Charge exchange processes between impurity ions and the host crystal in wide band-gap crystals

The point of this article is to investigate under what conditions the sum of a donor transition of an impurity ion, M(n+) --> M((n+1)+) + e(CB), and an acceptor transition M((n+1)+) + e(VB) --> M(n+) is equal to the band gap of the host. Our recent work on Ti3+, Ti4+ in aluminate host crystals provides the best of the few examples where both a donor and an acceptor transition are known.