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The ND3 group in Ni(ND3)6Br2 is orientationally disordered at room temperature: the D3 triangle gives rise to a density distribution with maxima on the corners of a square. We explain this observation as the consequence of a coupled rotational-translational motion of the molecule in a two-dimensional anharmonic single particle potential with tetragonal symmetry. Potential parameters are calculated from experimental data by a least squares procedure. From the total crystal potential of the D3 group we conclude that a combination of center of mass motion and rotation which is possible essentially without an energy barrier, is the basic process observed in quasielastic neutron scattering observations of the jump diffusion.

The ND3 group is in the high temperature phase of the title compound dynamically disordered. Fourier maps derived from the analysis of neutron single crystal Bragg data with phases based on a split atom and a Frenkel model show uniquely that each D3 group has four density maxima occupying the corners of a square. Neither one nor three dimensional models of purely rotational disorder can describe the observations adequately.

Single crystal- and powder X-ray diffraction, powder neutron diffraction at room temperature and 3 K, high resolution electron microscopy and electron diffraction were used to study the crystal- and defect-structure of YBa2(Cu0.93Fe0.05)3O7. Crystals of this compound are superconducting (T c ≈80 K) and appeart to be tetragonal down to at least 3 K. The structure resembles that of the undoped YBa2Cu3O7 phase with the oxygen content being very close to 7.0 and the Fe atoms preferentially occupying the Cu(1) site. A copper deficiency is shown to be present on this site too, leading to the approximate formula YBa2(Cu0.97Fe0.03)2(Cu0.86Fe0.1□0.04)O7. High resolution electron microscopy reveals the existence of orthorhombic micro-domains, 20–30 A in diameter, probably due to short range ordering of oxygen atoms on the O(1) site. In view of these results the structure has to be regarded as being tetragonal only in a statistical sense.