Orientational disordering in CsCo(ND₃)₆(CLO₄)₂Cl₂ crystals studied by single crystal neutron diffraction between 20 and 290 K

A member of the series M(I)M(III)(ND₃)₆(ClO₄)₂X₂, CsC₀(ND₃)₆(ClO₄)₂Cl₂, has been examined by single crystal time-of-flight neutron diffraction at 20, T(c)-10, T(c)+10, and 290 K, adding to previous X-ray diffraction studies of some compounds in this series. T(c) is the phase transition temperature of 250(1) K observed by DSC between closely related simple trigonal structures - twinned ordered R3 and disordered R3m, both with Z= 3 and the former with a =7.210(2) Å, c = 26.819 Å at 20K. The twin populations, orientational order parameter, and deuterium disordering are much more clearly observed by neutron than by X-ray diffraction. A simple model involving both twinning and order-disorder with differing ranges of ionic orientational correlations is appropriate for this crystal at all temperatures. This type of subtle transition, sometimes involving anharmonicity, floppy modes, and substantial disorder below the transition temperature, is easily missed by diffraction studies and yet the thermal and electrical consequences of such disorder may be severe. (C) 2000 Academic Press.