ORIENTATIONAL DISORDER IN NI(ND3)6BR2 - A 2-DIMENSIONAL THERMODYNAMIC ROTATION-TRANSLATION MODEL FOR THE DEUTERIUM DISORDER

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. © 1990 Springer-Verlag.