Photoconductivity Due to Autoionization of Divalent Rare Earth Impurities in Crystals Having the Fluorite Structure

Abstract

Photoionization of impurity ions in solids can lead to photo-conductivity or oxidation-reduction within the crystal. In the present case of divalent rare earth ions in alkaline earth fluorides, the photoconductivity occurs by exciting in the 4f→5d adsorption bands of the impurities. A good correlation exists between photoconduction signals and absorption spectra, indicating that some of the transitions lead to the release of electrons from the centres. Therefore, we are dealing with an autoionization process where impurity-induced discrete electronic states are in resonance with the conduction energy band, as confirmed by low temperature photoconductivity measurements. The emphasis is on a systematic study of photoionization thresholds in CaF2, SrF2 and BaF2 host crystals for the divalent rare earth ions. The threshold energy is interpreted as the energy mismatch between the ground state of the impurity ion and the bottom of the conduction band. A simple electrostatic model including the third ionization potential of the free rare earth ion and the influence on its surrounding in the crystal permits an explaination of the threshold shift upon changing the nature of the crystal and the nature of the impurity.