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 3T2→3A2 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.