The microscopic structures of the shallow and deep donor states of Ga and In donor impurities were identified by means of first-principles calculations. The deep state arose from a large [100]-axis atomic displacement of a donor. It possessed all of the properties of a DX center: it was negatively charged, and was separated from the metastable substitutional state by a large energy barrier that led to persistent photoconductivity.

First-Principles Study of Structural Bistability in Ga- and In-Doped CdF2. C.H.Park, D.J.Chadi: Physical Review Letters, 1999, 82[1], 113–6