Siting and mobility of deuterium absorbed in co-sputtered Mg0.65Ti0.35: A MAS 2H NMR study

Nanostructured magnesium titanium alloys are interesting lightweight materials for chemical hydrogen storage. We have therefore investigated the siting and dynamics of deuterium absorbed in a Mg0.65Ti0.35 alloy generated by magnetron cosputtering, and made a comparison to the corresponding features in bulk samples of deuterium-loaded Mg0.65Ti0.35 and Mg0.65Sc0.35 prepared by ball-milling and melt-casting, respectively. Magic-angle spinning 2H NMR of cosputtered Mg0.65Ti0.35D1.1 shows partly resolved signals of deuterium located in nonconductive domains at tetrahedral Mg4 and mixed MgnTi4-n sites (4 ppm) and deuterium at Ti4 sites in conducting TiD2 nanodomains (-29 and -68 ppm). No bulk TiD2 signal at -150 ppm is observed, in contrast to what we find in ball-milled Mg0.65Ti0.35D0.65, which is largely phase separated. The deuterium species with shift values of 4 and -29 ppm undergo complete exchange at a subsecond time scale in one- and two-dimensional exchange NMR and must therefore be close together in the lattice. In contrast, deuterium resonating at -68 ppm does not show deuterium exchange and thus appears to be located at more stable sites. The observed deuterium exchange and the reduced Knight shift compared to bulk TiD2 are explained using a model with TiD2 nanoslabs