REFINEMENT OF AMMONIUM PERRHENATE STRUCTURE USING A PSEUDO-SPIN MODELFOR THE AMMONIUM ION ORIENTATION

A new Rietveld refinement of previously reported powder neutron diffra ction data on ND4ReO4 taken over a wide range of temperatures [Powell, Brown, Harnden & Reid (1993). Acta Cryst. B49, 463-468] is reported. Rigid-body constraints are applied to the cation geometry and occupati on of a second cation orientation at higher temperatures is permitted. The resulting structure at higher temperatures is to be preferred to the previous structure with a single cation orientation and supports t he pseudo-spin theory of cooperative disordering without a phase trans ition.