Spectroscopy of Cr3+ activator: Tanabe−Sugano diagram and Racah parameter analysis

The Racah parameters in conjunction with the Sugano−Tanabe energy-level diagram are used as a means of describing the effects of electron−electron repulsion within the metal complexes in coordination chemistry. A determination of the excitation energies in the 3d3-configuration ions like Cr3+ and Mn4+ is possible from a set of Racah (B and C) and crystal-field splitting parameters (Dq), and vice versa. However, it is very difficult to exactly determine some excitation-state energies that leads to a large error in the Racah parameter determination. The present study focuses on this problem and proposes a new model that introduces an optional Racah parameter ratio of C/B ≡ r to make it possible without requiring an uncertain excited-state energy in the Racah parameter calculation. As a typical example, spectroscopic data of MgO:Cr3+ phosphor are investigated based on the present analysis model, where Cr3+ ions can be situated in cubic, tetragonal, and rhombic site of MgO lattice. The Racah parameters and related excited-state energies determined here are found to fall very well on each Tanabe−Sugano diagram curve (2Eg, 4T2g, 2T1g, 4T1g,a, 2T2g, and 2A2g). The present analysis model can be expanded to other 3d3-activated phosphor systems such as Mn4+ ion or fluoride hosts.