Refinement of the crystal structure of the high-temperature phase G{sub 0} in (NH{sub 4}){sub 2}WO{sub 2}F{sub 4} (powder, X-ray, and neutron scattering)
- Joint Institute for Nuclear Research, Frank Neutron Physics Laboratory (Russian Federation)
- Oak Ridge National Laboratory, Neutron Scattering Sciences Division (United States)
- Russian Academy of Sciences, Institute of Metal Physics, Ural Branch (Russian Federation)
- Russian Academy of Sciences, Institute of Chemistry, Far Eastern Branch (Russian Federation)
- Russian Academy of Sciences, Kirensky Institute of Physics, Siberian Branch (Russian Federation)
The (NH{sub 4}){sub 2}WO{sub 2}F{sub 4} compound undergoes a series of phase transitions: G{sub 0} {yields} 201, K {yields} G{sub 1} {yields} 160, and K {yields} G{sub 2}, with a significant change in entropy ({Delta}S{sub 1} {approx} Rln10 at the G{sub 0} {yields} G{sub 1} transition), which indicates significant orientational disordering in the G{sub 0} phase and the order-disorder type of the phase transition. X-ray diffraction is used to identify the crystal structure of the G{sub 0} phase as rhombohedral (sp. gr. Cmcm, Z = 4), determine the lattice parameters and the positions of all atoms (except hydrogen), and show that [WO{sub 2}F{sub 4}]{sup 2-} ions can form a superposition of dynamic and static orientational disorders in the anionic sublattice. A determination of the orientational position of [NH{sub 4}]{sup +} 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{sub 0} phase. Powder neutron diffraction shows that the orientational disorder of NH{sub 4} ions can adequately be described within the free-rotation approximation.
- OSTI ID:
- 22156462
- Journal Information:
- Crystallography Reports, Vol. 58, Issue 1; Other Information: Copyright (c) 2013 Pleiades Publishing, Ltd.; Country of input: International Atomic Energy Agency (IAEA); ISSN 1063-7745
- Country of Publication:
- United States
- Language:
- English
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