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

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.