A new description of the crystal structures of tin oxide fluorides

We present two new approaches to describe the structures of the tin oxide fluorides Sn2OF2, Sn2OF5 and Sn4OF6, 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 Sn2OF2 corresponds to that of the tin atoms in the rutile-like structure of SnO2 which, in turn, can be related to the structure of the high pressure phase γ-Sn. The anions in Sn2OF2 take positions like the O atoms in SnO2, but one fourth of them remain vacant. Sn2OF5 contains layers that match layers which can be cut out of the SnO2 structure parallel to (11¯1); compared to SnO2, the layers are mutually shifted, but within the layers the distribution of the tin atoms is alike. Sn4OF6 contains parallel columns that correspond to columnar fragments taken out of SnO2 parallel to b. These columns correspond to a composition [Sn3OF3]−; they are interconnected by [Sn2F6]2+ groups. The similarity of the tin atom arrangement in a [Sn3OF3]− column to that in SnO2 helps to recognize this. The other approach is to derive the structures of the oxide fluorides from the SnO2 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 P42/mnm, the space group of SnO2, to the space group of Sn2OF2, A112/m. All atomic positions occupied in Sn2OF2 can be derived from positions of SnO2, but one fourth of the anion positions remain vacant. Group-subgroup relations reveal only a partial agreement for Sn2OF5 (within the layers), but the Sn atoms between the layers cannot be related to positions in SnO2. The structure of Sn4OF6 cannot be derived group theoretically from SnO2 because of the interfering [Sn2F6]2+ groups between the [Sn3OF3]− columns.