First-principles study of structural bistability in Ga- and In-doped CdF2

We have identified the microscopic structures for the shallow and deep donor states of Ga and In donor impurities in CdF2 through first-principles calculations. The deep state arises from a large [100]-axis atomic displacement of a donor. It has all the properties of a DX center; i.e., it is negatively charged and is separated from the metastable substitutional state by a large energy barrier that leads to persistent photoconductivity. [S0031-9007(98)08097-1].