ORIENTATIONAL DISORDER IN NI(NH3)6I2 - EVIDENCE FOR ROTATION-TRANSLATION COUPLING

Patterson and Fourier analysis of neutron diffraction data of cubic Ni (HN3)6I2 taken at T = 295 K and T = 35 K show a planar proton density distribution with four maxima at the comers of a square. We show that this observation is the consequence of a coupled rotational-translatio nal motion of the NH3 group in a two-dimensional anharmonic single par ticle potential with tetragonal symmetry. Our potential ansatz gives a better fit to the data than conventional crystal structure analysis.