Stabilities, Stoichiometries and Site Occupancies in Hydrides of Intermetallic Compounds

Abstract

In the literature, one can find numerous attempts to explain the observed stabilities, stoichiometries and site occupancies in hydrides of the various families of intermetallic compounds. Some of the approaches to these problems are critically reviewed here. For some, but not all such hydrides, the stabilities have been shown by different researchers to correlate with the enthalpy for formation of the intermetallic compound, itself, or with cell size, or electronic properties, or elastic properties. It appears, therefore, that all of these effects may play a role, but none is dominant in all cases. The development of the procedure for qualitative and quantitative determinations of H-site occupancy from calculations of enthalpies for the formation of imaginary binary hydrides was reviewed. Such inspection raises the question of possible fortuitous agreement between experimental observations and predictions arising from the technique. The concepts of minimum hole size for H occupation and minimum H-H distance in stable hydrides of metals or intermetallic compounds have been discussed in terms of their importance to preferred H sites and to stoichiometry, and considerations necessary to a geometric model have been outlined. The model is used to rationalize observed H sites and stoichiometry of LaNi₅H_x. The review points up the need for theoretical treatment leading to fundamental understanding of such systems.