A new structural principle in anion-excess fluorite-related superlattices

A preliminary report on the rhombohedral structure of the ordered yttrofluorite mineral, tveitite (possible formulae: Ca_{13 + δ}Y_{6 - δ}F_{44 - 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.