The positive muon as a tracer for the study of dynamic correlation effects in metal hydrogen systems

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Abstract
In order to investigate whether the positive muon (μ+) can be used as a tracer in hydrogen diffusion studies, we have made what we believe to be the first systematic study of muon–hydrogen correlations in a metal hydride (NbHx) over a large range of hydrogen concentrations. The following observations were made from transverse field μSR experiments: (i) Loading with hydrogen has a strong impact on muon mobility in Nb, and shifts the motional narrowing region from 60 to above 170 K; (ii) in β‐NbHx, the inverse muon correlation times τ−1μ are of the same order as the hydrogen jump rates τ−1H. However, the activation energies are distinctly different (EHa≂230 meV, Eμa≂140–150 meV). Moreover, the concentration dependence of τ−1μ exhibits strong negative deviations from a (1−c) behavior; (iii) the H/D isotope effects as measured by μ+ diffusion in β‐NbH(D)x is significantly nonclassical. While the activation energy for τ−1μ is slightly larger in the deuteride, the ratio of jump rates is significantly larger than √2; (iv) at low temperature the μ+ mobility exhibits an irregular concentration dependence which can be associated with phase transitions to subhydride phases. They are characterized by an ordering of the vacancies on the H sublattice which apparently suppresses long‐range muon motion; (v) single crystal experiments support tetrahedral site occupancy as expected for a muon on a substitutional site of the H sublattice. The data are interpreted with the aid of Monte Carlo calculations, where the diffusion of tracer particles in a concentrated lattice gas was simulated. There is clear evidence that repulsive muon–hydrogen interactions are an important feature of the dynamic muon–hydrogen correlations.