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Crystallographic data for cubic Type II clathrate hydrate have been used to derive a hypothetical structure for Type V clathrate hydrate. The structures of Types II and V are related by different stacking arrangements of layers of six-membered rings of pentagonal dodecahedra, which make up the water framework. The structure of Type II has an … ABCABC … stacking sequence, whereas the structure of Type V has an … ABAB … arrangement. Hypothetical Type V is hexagonal,  P 6 3  mmc , with  a  = 12.19 Å and  c  = 19.91 Å. The general stoichiometry for Type V, assuming fully occupied cages, is 4 X · 8 Y · 68 H 2 O (X: H 2 O = 1:17). 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. The stability of Type V and the possibility of stacking disorder between Type II and V structures, as is found for the silica polymorphs, is discussed.

Clathrate hydrates as layer structures: comparison between the structures of Type II and hypothetical Type V