Luminescence spectroscopy of Cr3+ in an oxide: A strong or weak crystal-field phosphor?

Various Cr3+-activated oxide phosphors can be classified into two groups: a strong crystal field group [E(E-2(g))(ZPL) < E(T-4(2g))(ZPL)] and a weak crystal field group [E(E-2(g))(ZPL) > E(T-4(2g))(ZPL)] from an equality relation of the zero-phonon line (ZPL) energies of 2E(g) and T-4(2g). Obtaining reliable Racah parameters for the 3d(3) ion (Cr3+) usually needs accurate information on the three excited-state ZPL energies,E(E-2(g))(ZPL) > E(T-4(2g))(ZPL), and E(T-4(1g),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(T-4(1)g,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.

Automated summary

The article discusses the classification of Cr3+-activated oxide phosphors into two groups and the challenges in obtaining reliable Racah parameters. It emphasizes the importance of accurately determining excited-state energies.