Hydrogen desorption properties of FCC super-lattice hydride Mg 7NbH x prepared by ultra-high pressure techniques

Abstract

Preparation conditions of a FCC phase Mg 7NbH x have been investigated in detailed by means of an ultra-high pressure technique. The FCC phase Mg 7NbH x was obtained at 4 GPa or higher, but always coexisted with a monoclinic phase MgNb 2H y . Although the monoclinic phase MgNb 2H y releases only a small amount of hydrogen at around 623 K, the FCC phase Mg 7NbH x exhibits large amount of hydrogen desorption (ca. 4.5 mass%) around 573 K of which temperature is by 140 K lower than that of MgH 2. It was found that the hydrogen desorption temperature of the FCC hydrides tend to decrease in the following series: Mg 7TiH x > Mg 7NbH x > Mg 6VH x with increasing the vacancy ratio of the 4b site.