High-pressure synthesis of novel hydrides Mg7-xAxTiH16-x (A = Li, Na, K; x = 0-1.0) and their reversible hydrogen storage properties

Novel quaternary hydrides Mg7-xAxTiH16-x (A = Li, Na, K) have been synthesized by use of an ultrahigh-pressure (UHP) technique. The Rietveld refinements for synchrotron X-ray diffraction (XRD) patterns suggest that added LiH and NaH greatly enhanced the yield of the Mg7TiH16 phase. The unit cell volumes of the Mg7-xAxTiH16-x phases were increased with increasing the amount and the ionic radii of the alkali metals. The reversibility of the hydrogen releasing and restoring reactions was much improved by the addition of alkali metal hydrides. The reversible hydrogen storage capacities were around 3-4 wt.% for the Mg7-xAxTiH16-x phases on the pressure-composition isotherm (PCI) measurements. Obtained enthalpy values tend to be reduced with reducing the ΔH of the alkali metals.