Neutron scattering study of protonic species in ammonium pentamolybdate, (NH4)Mo5.33H3O18

Abstract

Incoherent inelastic neutron scattering (IINS) and quasielastic neutron scattering (QNS) have been used in an investigation of ammonium pentamolybdate, (NH₄) Mo_5.33H₃O₁₈. The IINS vibrational spectrum reveals the presence of NH₄⁺(in the tunnels of the ‘hexagonal MoO₃’ host framework), water molecules and Mo-coordinated hydroxy groups. Variable-temperature QNS measurements have enabled investigation of motions of the NH₄⁺ ion. Broadenings are observed in the QNS spectra over a remarkably large temperature range, consistent with a low activation barrier to reorientation. In analysis of the temperature dependence of the elastic incoherent structure factor, it was necessary to invoke a variable ‘static fraction’ of ammonium ions, arising from dynamical inequivalences of ammonium ions in different tunnel cavities. QNS data were treated using the isotropic rotational diffusion model for those reorienting ions giving discernible quasielastic broadenings, this diffusion being consequent on the interaction of the reorientations of the low-energy barrier with the lattice modes.