First-principles study of the stability and electronic structure of metal hydrides

A detailed analysis of the formation energies for alkali, earth-alkali, and transition-metal hydrides is presented. The hydriding energies are computed for various crystal structures using density functional theory. The early transition metals are found to have a strong tendency for hydride formation which decreases as one goes to the right in the transition-metal series. A detailed analysis of the changes in band structure and electron density upon hydride formation has allowed us to understand the hydriding energy on the basis of three contributions. The first is the energy to convert the crystal structure of the metal to the structure formed by the metal ions in the hydride (fcc in most cases). In particular, for metals with a strong bcc preference such as V and Cr, this significantly lowers the driving force for hydride formation. A second contribution, which for some materials is dominant, is the loss of cohesive energy when the metal structure is expanded to form the hydride. This expansion lowers the cohesive energy of the metal and is a significant impediment to form stable hydrides for the middle to late transition metals, as they have high cohesive energies. The final contribution to the hydride formation energy is the chemical bonding between the hydrogen and metal in which it is inserted. This is the only contribution that is negative and hence favorable to hydride formation.