I agree with Nicolau and jonsca, they are perfectly within our scope. Be the first to open up that can.

I unfortunately have no idea about this subject, there probably are a few questions about doping on the site which could fit the general description you gave. We currently have materials, which could be an intermediate tag, if you cannot create a new tag yet. (Either you wait until you can create it, but you can also ask some higher rep user in chat to create it for you.)

I would advise against donor/ acceptor, since they also have different meanings and could lead to some confusion, that might always have to be cleaned up. I see the same problem with impurities as this could lead to being used in conjunction with purification.
One of the most commonly wrongly applied tags is iupac instead of nomenclature for example. See the history of this question for a practical example. It would be better to avoid such confusions.

The choice defect-chemistry is probably the broadest it can get and might need a good tag wiki to define the scope. Instead on interstitials I would recommend interstitial-compounds, parallel to covalent-compounds, organometallic-compounds, aromatic-compounds, etc.. And I think that would be a very good choice as a tag.
Also a good choice would probably dopants and I guess, you could already find a few questions to give this tag some scope.

I agree with Nicolau and jonsca, they are perfectly within our scope. Be the first to open up that can.

I unfortunately have no idea about this subject, there probably are a few questions about doping on the site which could fit the general description you gave. We currently have materials, which could be an intermediate tag, if you cannot create a new tag yet. (Either you wait until you can create it, but you can also ask some higher rep user in chat to create it for you.)

I would advise against donor/ acceptor, since they also have different meanings and could lead to some confusion, that might always have to be cleaned up. I see the same problem with impurities as this could lead to being used in conjunction with purification.
One of the most commonly wrongly applied tags is iupac instead of nomenclature for example. See the history of this question for a practical example. It would be better to avoid such confusions.

The choice defect-chemistry is probably the broadest it can get and might need a good tag wiki to define the scope. Instead on interstitials I would recommend interstitial-compounds, parallel to covalent-compounds, organometallic-compounds, aromatic-compounds, etc.. And I think that would be a very good choice as a tag.
Also a good choice would probably dopants and I guess, you could already find a few questions to give this tag some scope.