Structure and motion of hydrogen in molybdenum bronzes HxMoO3 as studied by nuclear magnetic resonance

Abstract

¹H NMR solid state techniques have been used to study bonding properties, location, and mobility of hydrogen in various phases of the hydrogen bronze H_xMoO₃. Temperature‐dependent spectra characteristic of different degrees of intercalation have been observed, and furthermore, from measurements of the relaxation rate, dynamic properties have been derived. There is strong evidence of intralayer hydrogen positions on a quasi‐one‐dimensional zig–zag line connecting the vertex‐sharing oxygen atoms of the MoO₆ octahedra and these are first occupied if the degree of intercalation x is low. For x>0.85 the hydrogen in excess starts to fill up interlayer positions coordinated with terminal oxygen atoms at the van der Waals gap. Both isolated and paired protons have been detected in the interlayers, whereas clusters or pairs appear along the zig–zag lines. Hydrogen separations within the clusters, bonding with oxygen and charge transfer to the conduction band of the host lattice, are discussed. Hydrogen diffusion changes from being predominantly one‐dimensional to three‐dimensional in character as x increases. The activation energies of the motion are of the order of magnitude of 15 to 30 kJ/mol.