Investigation of the Structural and Luminescent Properties and the Chromium Ion Valence of Li2CaGeO4 Crystals Doped with Cr4+ Ions

Herein, we report on the growth of Cr4+-Li2CaGeO4 crystals by the flux growth method from the flux of LiCl, as well as on the effect of doping Li2CaGeO4 with Cr4+ ions on the NIR region spectral properties and crystal structure. The results quantified the occupancy of Cr4+ in Ge4+ sites. The emission spectrum presented broad bands in the NIR region, i.e., 1000-1500 nm excited by 980 nm, with maximum peaks at 1200 nm at room temperature caused by the transition of T-3(2)->(3)A(2) in Cr4+ ions. The lifetime decreased with the Cr4+ ion doping concentration, specifically from 14.038 to 12.224 ms. The chemical composition and the valence state of chromium in Li2CaGeO4 were analyzed using X-ray photoelectron spectroscopy, which showed that the chromium in Li2CaGeO4 was tetravalent and no trivalent chromium was found. Therefore, the Cr4+-Li2CaGeO4 crystal has a great potential and future in optical applications.