Structural study of the incommensurate and look-in phases of Rb2ZnCl4

The crystal structures of Rb2ZnCl4 at 210 K [incommensurate structure, a=7.253(5), b=12.646(9), c=9.221(7) A] and 115 K [threefold commensurate superstructure, a=7.230(5), b=12.608(9), c=9.199(7) A] have been investigated from X-ray single-crystal diffraction data. The refinement program REMOS was used. The final wR factors at 210 and 115 K are respectively 0.045 and 0.041 for 1073 and 1215 main reflections, and 0.187 and 0.136 for 587 and 1065 first-order satellite reflections. The superspace groups are designated as P(Pmcn):(s, s, anti 1) and P(P21cn):(anti 1, s, anti 1) in the incommensurate and lock-in phases, respectively. The modulation is interpreted as rotations of rigid ZnCl4 tetrahedra around two cristallographic directions. Calculations of polarization confirmed the choice of the superspace group in the incommensurate phase and showed a connection between the ferroelectricity of the crystal and structural elements of the threefold superstructure. The comparison of the structural results of both structures outlines the continuous character of the lock-in transition. (orig.)