Neutron scattering investigation of hydrogenic species in the ammonium molybdenum bronze (NH4)0.24H0.03MoO3

Abstract

Incoherent inelastic neutron scattering (IINS) and quasielastic neutron scattering (QENS) have been used in investigation of the ammonium bronze (NH₄)_0.24H_0.03MoO₃. The IINS vibrational spectrum reveals the presence of NH₄⁺(in the interlayer region of the α-MoO₃ host) and hydroxy groups (in intralayer bridging positions as in the parent hydrogen bronze, H_0.27MoO₃). Variable-temperature QENS measurements on instruments of differing elastic energy resolutions and differing ranges of elastic scattering vector magnitude have been used in investigation of motions of the NH₄⁺ ion. Broadenings are observed in QENS spectra over a remarkably large temperature range. In analysis of all spectra it was necessary to invoke a variable ‘static fraction’ of hydrogen, this arising from the temperature-dependent defect structures associated with incomplete filling of NH₄⁺ sites and consequent variations in local environment of NH₄⁺ ions. Variations in the scattering law [S(Q,ω)] at low temperature were consistent with reorientation of order >3 of a fraction of the NH₄⁺ present and were fitted assuming a four-fold barrier. At higher temperatures, spectra were treated using the isotropic rotational diffusion model for those reorienting ions giving discernible quasielastic broadenings, this diffusion being consequent on interaction of rotations of low-energy barrier with lattice modes.