capacity at constant volume (CV) and thermodynamic Grüneisen parameter (γTH) as a function of temperature. At ambient conditions, the calculated γTH is 1.055, which is in good agreement with the value (1.09) previously obtained from density functional theory (DFT). The obtained CV, however, is 13% larger than that calculated from the first-principles calculations, indicating that the dispersion correction in the DFT calculations may need to be further improved for describing intermolecular interactions of molecular crystals.