Structural transformation in HCP solid C<SUB>70</SUB> at low temperatures

The structural transformation of C<SUB>70</SUB> powder prepared from ideal h.c.p. grown single crystals has been investigated using X-ray diffraction. Data taken from 300 K to 77 K in steps of 5 K over a span of 1000 h indicate a direct transformation to a monoclinic phase without going through any other intermediate phase. This implies that the freezing of rotations of the C<SUB>70</SUB> molecules about their major and minor axes can be simultaneous and not necessarily only sequential. The transition is seen to proceed gradually with both the phases coexisting in the range 270 K to 170 K. The coexistence is rationalised in terms of internal dilatory stresses arising due to the nucleation and growth of the monoclinic phase and the consequent progressive suppression of the transformation temperature. Our monoclinic cell parameters support a model in which the polar cap interactions between the molecules in the basal plane and those in the midplane are significant