A STUDY OF BISTABLE (SHALLOW-DEEP) DEFECT SYSTEMS IN CDF2-M(3+) (M=IN, GA)

The bistable (shallow-deep) defect systems associated with trivalent impurities (In, Ga, Y and Sc) in CdF2 are studied. The equilibrium lattice relaxation around the defect and the wavefunction of the electron bound to the impurities are determined by minimizing the energy of the defects. The impurity-fluoride interatomic potentials determined using the electron-gas model of Gordon and Kim are used, and the defect electron is treated by the extended-ion method. In order to compare the deep and shallow states using the same discrete lattice model, a very large cluster of atoms is treated. Two groups of trivalent impurity centres are found. With In and Ga, there is a low, but clearly identified, potential barrier which separates the deep level from the shallow one. In Sc and Y, only a simple shallow level state is obtained. The analysis of the result shows that the difference is to be attributed to the short-range potential of the trivalent impurity centres. On the basis of present work, we predict that Tl3+ would exhibit similar bistable behaviour.