Structures and stabilities of the group IIIa dihydrides

The CaF2-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 HoH2 and DyH2 with metal-hydrogen distances of 2.237 A and 2.252 A respectively. A modified Born-Mayer ionic model is applied to this family of dihydrides. The results obtained are compared with the thermochemical data using the Born-Haber cycle. 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 dihyrides. This similarity is used to rationalize observed correlations between stability and tetrahedral hole sizes in intermetallic compound hydrides. (Auth.)