The Crystal Structures at 10 K of the Trigonal Salts KMIII(NH3)6(ClO4)2Cl2, MIII = Os and Cr, and the Possibility of Phase Transitions

Abstract

The crystal structures of KOs(NH₃)₃(ClO₄)₂Cl₂ and KCr(NH₃)₆(ClO₄)₂Cl₂ were determined at 10 K by X-ray diffraction, and for the osmium salt also at 293 K. At 293 K the osmium salt is trigonal, space group R 3m, with the same simple structure as others of this class of double salt. At 10 K, in agreement with previous radius ratio predictions, both crystals are best described as remaining R 3m. All previously studied members, with larger alkali metal cations, are twinned R 3 at low temperatures, with small, symmetry-breaking rotations of the hexaamminemetal(III) and perchlorate ions about the threefold axis. Differential scanning calorimetry on CsRu(NH₃)₆(ClO₄)₂Cl₂ suggests that the R 3m to R 3 change is very extended in temperature with only a small discontinuity at the transition temperature.