In pursuit of light intermetallic hydrides

The search for a "magic" intermetallic compound, absorbing reversibly and easily copious quantities of hydrogen, has been pursued since the discovery of the first intermetallic hydrides, forty-fifty years ago. Obviously, such intermetallic compounds should contain light metal elements like Li, Mg, Al, Ca, etc. We summarize in this work the experimental evidence that sheds light on the hydrogen absorption abstinence of Al-rich intermetallic compounds. The elastic moduli and/or bonding properties of Al-containing pseudobinary compounds initially decrease and then increase as a function of the Al composition. The shear and the bulk moduli behave differently in the Al-rich portion, where they present significantly higher and lower values, respectively, than the corresponding moduli in the Al-poor region. We claim that the extensive shear stiffening inhibits the hydrogen absorption in the Al-rich compounds. An explanation for the choice of the LaSn_0.22Ni_0.78 compound as a sorption cryocooler in the Planck mission is suggested in view of its shear softening with respect to LaNi₅. Recent attempts to find pseudobinary intermetallic hydrides, containing Li and Ca, are also presented. These attempts are conducted by considering some of the factors which regulate hydrogen absorption in intermetallic compounds.