Clathrate III (Bromine Hydrate): Structural Relationship with Clathrate I.

Abstract

Tetragonal structure of clathrate III (TS-I) is found in bromine hydrate, ionic hydrates, in particular, in hydrates of tetraalkylammonium salts, as well as in clathrate compounds of the group IV elements ('Zintl clathrates'). In this paper new description of this clathrate structure is suggested. Main structural unit in this description is '4-column', the set of four nearest columns of 14-hedral T-cages together with its internal filling by the pillar of mutually orthogonal pairs of T-cages. Comparison based on this new description of clathrate III and clathrate I (CS-I) shows that 4-column is maximal common structural element of these two structures. On this ground it is argued that clathrate III can be considered as a modification of the clathrate I, formed as a result of multiple interruption of clathrate I crystal growth. At the same time, it is shown that due to crucial differences between these two structures there does not exist structural or reconstructive phase transition between them. Integral character of 4-columns as the structural units of clathrate III is confirmed by the analysis of the most studied case of ionic hydrates of tetraalkylammonium salts in which guest molecules occupy simultaneously T-cages in outer columns and inner T-cages. This analysis allows also to state important hypothesis that in general case for formation of clathrate III crystals presence of special guest molecules occupying several cages (or special ions in the host lattice) is necessary.