Crystal field and molecular orbital calculation of the optical transitions in Ce doped alkaline earth sulfide (MgS, CaS, SrS, and BaS) phosphors

The effect that different alkaline earth sulfide host materials have on the Ce+3 5d-->4f optical transitions is investigated using self-consistent field-configuration interaction calculations. The results are in excellent agreement with the experimentally determined transition energies for MgS : Ce, CaS : Ce, SrS : Ce, and BaS : Ce. The red shift of the spectroscopy of BaS : Ce relative to that of SrS : Ce was correctly predicted. In addition, calculated oscillator strengths of the electronic transitions in SrS : Ce and BaS : Ce were fit with Gaussian shapes to model the vibronic structure. Both absorption and emission spectra were generated, and were found to have good agreement with experimental spectra. Point charge crystal-held calculations were employed in an attempt to account for the experimentally observed trend, but failed to do so. A population analysis of the configuration interaction states indicates that the trend in emission energies is caused by differing extents of covalency experienced by the Ce 5d orbitals in the various host materials. (C) 1998 Elsevier Science B.V. All rights reserved.