CLATHRATE HYDRATES AS LAYER STRUCTURES - COMPARISON BETWEEN THE STRUCTURES OF TYPE-II AND HYPOTHETICAL TYPE-V

Crystallographic data for cubic Type II clathrate hydrate have been us ed to derive a hypothetical structure for Type V clathrate hydrate. Th e structures of Types II and V are related by different stacking arran gements of layers of six-membered rings of pentagonal dodecahedra, whi ch make up the water framework. The structure of Type II has an... ABC ABC... stacking sequence, whereas the structure of Type V has an...ABA B... arrangement. Hypothetical Type V is hexagonal, P6(3)mmc, with a = 12.19 Angstrom; and c = 19.91 Angstrom. The general stoichiometry for Type V, assuming fully occupied cages, is 4 X 8 Y 68 H2O (X:H2O = 1:1 7). Theoretical oxygen and guest molecule positions are given for the structure of the hypothetical Type V hydrate. The relationship between the structures of Types II and V are analogous to that of the silica polymorphs tridymite and cristobalite, and the layered clathrasils. Th e stability of Type V and the possibility of stacking disorder between Type II and V structures, as is found for the silica polymorphs, is d iscussed.