Dynamics of positive muons in CaB6

We report on a muon spin relaxation and rotation (mSR) study in cubic Ca0.995La0.005B6. The oriented sample has practically the lattice parameters of CaB6 - space group Pm3m. A muSR investigation of CeB6, isostructural with CaB6 and with nearly the same lattice constants, has shown that the implanted m+ are located at the crystallographic d site and are immobile up to at least 200 K. Surprisingly, this is different for the m+ in CaB6: they are at the interstitial 8 g site, and already for the lowest temperature range of the study, between 1.8 and -7 K, an apparently constant muon-spin relaxation of dynamical origin is present. This relaxation can only be explained by a mobile mu+, e.g., by quantum diffusion, implying a hopping rate of approx 2 x 10(5)s-1. Between -7 and -70 K one observes an intermediate regime with slowly increasing mobility. True long range m+ diffusion appears to set in above -70 K. This picture describes in a consistent way a comprehensive set of zero-field, longitudinal-field and transverse-field mSR measurements performed for sample temperatures between 1.8 and 300 K.