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

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 th resolution of the present diffraction data, our results do not reveal evidence for a pressureinduce structural phase transition near 2 GPa, previously observed in several vibrationa spectroscopy experiments. Based on unit-cell volume measurements, the least-squares fi using the third-order Birch–Murnaghan equation of state (EOS) yields K0 = 12.6 ± 1.4 GP and K0 = 11.3 ± 2.1 for the α-phase of FOX-7, which are in good agreement with recentl reported values for the deuterated sample, indicating that the effect of hydrogen–deuteriu substitution on the compressibility of FOX-7 is negligibly small. A thermal EOS is als 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 capacit at constant volume (CV) and thermodynamic Gruneisen parameter (γTH) as a function o temperature. At ambient conditions, the calculated γTH is 1.055, which is in good agreemen with the value (1.09) previously obtained from density functionalmore » theory (DFT). The obtaine 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 furthe improved for describing intermolecular interactions of molecular crystals.« less