A STUDY OF MUON LOCALIZATION AND DIFFUSION IN HF2CO AND HF2COH3

Muon spin relaxation (mu-SR) experiments were carried out in zero field on polycrystalline samples of Hf2Co and Hf2CoH3. In the unhydrided alloy the muon preferentially occupies octahedral interstices and diffuses through a tunnelling mechanism within the network made of adjacent octahedra above 220 K. In contrast, the muon occupies tetrahedral interstices in the hydrided alloy. Muon diffusion takes place through two distinct phonon-assisted tunnelling mechanisms with a possible contribution of over-barrier hopping above 250 K, and appears to be strongly enhanced by the presence of hydrogen. This effect is attributed to the different pathways taken by the muon in hydrided and unhydrided Hf2Co. A comparison with the hydrogen jump rate measured by perturbed angular correlation in the same hydride provides confirmation that the hydrogen mobility is indeed faster than that of the muon, which appears to be a quite general result in concentrated metallic hydrides.