Orientational disordering in CsCo(ND3)(6)(ClO4)(2)Cl-2 crystals studied bysingle crystal neutron diffraction between 20 and 290 K

A member of the series M(I)M(III)(ND3)(6)(ClO4)(2)X-2, CsCo(ND3)(6) (ClO4)( 2)Cl-2, has been examined by single crystal time-of-flight neutron diffract ion at 20, T-c-10, T-c + 10, and 290 K, adding to previous X-ray diffractio n studies of some compounds in this series. T-c is the phase transition tem perature of 250(1)K observed by DSC between closely related simple trigonal structures-twinned ordered R (3) over bar and disordered R (3) over bar m, both with Z = 3 and the former with a = 7.210(2) Angstrom, c = 26.819 Angs trom at 20 K, The twin populations, orientational order parameter, and deut erium disordering are much more clearly observed by neutron than by X-ray d iffraction, A simple model involving both twinning and order-disorder with differing ranges of ionic orientational correlations is appropriate for thi s crystal at all temperatures. This type of subtle transition, sometimes in volving anharmonicity, floppy modes, and substantial disorder below the tra nsition temperature, is easily missed by diffraction studies and get the th ermal and electrical consequences of such disorder may be severe. (C) 2000 Academic Press.