J. Phys. I France
Volume 3, Numéro 4, April 1993
Page(s) 987 - 1006
DOI: 10.1051/jp1:1993179
J. Phys. I France 3 (1993) 987-1006

Orientational disorder in $\mathsf{Ni(NH_3)_6I_2}$. Evidence for rotation-translation coupling

P. Schiebel1, A. Hoser1, W. Prandl1, G. Heger2 and P. Schweiss2

1  Institut für Kristallographie, Universität Tübingen, D-7400 Tübingen, Germany
2  Laboratoire Léon Brillouin, CEA-CNRS Saclay, F-91191 Gif-sur-Yvette Cedex, France

(Received 30 September 1992, accepted in final form 16 November 1992)

Patterson and Fourier analysis of neutron diffraction data of cubic Ni(HN 3) 6I 2 taken at T = 295 K and T = 35 K show a planar proton density distribution with four maxima at the corners of a square. We show that this observation is the consequence of a coupled rotational-translational motion of the NH 3 group in a two-dimensional anharmonic single particle potential with tetragonal symmetry. Our potential ansatz gives a better fit to the data than conventional crystal structure analysis.


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