Crystal electric field next to a hydrogen-like interstitial - mu(+) in PrNi5

Muon spin rotation measurements of the temperature dependence and the anisotropy of the mu(+) Knight shift in single crystals of the crystal electric field singlet ground state system PrNi5 reveal pronounced deviations from a linear scaling of the Knight shift with the bulk magnetic susceptibility at T <= 50 K. They are explained by a mu(+) induced modification of the atomic susceptibility of neighboring Pr3+ ions. From the Knight shift anisotropy at T > 50 K it is determined that the implanted mu(+) occupy a single interstitial site, namely the 6i site (0.5, 0, 0.21 +/- 0.02). Using this site information, good model fits to the measured data are obtained assuming a mu(+) induced perturbation of the crystal electric field at the Pr3+ ions next to the mu(+). Apparently, the presence of the mu(+) leads to a lowering of the local symmetry, causing a lifting of the degeneracy and a pronounced rearrangement of the low lying crystal electric field levels for these ions.