Structure and Phase Transitions of Solid Heavy Methane (CD4)

The orientational structures and the phase transitions (27.0 and 22.1°K) of solid CD4 are investigated by coherent neutron scattering from powder samples as well as from single crystals. In its high temperature phase, CD4 is found to crystallize within the space group Fm3m with four molecules per unit cell and a lattice constant a0=5.96 Å at 77°K. The molecular orientations are completely disordered at 35°K, while an indication of partial order was detected at 77°K. The transition to phase II is accompanied by critical fluctuations with a correlation length of about 24 Å (at 27.7°K) and appears to be of second order (critical exponent β =0.4± 0.1. In phase II six of eight molecules order with a local symmetry 42m, while the remaining two are orientationally disordered. The space group is cubic Fm3c with 32 molecules per unit cell and a0=11.64 Å at 24.5°K. The transition to phase III is predominantly of first order. With the present data the structure of phase III cannot be determined unambiguously. Superlattice reflections can be indexed cubic primitive. The number of molecules per unit cell remains 32, with a0=11.61 Å at 17.5°K. The data suggest, that the low temperature structure is to be understood by an ordering within the sublattices of molecules disordered in phase II together with slight distortions of the other sublattices. The structures of phases I and II agree with the ones predicted by James and Keenan under the assumption of octupole—octupole interaction between nearest neighbour molecules. For phase III the model of James and Keenan, as well as other predictions, hitherto published, must be ruled out.