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High pressure; organic compounds; p-dichlorobenzene; phase transitions; x-ray diffraction

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 constantsa =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 initialα phase which has aβ-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.

Hema Sankaran¹ Surinder M Sharma¹ S K Sikka¹ R Chidambaram¹

1. Neutron Physics Division, Bhabha Atomic Research Centre, Bombay - 400 085, India

Published

December 1986

Manuscript received

3 July 1986

Manuscript revised

24 September 1986