First-Principles Study of Structural Bistability in Ga- and In-DopedCdF2

We have identified the microscopic structures for the shallow and deep donor states of Ga and In donor impurities in ${\mathrm{CdF}}_2$ through first-principles calculations. The deep state arises from a large $\left[100\right]$-axis atomic displacement of a donor. It has all the properties of a $\mathrm{DX}$ center; i.e., it is $\mathrm{negatively}$ charged and is separated from the metastable substitutional state by a large energy barrier that leads to persistent photoconductivity.