Let's imagine we have two polycrystalline samples: $\ce{Ca3(PO4)2}$ and $\ce{Na3PO4}$. A free electron is added to both. Some heat is released.
I wanted to check whether my reasoning about this scenario is correct. Could not find any data about this, though.
I already know that, for some other materials, surfaces' electron affinities depend on the crystal facet the electron went to. If this is the case here as well, I would accept this as a useful piece of information. However, maybe this is a negligible effect here.
Here's my attempt:
- the free electron to be added to the surface cannot go to the negatively charged phosphate ion. It must be only attracted by $\ce{Na^+}$ or $\ce{Ca^2+}$ .
- the ionic radii are similar.
- $\ce{Ca^2+}$ has more charge per ion, therefore it may attract the added electron more.
Here's my issue:
- There are twice as many $\ce{Na^+}$ ions per one phosphate compared to $\ce{Ca^2+}$ ions per phosphate. That makes me suspect, the added electron may rather be attracted to multiple $\ce{Na^+}$ simultaneously and occupy a larger volume or even conduction band instead. Larger orbital may lower its kinetic energy more, assuming this does not happen in $\ce{Ca3(PO4)2}$.
Which explanation is more plausible?