Journal
JOURNAL OF ALLOYS AND COMPOUNDS
Volume 350, Issue 1-2, Pages 113-122Publisher
ELSEVIER SCIENCE SA
DOI: 10.1016/S0925-8388(02)00932-5
Keywords
europium clathrate; phase transition; atomistic model; hierarchical relationships
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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.
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