The structure of deuterated methane-hydrate

We present the results of a high-resolution neutron diffraction experiment with a fully deuterated methane hydrate type I at temperatures of 2, 100, a nd 150 K. Precise crystallographic parameters of the ice-like D2O lattice a nd the thermal parameters of the encaged methane molecules have been obtain ed. The parameters of the host lattice differ only slightly from values fou nd for hydrates with asymmetric guests included, which leads to the conclus ion that the host lattice of structure I is only a little adaptive. At low temperatures (2 K) the methane molecules in both types of cages present in structure I occupy positions in the center of the cages. At higher temperat ures the thermal parameters in both types of cages reflect the surrounding cage geometries or more precisely the translational potentials of the cages . The orientational scattering length density of the CD4 molecules has been analyzed in terms of a multipole expansion with symmetry adapted functions [Press and Huller, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor . Gen. Crystallogr. A29, 252 (1972); Press, ibid. A29, 257 (1972)]. In both types of cages we found only small modulations of a spherically symmetric scattering density accounting for almost free rotations of the methane mole cules. The large and asymmetric cage leads to a somewhat more pronounced mo dulation of the orientational density than in the small dodecahedral cage. The orientational probability distribution function (PDF) remains nearly un changed from 2 to 150 K. At 200 K we observed the time-resolved decompositi on of the hydrate structure I into hexagonal ice Ih. (C) 2000 American Inst itute of Physics. [S0021- 9606(00)00735-2].