Figure B-1. In this energy-level diagram of impuritiesin corundum, trivalent donor ions are shown on theleft, with divalent acceptor ions on the right. Acharge-compensating donor-acceptor pair can formwhen the donor level is above the acceptor level.There is no significance to the lateral positions of theenergy levels; the vertical positions are absolute,measured from the top of the valence band. Datafrom Kröger (1984) and references therein.Figure B-2. In A, the Ti3+ donor exchanges an electronwith the Fe2+-hole acceptor, thus producing the Fe2+-Ti4+ pair responsible for blue in sapphire. In B, whenberyllium is diffused into A, the Be2+-hole acceptorpresents a more attractive site (lower energy) for theTi3+ donor electron than Fe2+, causing the electron tobe transferred to the Be2+-hole acceptor (or the Beinduced hole is transferred to the Fe2+). The transferresults in Fe2+ becoming Fe3+, eliminating the Fe2+-Ti4+pair and the blue color. The Be2+-Ti4+ pair formed doesnot absorb light in the visible region of the spectrumand thus does not contribute color to the sapphire.