Sites and diffusion of muons in FCC metal hydride systems

A positive muon can be considered an isotope of hydrogen due to similarities in spin and charge. For metal hydride systems, the muon enters the sample as the last hydrogen added, and competes for the same sites as the hydrogen atoms. to observe the site competition and diffusion of both particles (muon and proton), several FCC metal hydrides, TiH{dollar}\sb{lcub}1.83{rcub}{dollar}, TiH{dollar}\sb{lcub}1.97{rcub}{dollar}, TiH{dollar}\sb{lcub}1.99{rcub}{dollar}, YH{dollar}\sb{lcub}1.77{rcub}{dollar}, YH{dollar}\sb2{dollar}, ZrH{dollar}\sb{lcub}1.94{rcub}{dollar}, and LaH{dollar}\sb{lcub}2.06{rcub}{dollar}, were studied using transverse-, zero-, and low longitudinal-field {dollar}\mu{dollar}SR. The low temperature region results indicate that the muon predominately occupies octahedral sites for the FCC metal hydrides in this study. The probability for a muon to occupy a tetrahedral site in titanium and zirconium hydrides at these temperatures is proportional to the vacancy concentration. Whereas the probability for T site occupation in yttrium hydride is proportional to the number of protons not occupying these sites which increases with hydrogen concentration. Muon T site occupancy below room temperature for LaH{dollar}\sb{lcub}2.06{rcub}{dollar} was not observed and was not expected since these sites are occupied by protons. Around 300 K, the muon diffuses over interstitial O sites to vacancies in the H sublattice of TiH{dollar}\sb{lcub}1.99{rcub}.{dollar} The vibration of the hydrogen lattice is found to be the mechanism responsible for the activation of the muon out of the O site. Above room temperature, the muon occupies tetrahedral sites in yttrium and titanium hydrides. at high temperatures, the field-correlation time for a muon in titanium and yttrium hydrides is approximately one to two orders of magnitude greater than for a proton as measured by NMR. The results of a Monte Carlo simulation indicate that the presence of the muon inhibits the motion of the nearest-neighbor protons at high temperatures. The dynamics of the proton spins are observed by zero- and low longitudinal-field {dollar}\mu{dollar}SR through the oscillation of the muon polarization at long times for a static muon in a T or O site. This observation is not predicted by the Kubo-Toyabe treatment for a stationary muon