Appendix VII

The Ratio of A to B in Metal Hydrides.

The final factor to mention concerning bond strength is the overall ratio of A to B metals. Generally an IMC has a particular chemical formula (LaNi₅) that corresponds to a certain lattice structure. However, IMCs can be produced to be “rich” in one of the components. H₂ capacity is roughly based on the A/B (atomic weights of A and B) composition ratio. As the value of A/B increases, the material’s capacity increases. This is why there is interest in increasing the amount of A component. Unfortunately, depending on the material, as A/B becomes too large, a disproportionate reaction will come to dominate and the hydride can become to stable. As one may guess, the hydride will now require higher temperatures to dissociate the absorbed hydrogen. As we know, an example of the desired reaction:

 

LaNi₅ + 3H₂ → LaNi₅H₆

 

Now, the undesirable disproportionate reaction (occurring in an environment rich in A : La):

 

LaNi₅ + H₂ → LaH₂ + 5Ni

 

The La rich environmental pushes towards a hydrogen storage medium with undesirable characteristics as well as a hydriding reaction that acts to further segregate the hydride (locally separate the La from the Ni). The following figure shows a shaded grey region that represents a favourable relationship between the bond order ratio of the A and B components and the composition ratio.

 

 

 

Fig. ff   – Illustration of the A/B component factors significant to hydrides operating in the geothermal temperature range.  The grey region refers to hydrides with ideal bond strengths.  Bond order is a measure of the number of bonding electron pairs between atoms (Yukawa et al 2000)

 

For a given hydride the bond order ratio is constant, since it depends upon the components used, not the total composition. From fig ff one can see that attractive hydrides can easy be pushed into stable or unstable territory by simply changing the A/B composition ratio. We will devote more attention to the dynamics of hydrides in Appendix VI.