Dynamics of positive muons in CaB6

We report on a muon spin relaxation and rotation (μ SR) study in cubic Ca0.995La0.005B6. The oriented sample has practically the lattice parameters of CaB6 - space group Pm3m. A μ SR investigation of CeB6, isostructural with CaB6 and with nearly the same lattice constants, has shown that the implanted μ+ are located at the crystallographic d site and are immobile up to at least 200 K. Surprisingly, this is different for the μ+ 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 μ+, e.g., by quantum diffusion, implying a hopping rate of ≈ 2 ×105  s−1. Between ∼ 7 and ∼ 70 K one observes an intermediate regime with slowly increasing mobility. True long range μ+ 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 μ SR measurements performed for sample temperatures between 1.8 and 300 K.