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

Abstract

A great number of anion-excess fluorite-related phases MX_2+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 M_m+6X_2m+14 (0 ≤ m < ∞) or more generally as M_m′X_2m′+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 M_nX_2n+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(_→a_F + _→b_F)/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.