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 = 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. © 2016 IOP Publishing Ltd.
Equation of state and thermodynamic Grüneisen parameter of monoclinic 1,1-diamino-2,2-dinitroethylene.
Journal of Physics Condensed Matter, 28(39),