Muon-proton correlated diffusion in TiHx

The diffusion of positive muons embedded in the hydrogen sub-lattice of metal hydrides becomes highly correlated with the motion of the protons at temperatures high enough to cause rapid diffusion of the hydrogen. Hydrogen blocking and hopping diffusion dictate that the motion of the lighter particle should be more rapid than that of the heavier but the contrary has been reported in many experiments on metal hydrides. A convincing explanation of this 'slow muon problem', in which the muon diffusion rate is apparently less than the proton diffusion rate, has not been given so far. The present experiment confirms the existence of slow muons in TiHx for x similar to 2 but show that as x is reduced the correlation times associated with the motion of the muons and hydrogen atoms became more comparable. (C) 1999 Elsevier Science S.A. All rights reserved.