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

Abstract

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.

Valence state control of Cr4+-activated Li2SrGeO4for NIR-II light source to distinguish deuterium and non-deuterium reagents

“…[30][31][32] Li 2 M II M IV O 4 -type structure (M II = Mn 2+ , Ca 2+ , Zn 2+ , Cd 2+ , and M IV = Si 4+ , Ge 4+ ) is also a good choice for Cr 4+ host materials such as Li 2 MgSiO 4 (l em E 1200 nm), 33 Li 2 ZnGeO 4 (l em = 1380 nm), 16 Li 2 CaGeO 4 (l em = 1200 nm). 34 Except for Cr 3+ , the production of Cr 6+ also lead to the reduction of Cr 4+ doping. In most studies, Cr 4+ -activated phosphors are synthesized in air to avoid the formation of Cr 3+ while it also inevitably produces Cr 6+ ions.…”

“…In most studies, Cr 4+ -activated phosphors are synthesized in air to avoid the formation of Cr 3+ while it also inevitably produces Cr 6+ ions. 34 Hence, controlling the valence state of Cr 4+ ions to avoid the generation of other ions is an effective way to obtain highly efficient NIR-II luminescent phosphors.…”

Achieving Ultra-Wideband NIR-II Emission in Cr3+-Doped Li(Sc,In)(Si,Ge)O4 Phosphors Based on Host Composition Engineering

“…Recently silicates and germanates with the olivine structure have received much attention as host materials for Cr 3+ /Cr 4+ -activated NIR phosphors. The common olivine-structure phosphors like Mg 2 (Si,Ge)­O 4 , Li 2 CaGeO 4 , and Ca 2 GeO 4 often possess the luminescence from Cr 4+ occupied tetrahedral site, which is of low luminescence efficiency. − However, in case of chromium-doped LiScGeO 4 and LiInGeO 4 , broadband emissions at 1110 and 1176 nm are assigned to Cr 3+ locating in large octahedron sites with great octahedral distortion. , Nevertheless, further investigations are necessary to realize spectral tuning and clarify the corresponding luminescence mechanism.…”

A non-rare earth ion doped broadband emission phosphor located in NIR-II for ethanol concentration detection

“…Wang developed Li 2 CaGeO 4 crystals doped with Cr 4+ ions; their emission spectrum presented broad bands in the NIR region (1000–1500 nm) with excitation at 980 nm. 16 Liu developed a chromium valence-controlled single-phase phosphor Mg 2 GeO 4 :Cr 3+ ,Cr 4+ to achieve ultra-broadband NIR emission. The ultra-broadband double emission from 650 nm to 1600 nm was achieved by fully controlling the concentration ratio of Cr 3+ to Cr 4+ in a single host.…”