Modelling of the α (clathrate VIII) ⇆ β (clathrate I) phase transition in Eu8Ga16Ge30

The title compound has been reported to be dimorphic. The structure of the high-temperature phase (P) corresponds to clathrate I (space group Pm (3) over barn, Pearson symbol cP54), that of the low-temperature phase (a) is of clathrate VIII type (space group I (4) over bar 3m, cI54). The structural relationship between the clathrate I and clathrate VIII structure types is investigated with a topological approach, based on periodic minimal surfaces approximants. The description of the structural transition is formulated in terms of a global transformation of the two four-connected covalent frameworks, and appears as a rotation around a distinct cubic three-fold axis. Locally, this involves the opening of a bond at a few tetrahedral nodes in the starting arrangement, the formation of three-connected nodes and the formation of new connections in the final arrangement, in order to restore the four-connected nodes. A picture of the transition in terms of distortion of polyhedra is also proposed. Through the derivation of both limiting networks, by hierarchical expansion, from a J* and an I net, a structural motif underlying the entire transition is discovered. (C) 2002 Elsevier Science B.V. All rights reserved.