P. Schiebel et al., ORIENTATIONAL DISORDER IN NI(NH3)6I2 - EVIDENCE FOR ROTATION-TRANSLATION COUPLING, Journal de physique. I, 3(4), 1993, pp. 987-1006
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.