Relaxation/Polarization with Semiconductor Electrons

citw

Please help me understand the following (general) statement, referring to electrons in a full valence band of an n-type semiconductor:

"An electron filling up the last empty state in the valence band will in doing so prevent the other valence electrons from reducing their energy through polarization and relaxation around the impurity center"

I don't understand what is meant by the last part of this. I know that in a full band, electrons have no empty state to move into, but I don't know what is meant by polarization and relaxation "around the impurity."

The impurity referred to is a donor (n-type SC). Another statement (which I think is basically the same) says:

"In the case of a completely filled valence band, no (impurity, I assume) scattering can take place, because all states are occupied. Now, if a hole is created at, for example, the top of the valence band, the remaining valence electrons will relax around the impurity ions... this new flexibility of the system makes the system lower in its energy."

What does this mean "relax around impurity ions"?

To sum it up, what I'm not understanding here is:

a) what is meant by "relax around the impurity" (I have no idea what they mean by "relax"), and

b) reducing energy by polarization or relaxation (What/how/why? What do they mean by polarization?)