Crystallography Journals Online : IUCR Backfile Archive 1948-2001
Chemistry and Pharmacology
A preliminary report on the rhombohedral structure of the ordered yttrofluorite mineral, tveitite (possible formulae: Ca13 + δY6 - δF44 - δ, Ca14Y5F43 or Ca13Y6F42O), is given. In this structure discrete M6X37 groups occur, which consist of an octahedral arrangement of cation-centred square antiprisms sharing corners around the threefold axis to enclose a filled cuboctahedron of anions. The same group is found in other known fluorite-related superlattices, which suggests that it is a basic structural unit. Its formal derivation from the analogous element of fluorite is described.
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