Refinement of the crystal structure of the high-temperature phase G in (NH)WOF (powder, X-ray, and neutron scattering).

The (NH)WOF compound undergoes a series of phase transitions: G → 201, K → G → 160, and K → G, with a significant change in entropy (Δ S ∼ Rln10 at the G → G transition), which indicates significant orientational disordering in the G phase and the order-disorder type of the phase transition. X-ray diffraction is used to identify the crystal structure of the G phase as rhombohedral (sp. gr. Cmcm, Z = 4), determine the lattice parameters and the positions of all atoms (except hydrogen), and show that [WOF] ions can form a superposition of dynamic and static orientational disorders in the anionic sublattice. A determination of the orientational position of [NH] ions calls for the combined method of elastic and inelastic neutron scattering. Inelastic neutron scattering is used to determine the state of hindered rotation for ammonium ions in the G phase. Powder neutron diffraction shows that the orientational disorder of NH ions can adequately be described within the free-rotation approximation. [ABSTRACT FROM AUTHOR]