Proton mobility in molybdenum bronzes

Abstract

Molybdenum bronzes were prepared by exposing microcrystalline MoO 3, covered with small particles of Pt, to gaseous hydrogen. A pulsed proton magnetic resonance study was made of H1.6MoO3 bronzes between 160 and 320 K at 90, 40 and 15 Mhz. The bronzes behave as metallic conductors and, from the point of view of relaxation, are comparable to metallic hydrides. The longitudinal relaxation rate (T1)-1 has three main contributions : one from the interaction between protons and conduction electrons (T 1)-1e, and the others from the proton-proton dipolar (T1)-1d and proton-paramagnetic centre interactions (T1)-1ed. T1e and T1d are modulated by the proton mobility in the oxide lattice, and T1 has been treated using the theory of Torrey, modified by Krüger. The ratio of the mean square displacement to the square of the distance of closest approach is much greater than 1, which implies diffusion. The activation energy derived from the variation of the correlation time with temperature is about 0.3 eV. The NMR line is asymmetric and is displaced from that of water absorbed on the oxide. The displacement is very small due to the small electron density seen by the nucleus as a result of diffusion