High pressure phase transitions in organic solids II: X-ray diffraction study of p-dichlorobenzene at high pressures

Abstract

X-ray diffraction experiments onp-dichlorobenzene at high pressures show a transition at ~ 0.3 GPa, to a new phase, the diffraction pattern of which cannot be indexed on the anticipated low temperature monoclinic crystal structure. We have instead found an orthorhombic cell, very closely related to the low temperature monoclinic cell, for this new phase. This structure, which also occurs inp-diiodobenzene at ambient conditions, has cell constants a =14.02,b = 6.06,c = 7.41Å andZ = 4. The space group is Pbca. This new phase has a non-β herring-bone structure, in contrast with the initial α phase which has a β -structure with ribbon-like arrangement of molecules, with Cl-Cl contacts of ~ 4A between adjacent molecules. This implies that with pressure the halogen-halogen interaction in this compound plays a less dominant role in crystal engineering.