Influence of Mn2+ ions on the structure, spectral characteristics and optical thermometry performances of ZnAl2O4:Cr3+ multifunctional phosphors

Cr3+/Mn2+ co-doped ZnAl2O4 phosphors with normal spinel structure were successfully synthesized by using the common sol-gel method. The crystal structure was characterized by XRD, confirming the single spinel phase even 30 mol% Mn2+ doping in ZnAl2O4 lattices. The Mn2+ ions caused the cations in spinel lattices deviating from the equilibrium position, which furtherly resulted in the higher spinel inversion rate. But it revealed a positive effect on the luminescence of ZnAl2O4:Cr3+. The energy transfer from Mn2+ to Cr3+ in the ZnAl2O4 phosphors was confirmed. With Mn2+ doping in ZnAl2O4 lattices, the emission peaks at 685 nm (R3 line) and 689 nm (R2 line) revealed an obvious increase until the concentration of Mn2+ increased up to 18 mol%. While the emission at 687 nm (R1 line) became weaker comparing with the emissions at 685 nm and 689 nm. The position, line width and the intensity of R-line revealed an obvious temperature dependence (80–100 K). The thermometer based on the fluorescence intensity ratio of Cr3+ was proposed, and the absolute (Sa) and relative sensitivity (Sr) were calculated, respectively, which values revealed an obvious dependence on the concentration of Mn2+ ions. 0.25 %K−1 of Sr was obtained with Mn2+ ions dopant, which was twice than that without Mn2+ ions dopants at 310 K, indicating its potential application in the optical thermometry.