New Structural Models for Anion-Excess Fluorite-Related Vernier Phases

International audienceA great number of anion-excess fluorite-related phases MX2+y correspond to ordered distributions, within a fluorite matrix, of the same M6X14 columnar cluster generated by the insertion at the centre of six primitive cubes MX8 of an extra zigzag row of anions. They can therefore be formulated as Mm+6X2m+14 (0 ≤ m < ∞) or more generally as Mm′X2m′+2 (m′ = m+6). It has been shown that, among all these structures, some, in which the clusters are systematically aligned and directly connected along rows perpendicular to the densified anion layers, are classical vernier structures formulated as MnX2n+1 (n = m′/2 = integer). Some are new vernier structures that can be called shifted vernier structures since the M6X14 clusters are shifted, from one densified plane to the other, by a vector →T = p(→aF + →bF)/2 (p = integer) and for which non-integer values of n are possible. Bi7O5F11 constitutes the first isolated member (n = 3.5, m' = 7) of this new structural series. Some others are completely different structures like the K2NbF7 compound