Acta Crystallographica Section B

Structural Science

Volume 53, Part 1 (February 1997)

research papers

Acta Cryst. (1997). B53, 76-81 [ doi:10.1107/S0108768196011160 ]

Refinement of Ammonium Perrhenate Structure Using a Pseudo-Spin Model for the Ammonium Ion Orientation

I. P. Swainson and R. J. C. Brown

Abstract: A new Rietveld refinement of previously reported powder neutron diffraction 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 occupation 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 the pseudo-spin theory of cooperative disordering without a phase transition.