Hydrogen absorption and desorption properties of AB₂ laves-phase pseudobinary compounds

The formation and properties of the hydrides of the pseudobinary intermetallic compounds Zr(Co_xM_1-x)₂ and Zr(Fe_xM_1-x)₂ (M = V, Cr, Mn; 0 < x < 1) have been studied at various temperatures and in the pressure range up to 70 atm. It is demonstrated that many of the pseudobinary compounds exhibit excellent sorption properties, i.e. they absorb approximately three hydrogen atoms per molecule and exhibit a plateau dissociation pressure around 1 atm at room temperature. The hydrogen capacity N_h decreases with increase of the 3d occupation number across the 3d series. Our results are discussed in terms of the "rule of reversed stability" of Van-Mal and the theory of Miedema. There is qualitative agreement between experiment and theory. A list of new hydrides based on AB₂ Laves-phase compounds which contain large quantities of hydrogen is given in the Appendix. In view of our work on the pseudobinary compounds of Zr, a general method of increasing the plateau pressure of intermetallic compounds is suggested.