Light hydrogen isotope mu(+) in Sc and alpha-ScHx solid solutions

The response of Sc and alpha-ScHx, systems (0 less than or equal to x less than or equal to 0.25) to the implantation of positive muons, i.e., light H isotopes, has been studied by mu(+)SR spectroscopy in high and low transve rse magnetic field, and in zero field (ZF). The investigation covered a tem perature range from 300 K down to 2 K, for various orientations of the sing le-crystal samples with respect to the field or initial mu(+) polarization. Below 30 K the positive muons occupy, in pure Sc, a localized "fast-tunnel ing" state, extending over two adjacent, interstitial-tetrahedral (T) sites above and below the ab-atomic plane of the hcp lattice. In alpha-ScHx, a f raction of the muons stays in this tunneling state, while for the other par t, the muons pair with a H atom, to form the analog of the T-pair state H-S c-H aligned along the c axis, specific to the alpha* phase. The electric-fi eld gradients, of muonic as well as intrinsic-crystalline origin, acting on the nuclei near the mu(+) influence the results at low and zero magnetic f ields. Their characteristics and the H-concentration dependences are evalua ted. For the ZF-data analysis a newly developed uniaxial version of the Kub o-Toyabe formula was applied. Above 30 K the long-range diffusion of positi ve muons becomes noticeable; a two-state model explains very well the data. It assumes the mu(+) to be, alternatively, in a state of free diffusion be tween localized-tunneling states or in a mu(+)-Sc-H trap. The model describ es the repeated mu(+)-capture and release processes with three characterist ic correlation times, which follow Arrhenius laws. The parameters, e.g., ac tivation energies and preexponential factors, are compared to the parameter s of the H and D dynamics in alpha-ScH(D)(x). From mu SR measurements in a deuterated sample (alpha-ScD0.10) one infers very similar behaviors of the mu(+) in D and H solid solutions in the Sc host.