The bistable (shallowdeep) defect sysiems associated with trivalent impurities (In, Ga, Y and Sc) in CdFz 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 intemomic potentials determined using the electron-gas model of Gordon and Kim are used, and the defect electron is mated by the extended-ion method. In order to compare the deep and shallow states using the same discrete lattice model, B very large cluster of atom is treated. Two groups of vivalent impurity centres are found.With In and Ga, there is a low. but dearly 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 hasis of present work. we predict that Ti3+ would exhibit similar bistable behaviour.

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