Figure 12 shows the density of states (DOS) of zirco­nium hydride, determined by DV-Xa molecular orbital (MO) calculations. Here, 0 eV corresponds to the Fermi energy. The new band resulting
from the hydrogen is generated immediately below the d-band of the hydride cluster, in the region of ^5-7 eV. Figure 13 shows the bond order of zirconium hydride. With increasing hydrogen con­tent, there is a marked decrease in the bond order of Zr-Zr metallic bonds, whereas the bond order of Zr-H covalent bonds does not change. This reduction in bond order is likely due to a decrease in the electric charge of the matrix of Zr bonds.

-10 -5 0

Electron energy, E (eV)

Figure 12 Hydrogen content dependence ofthe DOS of zirconium hydride.

Since bond order can be thought to be related to the spring constant of interatomic bonds, these results can be understood to mean that the effective spring constant of zirconium hydride, as a whole, decreases with increasing hydrogen content. This hypothesis offers a good explanation for the hydrogen content dependence of the various properties of zirconium hydride.