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

Abstract

Experimental studies on the series of ternary intermetallic compounds, Zr6MeX2, where Me=Fe, Co, and Ni, and X=Al, Ga, and Sn, show that they have large 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 P6m2 to P62c. Full profile (Rietveld) refinement using X-ray powder diffraction data indicates that the crystal structures of the new hydrides, Zr6CoAl2H10, Zr6NiAl2H9.7 and Zr6NiSn2H10.8 are analogous to Zr6FeAl2D10, which was examined by neutron powder diffraction. Thermal desorption of hydrogen in all hydrides occurs in the temperature range between 400 and 900 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≤x≤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 states are evaluated. Site energies and metal–metal bonding largely affect the maximum hydrogen content, while H–H repulsions constitute the greatest driving force for the change in space group upon hydrogenation.