The diffusion of positive muons which were embedded in the H sub-lattice of metal hydrides became highly correlated with the motion of protons at temperatures which were high enough to cause rapid H diffusion. The occurrence of H-blocking and hopping diffusion required that the motion of the lighter particles should be more rapid than that of the heavier ones, but the opposite had been reported for many experiments on metal hydrides. No convincing explanation of the so-called slow-muon problem, in which the muon-diffusion rate was apparently lower than the proton-diffusion rate, had been given. The present work confirmed the existence of slow muons in TiHx, when x was about 2, but showed that, as x was reduced, the correlation times which were associated with the motion of the muons and H atoms became more comparable.

Muon-Proton Correlated Diffusion in TiHx. J.Lam, J.M.Titman: Journal of Alloys and Compounds, 1999, 293-295, 266-9