Electrons trapped at a vacancy depend on the atoms around it.
What is it about?
If an oxygen atom is removed from its position in a crystal - a vacancy is formed. Such a site can attract electrons. In cubic lutetium oxide, four Lu atoms surround the vacancy. Replacing one of them with Hf or Zr affects the properties of electron gas blob trapped at the vacancy. The size of the blob is similar to the oxygen ion size: unlike conduction band electrons, it is localized. At the same time, it is also not an atomic density, it sits in between atoms. The trapped electron density can absorb light via electronic transitions - just like atoms do.
Why is it important?
The research is another step towards deeper understanding of solid state materials and matter in general. We are used to electrons sitting at atoms or spread in multiatomic bands. Here, a specie is shown that is neither of the two. A blob of electron gas sitting at an oxygen vacancy is not at atom - however, it is a localized piece of electron density of surprisingly atom-like shape. It depends on the atoms next to it and yet remains a thing in itself.