Crystal structure and symmetry mode analysis of the threefold superstructure of Cs2ZnI4 at 104 K

Abstract

Abstract In this work we determined the structure of Cs2ZnI4 at 104 K. Lattice parameters at this temperature are a = 32.33(4) Å, b = 14.291(7) Å and c = 8.220(4) Å. With respect to the high temperature Pnam phase the a axis is triplicated and the symmetry is lowered to Pna21. The structure is thus similar to the lock-in phases observed in other A2BX4 compounds of β-K2SeO4 type. In comparison to the high temperature phase the main change consists in shifts of the atoms in the direction of c while movements in the direction of b are rather small. The volume of the cell of the threefold phase is slightly smaller than three times the volume of the Pnam phase. Yet the volume and the average bond distances within the [ZnI4]2− remain more or less unchanged. The three symmetrically independent tetrahedra in the Pna21 phase are geometrically very similar and show comparative distortion parameters. Like in other A2BX4 compounds the distortion of the tetrahedra is increased with respect to the high temperature phase. The degree of fitting of the tetrahedra in Cs2ZnI4 to rigid bodies is 87%. The average bond distances and the volume of all A-coordination polyhedra in the Pna21 phase are decreased with respect to the Pnam phase and accordingly bond valence sums for the A[9] and especially for A[11] cations are increased. Symmetry mode analysis of the structural distortions with respect to the Pnam phase shows striking similarities to other A2BX4 compounds, particularly to Cs2CdBr4.