Hydrogen desorption properties of FCC super-lattice hydride Mg7NbHx prepared by ultra-high pressure techniques

Preparation conditions of a FCC phase Mg7NbHx have been investigated in detailed by means of an ultra-high pressure technique. The FCC phase Mg7NbHx was obtained at 4 GPa or higher, but always coexisted with a monoclinic phase MgNb2Hy. Although the monoclinic phase MgNb2Hy releases only a small amount of hydrogen at around 623 K, the FCC phase Mg7NbHx exhibits large amount of hydrogen desorption (ca. 4.5 mass%) around 573 K of which temperature is by 140 K lower than that of MgH2. It was found that the hydrogen desorption temperature of the FCC hydrides tend to decrease in the following series: Mg7TiHx > Mg7NbHx > Mg6VHx with increasing the vacancy ratio of the 4b site.