Structures and stabilities of the group IIIa dihydrides

Abstract

The CaF 2-type dihydrides of the group IIIa elements represent a relatively large family of similar metallic hydrides for which relationships between interatomic distances and stabilities can be derived. Available data for the thermodynamic properties of formation of these dihydrides were assembled and the enthalpies and free energies of formation are compared with the metal-hydrogen distances in the hydrides. The data show that for these hydrides there is a maximum in the stability-distance curve; the most stable dihydrides are HoH 2 and DyH 2 with metal-hydrogen distances of 2.237Å and 2.252 Å respectively. A modified Born-Mayer ionic model is applied to this family of dihydrides. In addition to the usual Coulomb (attraction) and overlap (repulsion) terms, an additional attraction term is introduced to take into account residual metallic cohesion in the hydride lattice. The results obtained from this model are compared with the thermochemical data using the BornHaber cycle. In making this comparison a parameter β (which has units of length) is introduced which accounts for the influence of the 4f electronic structure on the repulsive potential for the lanthanide dihydrides. It is shown that the relation between stability and the sum of the metallic radius and the tetrahedral hole radius (in the metal) is similar to that between stability and the actual metal-hydrogen distance in these dihydrides. This similarity is used to rationalize observed correlations between stability and tetrahedral hole sizes in intermetallic compound hydrides.