Ammonium-ion motions in the hexagonal tungsten trioxide framework. A neutron scattering study of the bronze (NH4)0.22WO3 and of [(NH4)2O]0.085WO3

Abstract

Variable-temperature quasielastic neutron scattering (QENS) measurements on instruments of differing elastic energy resolutions and differing ranges of elastic scattering vector magnitude have been used in investigation of the motions of the NH₄⁺ ion in the h-WO₃ framework [in the ammonium tungsten bronze (NH₄)_0.22WO₃ and the hexagonal polytungstate [(NH₄)₂O]_0.085WO₃]. Broadenings are observed in QENS spectra over a remarkably large temperature range. Constant-energy window experiments reveal very-low-energy barriers for the ions/motion responsible (E_a≈ 2.1 and 1.7 kJ mol^–1 for the bronze and the polytungstate, respectively). In analysis of all spectra it was necessary to invoke a ‘static fraction’ of NH₄⁺ ions, this arising from the temperature-dependent defect structures associated with incomplete filling of NH₄⁺ sites (in the hexagonal tunnels of the h-WO₃ host) and consequent variations in local environment of NH₄⁺ ions. Variations in the scattering law [S(Q, ω)] at low temperature were consistent with a classical description of jump reorientation of NH₄⁺(interchanging three or four of the H-atom positions) of a fraction of the ions present. At higher temperatures non-classical behaviour was detected (half-widths for the quasielastic components in scattering spectra decreased at the highest temperatures), this arising from interactions of rotations with lattice modes. Spectra were then treated using the isotropic rotational diffusion model for reorienting ions giving discernible quasielastic broadenings.