An X-ray diffraction study of the molecular complex system hexaflurobenzene-deuterobenzene at 279K is reported. The structure has been determined for the first time as rhombohedral, space group R3m or R3m, and of unit-cell dimensions a = 7.310 (19)/k and a = 109.67 (43) °, giving a unit-cell volume of 299 (5) A ~. R = 0.098 for 150 independent reflections. There is one molecule of each species within the unit cell and they are alternately arranged in infinite stacks along the unique [ 111 ] crystallographic direction, the molecular planes being perpendicular to this direction. Both constrained and unconstrained refinements have been done and the results of both are presented. The constraints consisted of rigidly fixing the bond lengths and geometry of the molecules such that the resulting molecular hexad symmetry axis was coincident with the unique axis of the structure. The unconstrained model relaxes the fixed bond lengths but the hexad symmetry is retained. The possibility of pairing of the molecules along the unique direction is considered for this phase, and crystallographic evidence is given for at least two other solid-state phases of this system.

The crystal structure of carbonyl sulphide (COS) at 90 K has been reinvestigated using the neutron powder diffraction technique. The structure is rhombohedral, space group R3m, with unit-cell parameters a ---4.063 (3)A and a = 98.81 (3)°; R = 0.11 for 742 independent observations. There is one molecule per unit cell, aligned along the [111] crystallographic axis.

No abstract

No abstract