CRYSTAL HELD 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 o n the Ce+3 5d-->4f optical transitions is investigated using self-cons istent field-configuration interaction calculations. The results are i n excellent agreement with the experimentally determined transition en ergies for MgS : Ce, CaS : Ce, SrS : Ce, and BaS : Ce. The red shift o f the spectroscopy of BaS : Ce relative to that of SrS : Ce was correc tly predicted. In addition, calculated oscillator strengths of the ele ctronic transitions in SrS : Ce and BaS : Ce were fit with Gaussian sh apes to model the vibronic structure. Both absorption and emission spe ctra were generated, and were found to have good agreement with experi mental spectra. Point charge crystal-held calculations were employed i n an attempt to account for the experimentally observed trend, but fai led to do so. A population analysis of the configuration interaction s tates indicates that the trend in emission energies is caused by diffe ring extents of covalency experienced by the Ce 5d orbitals in the var ious host materials. (C) 1998 Elsevier Science B.V. All rights reserve d.