Energy expression of the chemical bond between atoms in hydrides and oxides and its application to materials design

Using energy density analysis, total energy is partitioned into the atomic energy densities of constituent elements. The atomization energy of each element is then evaluated by subtracting the atomic energy density from the energy of the isolated neutral atom. This recent approach to the energy expression is reviewed of the chemical bond between atoms in hydrides and oxides. For various hydrides, the atomization energies, ΔE _H for H atom and ΔE _M for metal atom, are evaluated and the ΔE _H vs. ΔE _M diagram called atomization energy diagram is made. All the hydrides including complex hydrides and metal hydrides can be located on this diagram, although there are significant differences in the nature of the chemical bond among them. Also, for hydrocarbons, C_mH, the atomization energy for carbon, ΔE _C, increases linearly with the ratio of carbon number to hydrogen number, m/n, while keeping ΔE _H constant. It is no longer needed for us to give expression for the C-C bond to be either single, or double or triple bond in C_mH_n. For metal oxides, the atomization energies, ΔE _M for metal atom and ΔE _O for O atom, reflect the average structure as well as the local structure. As a result, their values change with the overall density of binary metal oxides. For perovskite-type oxides, the ΔE _O value increases by the phase transition from cubic to tetragonal phase, regardless of the tilting-type or the <100> displacement-type transition. One of the applications of this approach is the quantitative evaluation for the catalytic activities of metal oxides (e.g., Nb₂O₅) on the dehydrogenation reaction of magnesium hydride (MgH₂), MgH₂ → Mg + H₂. The atomization energy concept will provide us a new clue to materials design, for example, catalysts design.