NH3 reorientation in phases I, II, and III of Ni(NH3)6(NO3)2

The inelastic neutron scattering (INS) spectra of rotational tunnelling and librations of NH3 ligands in phase III of Ni(NH3)6(NO3)2 are not consistent with a rotational hindrance potential containing only cos [Formula: see text] terms owing to strong interaction between neighboring cations. This type of interaction, and motion of the whole cation, also influences the classical reorientational motions involving displacement of the H atoms, where the overall radius of rotation is consistently greater than the 0.9 Å expected for isolated NH3 rotors. Quasielastic neutron scattering (QNS) suggests that in phase III there are two sublattices of cations, one of which becomes completely disordered (with respect to the NH3 rotors) at the III–II phase transition. Disorder of the second sublattice marks the II–I transformation where only a single type of rotational motion is found.