A new description of the crystal structures of tin oxide fluorides

Abstract

We present two new approaches to describe the structures of the tin oxide fluorides Sn 2OF 2, Sn 2OF 5 and Sn 4OF 6, formerly described in terms of linked, cation-centered anion polyhedra. One approach considers the structures as cation arrays with inserted anions. The arrangement of the tin atoms in Sn 2OF 2 corresponds to that of the tin atoms in the rutile-like structure of SnO 2 which, in turn, can be related to the structure of the high pressure phase γ-Sn. The anions in Sn 2OF 2 take positions like the O atoms in SnO 2, but one fourth of them remain vacant. Sn 2OF 5 contains layers that match layers which can be cut out of the SnO 2 structure parallel to ( 1 1 ¯ 1 ); compared to SnO 2, the layers are mutually shifted, but within the layers the distribution of the tin atoms is alike. Sn 4OF 6 contains parallel columns that correspond to columnar fragments taken out of SnO 2 parallel to b. These columns correspond to a composition [Sn 3OF 3] −; they are interconnected by [Sn 2F 6] 2+ groups. The similarity of the tin atom arrangement in a [Sn 3OF 3] − column to that in SnO 2 helps to recognize this. The other approach is to derive the structures of the oxide fluorides from the SnO 2 structure by symmetry reduction, i.e., with the aid of group–subgroup relations between their space groups. It takes three steps of symmetry reduction from P 4 2 / m n m , the space group of SnO 2, to the space group of Sn 2OF 2, A 112 / m . All atomic positions occupied in Sn 2OF 2 can be derived from positions of SnO 2, but one fourth of the anion positions remain vacant. Group–subgroup relations reveal only a partial agreement for Sn 2OF 5 (within the layers), but the Sn atoms between the layers cannot be related to positions in SnO 2. The structure of Sn 4OF 6 cannot be derived group theoretically from SnO 2 because of the interfering [Sn 2F 6] 2+ groups between the [Sn 3OF 3] − columns.