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.