First-principles study on 3d transition-metal dihydrides -: art. no. 155114

The ultrasoft pseudopotential calculations are performed for the CaF2-type dihydrides TiH2, VH2, and CrH2. The calculated lattice constants agree well with the experimental data except for CrH2. We also perform calculations for a solution of hydrogen in Fe and Ni and obtain the site preferences correctly. The results for the heats of formation and heats of solution show that the calculations give the correct trends for exothermic through endothermic reactions but overbind hydrogen with transition metals, typically about 20 kJ/mol H-2. This overbinding is improved by including the zero-point energy correction. The energetics of binary dihydrides including hypothetical FeH2 and NiH2 is discussed in term of three contributions: namely, the structural transformation energy, the lattice expansion energy, and the hydrogen insertion energy. The former two contributions are understood from the nature of the host metals. The hydrogen insertion energy can be represented by a simple geometric model. The extension of this model is also suitable for alloy dihydrides.