In-situ pressure crystallization and X-ray diffraction study of 1,1,2,2-tetrachloroethane at 0.5 GPa

1,1,2,2-Tetrachloroethane, C2H2Cl4 (denoted TCE, m.p. 230 K) has been in-situ pressure crystallized in a Merrill-Bassett diamond-anvil cell, and its structure has been determined at 0.5 GPa and 295 K from the single-crystal X-ray diffraction data. TCE crystallizes in the monoclinic space group P21 /c with the molecules located at the inversion centers. The molecules are in the s-trans conformation, while they assume the gauche conformation in the crystal obtained by cooling. This implies that a phase transition may exist between the low-temperature and high-pressure phases of TCE. In the high-pressure phase the HC–CH moiety of the C2H2Cl4 molecule is disordered in two sites, one related to the other by the non-crystallographic mirror plane approximately through the four Cl atoms of one molecule. The refined occupancy factors for the disordered sites are 0.77 and 0.23. The role played by the Cl…Cl intermolecular interactions for the disorder, and the phase transition between TCE phases built of s-trans (high-pressure) and gauche (low-temperature) molecules have been discussed.