Abstract
Various Cr3+-activated oxide phosphors can be classified into two groups: a strong crystal field group [E(2Eg)ZPL < E(4T2g)ZPL] and a weak crystal field group [E(2Eg)ZPL > E(4T2g)ZPL] from an equality relation of the zero-phonon line (ZPL) energies of 2Eg and 4T2g. Obtaining reliable Racah parameters for the 3d3 ion (Cr3+) usually needs accurate information on the three excited-state ZPL energies, E(2Eg)ZPL, E(4T2g)ZPL, and E(4T1g,a)ZPL. However, an exact determination of such ZPL energies is a very difficult problem. Because of an accumulation with adjacent excited states along with the presence of charge transfer band, this is especially for E(4T1g,a)ZPL. An importance of accurate determination of the excited-state energies to be used in the Racah parameter calculation is discussed in detail with demonstrating a risk of erroneous conclusion of whether Cr3+-activated oxide phosphor belongs to a strong or weak crystal-field phosphor.
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