Extended Solubility Limits and Nanograin Refinement in Ti/Zr Fluoride-Catalyzed MgH2

Catalyzing magnesium hydride by 5 mol % titanium fluoride is observed to have a pronounced impact on the materials nanoscaling and the thermodynamics of H sorption. Surprisingly small similar to 30 nm MgH2 crystallites result after hydrogen uptake, which are apparently stabilized by abundant interface interactions, arresting Ostwald ripening. Rapid hydrogen uptake is observed from temperatures as low as -10 degrees C, with for the first time large quantities of H inserted into the hexagonal Mg metal phase before formation of the tetragonal hydride phase. Temperature dependent equilibrium pressure measurements at different overall H compositions reveal strongly reduced enthalpy and entropy changes upon hydride formation for the most reactive part of the sample, with a close to perfect linear correlation between them. It is argued that the origin of this effect is in the abundant hydrogen dissolved in the alpha-phase and abundant vacancies present in the beta-phase due to the interaction between alpha- and beta-phase domains in the nanograins, i.e., strongly altered solubility limits. The titanium/zirconium fluoride additives are converted to respectively titanium/zirconium dihydride and MgF2, which apparently stabilize by their closely matching lattices respectively the Mg metal and hydride phase at their interfaces. Being nanodispersed, this leads in addition to nanoscale grain refinement. The formation of a new stable [Mg, T](2)TH6 (T = Ti or Zr) phase with a large cubic unit cell is observed after prolonged cycling and annealing times.