Muon sites and diffusion in lithium oxide

Muon spin relaxation (μSR) measurements have been performed on the alkali metal oxides Li2O and Na2O in order to model the behaviour of hydrogen impurities in these materials. Lithium oxide has been suggested as a suitable blanket material for fusion reactions, hence the need to understand the location and dynamics of T+ ions in the crystalline lattice. This oxide becomes a fast ion conductor at high temperatures (T ∼1000 K) and our experiments also provide information on the Li+ ion dynamics. Muon implantation into these oxides at low temperatures (T∼10 K) creates mainly neutral muonium (the analogue of the hydrogen atom) with a smaller fraction (∼20%) of positive muons μ+ at interstitial sites closely associated with O2- ions. At higher temperatures (T>100 K) the proportion of μ+ formed increases, and there is an increased tendency for trapping at substitutional lattice sites. A sharp decrease in relaxation rate was observed in Li2O at temperatures above 250 K and from this temperature dependence we calculate the activation energy for muon hopping relative to the Li+ ions, which are also mobile, to be approximately 90 meV. (author)