Hydrogenation properties and crystal structure of the single BCC (Ti0.355V0.645)100-xMx alloys with M = Mn, Fe, Co, Ni (x=7, 14 and 21)

The structural and hydrogenation properties of the (Ti0.355V0.645)(100-x)M-x (x=7, 14 and 21) alloys with M=Mn, Fe, Co and Ni and their hydrides have been investigated. The limit of solubility of the M element in the solid solution at constant TiN ratio (0.55) increases from Ni to Mn, respectively, from 7 at.% to more than 14 at.% at 1273 K. For the single phase BCC alloys, the hydrogen capacities reach about 4 wt.% at room temperature under 2 MPa but the reversible capacity at 298 K is low for the (Ti0.355V0.645)(93)M-7 alloys. However, the cell volume reduction induced by the increasing substitution of the M element leads to a reversible capacity of 2 wt.% at room temperature for the (Ti0.355V0.645)(86)Fe-14 alloy. Moreover, the influence of the M element is observed on the equilibrium pressure since destabilization of the hydride is larger for Fe than Mn beyond geometrical consideration. Evolution of the structural parameters during absorption and desorption have been studied by neutron and X-ray diffraction for the Mn (x = 7) and Fe (x = 14) alloys, respectively. For the system (Ti0.355V0.645)(93)Mn-7-H-(2), only one hydride is observed with a FCC-type structure (Fm (3) over barm). The hydrogen sites have been determined by neutron diffraction for the fully charge dihydride (Ti0.355V0.645)(93)Mn7D1.88. For the system (Ti0.355V0.645)(86)M-14-H-2, the fully charged hydride exhibits a FCC-type structure (Fm (3) over barm) whereas the intermediate one shows a tetragonal symmetry (Wrrunm). The stability domain of the tetragonal phase ranges from 0.75 to 0.92 H/M. (c) 2006 Elsevier B.V All rights reserved.