CE(N-LEVELS IN ALKALINE-EARTH FLUORIDES AND CERIUM ELECTRON, CERIUM HOLE INTERACTIONS() ENERGY)

The energy levels of three cerium centres in CaF2, SrF2 and BaF2 were calculated in the framework of the relativistic Dirac-Hartree-Fock for malism. In this study cerium was taken to be in various ionic states a nd lattice relaxation was included. The centres were the following: th e charge-uncompensated cerium centre (Ce) and two charge-compensated c erium centres. The charge-compensating fluorine ion was situated at th e (111) next-nearest-neighbour interstitial site (CeF(nnn)) or at the (100) nearest-neighbour interstitial site (CeF(nn)). We did not work w ith crystal field parameters, our method being ab initio. The results are compared with a variety of experimental data. Relaxation of the la ttice around the cerium centres is crucial for the results. From the l ocation of the energy levels, we conclude that only trivalent and tetr avalent cerium centres are stable in the alkaline-earth fluorides. If free electrons are present, only the trivalent cerium centre is stable . The divalent centre without charge compensation is metastable. This is in agreement with experiment. Further, it is found that excitation of trivalent cerium centres by capture of an electron and a hole at th e cerium ion is improbable. This agrees with earlier observations of t he scintillation decay of Ce3+-doped BaF2.