Muon spin dynamics and sites in GdNi5

In GdNi5, the muon localizes at the 3f interstitial site and below ∼80K a second muon site becomes populated, the so-called 6ring site. This ring site is metastable and the muon hops to the 3f site. We determine the mean time of stay of the muon in the ring site and show that it is governed by a multi-phonon quantum diffusion process. The coincidence energy is measured to be E a =272(10)K and the tunneling matrix element J=0.11(2)meV.