First-Principles Analysis of the Effects of Covalency and Ionicity on the 4f-5d Transition Energy of Ce³⁺ in Garnet-Type Oxides

A blue light-emitting diode (LED) and a yellow phosphor are frequently combined to create white LEDs, with cerium ion (Ce3+)-doped yttrium aluminum garnet (YAG) as a common phosphor utilized in this process. A yellow light is produced when Ce3+ ions are excited by blue LEDs. This yellow light is combined with the direct blue light from blue LEDs to form white light. In this study, the effects of electronic characteristics, such as covalency and ionicity, on the 5d level energies and the 4f level energies of Ce3+ in various garnet-type crystals were investigated using first-principles relativistic discrete variational-X & alpha; (DV-X & alpha;) molecular orbital (MO) calculations. The purpose of this study is to elucidate a detailed mechanism for the centroid shift of the 5d level energies of Ce3+ in crystals based on the MO theory. The theoretical 4f-5d transition energies agreed well with the experimental ones and according to the electronic structure analysis, it was found that there is a high correlation between the centroid shift and the net charge of Ce3+. The detailed analyses of covalency and ionicity indicated that the primary cause of the centroid shift of the 5d level energies relative to the lowest 4f level of Ce3+ in crystals is an increase of the 4f level energies caused by a reduction of the net charge of Ce3+. These results would provide a theoretical foundation for the creation of novel Ce3+-doped garnet phosphors for usage in displays and solid-state lighting.