Refinement of the crystal structure of the high-temperature phase G0 in (NH4)2WO2F4 (powder, x-ray, and neutron scattering)

Abstract

The (NH4)2WO2F4 compound undergoes a series of phase transitions: G0 -> 201 K -> G1 -> 160 K -> G2, with a significant change in entropy (ΔS1 ~ Rln10 at the G0 -> G1 transition), which indicates significant orientational disordering in the G0 phase and the order–disorder type of the phase transition. X-ray diffraction is used to identify the crystal structure of the G0 phase as rhombohedral (sp. gr. Cmcm, Z = 4), determine the lattice parameters and the positions of all atoms (except hydrogen), and show that [WO2F4]2– ions can form a superposition of dynamic and static orientational disorders in the anionic sublattice. A determination of the orientational position of [NH4]+ 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 G0 phase. Powder neutron diffraction shows that the orientational disorder of NH4 ions can adequately be described within the free rotation approximation.