High pressure structural study of TATB: Evidence of a structural phase transition.

High pressure unreacted isothermal equations of state (EOS) for energetic materials are needed for accurate modeling of detonation and shock initiation. EOS determination through conventional powder X-ray diffraction (XRD) experiments is often problematic due to the low-Z of constituent elements (typically CHNO) and the low symmetry of the corresponding crystal structure. To overcome this barrier for TATB, we performed a concomitant powder and single crystal (SC) XRD study. Our SC XRD results reveal a structural phase transition, reported for the first time, towards a monoclinic C-centered structure above 4-5GPa. Experimental results are supported by calculations using the USPEX evolutionary crystal structure prediction algorithm. The phase transition involves a pressure-induced alteration of the stacking of the layers of the TATB molecules. This results to a higher crystal symmetry, from triclinic to monoclinic, without an abrupt change of the volume per formula unit.