Hydrogenation of Zr6MeX2 intermetallic compounds (Me=Fe, Co, Ni; X=Al, Ga,Sn): Crystallographic and theoretical analysis

Experimental studies on the series of ternary intermetallic compounds, Zr6M eX2, where Me=Fe, Co, and Ni, and X=Al, Ga, and Sn, show that they have lar ge hydrogen storage capacities ranging from 9.3 to 10.8 H-atoms per formula unit. The crystal structure of the parent intermetallic compounds changes upon hydrogenation with a two-fold increase of the crystallographic c-axis length and a corresponding change in space group from P (6) over bar m2 to P (6) over bar 2c. pull profile (Rietveld) refinement using X-ray powder di ffraction data indicates that the crystal structures of the new hydrides, Z r6CoAl2H10, Zr6NiAl2H9.7 and Zr(6)NiSn(2)H(10.)8 are analogous to Zr6FeAl2D 10, which was examined by neutron powder diffraction. Thermal desorption of hydrogen in all hydrides occurs in the temperature range between 400 and 9 00 K and is characterized by two similar events. The compounds Zr6FeAl2Hx ( x=0, 10 and 0.1-0.2) order magnetically at 45, 155 and 10 K, respectively. Electronic structure calculations are carried out on Zr6FeAl2Hx (0 less tha n or equal to x less than or equal to 10) to interpret their structural and thermal behavior. Site energies for hydrogen incorporation, energetics of hydrogen desorption, as well as quantitative details of the densities of st ates are evaluated. Site energies and metal-metal bonding largely affect th e maximum hydrogen content, while H-H repulsions constitute the greatest dr iving force for the change in space group upon hydrogenation. (C) 1999 Else vier Science S.A. All rights reserved.