On the excitation spectra of Cr³⁺/Cr²⁺ and V³⁺ co-doped ZnAl₂S₄ single crystals

Abstract The excitation spectra of the ZnAl₂S₄ spinel crystals codoped with chromium and vanadium are investigated in order to explain some features of the Cr³⁺ ions optical spectra and the inconsistency of the experimental absorption/emission bands position with the Tanabe-Sugano diagram, as well as the "missing" band in the absorption spectrum of V³⁺ ion although this band should be present as it is due to a spin allowed transition. The unusual high Racah parameters of the C/B ratio for Cr³⁺ ions have been induced to recalculate the Tanabe-Sugano diagram for the d³ electron configuration and the ratio C/B=8 in order to use it for the experimental results interpretation. The presence of Cr²⁺ ions in the low spin electron configurations in the octahedral coordination was confirmed; the temperature dependence of the Cr²⁺ emission at about 1.9 eV was studied and the Huang-Rhys factors were estimated for different temperatures. Despite the fact that the V³⁺ C/B ratio has a different value compared to the case of Cr³⁺ ions, the electron-phonon interaction is similar for both ions, with the Huang-Rhys parameter equal to 10 in both cases. Further research into optimizing the ZnAl₂S₄:Cr, V system to get an efficient enhancement of the vanadium emission on account on re-absorbing the chromium emission is suggested.