Abstract
There are many applications that utilize the 4f-5d transitions of lanthanide ions, including (Sr,Ca)AlSiN3:Eu2+ (SCASN), which is widely used as a red phosphor for white LEDs. Also, research on the f-d transitions of Eu2+ in such compounds has been actively conducted both experimentally and theoretically. Dorenbos systematically analyzed the red shift of the 4f-5d transitions of lanthanide ions in crystals from those of the free ions based on large amount of literature data. He indicated that the red shift is a property unique to each host crystal and almost independent of the species of the lanthanide ions. Based on this fact, he created the energy level diagrams of lanthanide ions in various crystals. The red shift is mainly determined by two factors, the crystal field splitting and the centroid shift which is a shift of the barycenter of the 5d levels relative to the lowest 4f level. Therefore, prediction of these parameters for a given host crystal is important for theoretical design of novel lanthanide-doped phosphors. In this work, we aimed to analyze the electronic and structural origin of the centroid shift in 4f-5d transition energies of Eu2+ in oxides and fluorides. Small clusters consisting of the substituted Eu2+ ions and first-neighbor anions were constructed and the first-principles multiplet calculations using the relativistic DVME method were performed. In order to clarify the origin of the centroid shift, the correlations between the centroid shift and the various electronic and structural parameters were investigated in detail.
Published Version
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