Equation of state and thermodynamic Grüneisen 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 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 K ₀  =  12.6  ±  1.4 GPa and $K_0^{\prime }$  =  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, (∂K _T/∂T)_P  =  −1.1  ×  10⁻² GPa K⁻¹. From these EOS parameters, we calculate heat capacity at constant volume (C _V) and thermodynamic Grüneisen parameter (γ _TH) as a function of temperature. At ambient conditions, the calculated γ _TH is 1.055, which is in good agreement with the value (1.09) previously obtained from density functional theory (DFT). The obtained C _V, 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. (paper)