Fluorescence temperature dependent behaviors of Eu3+/Mn4+ co-doping cubic and hexagonal ZnO-TiO2 compounds: Application in high sensitive optical thermometers
ZnTiO 3 :Eu 3+ ,Mn 4+ phosphors with hexagonal and cubic structure are synthesized by solvothermal method. The structure of ZnTiO 3 depends on precursors and the annealing temperature. Inverse spinel Zn 2 TiO 4 phase reveals the highest thermostability compared with cubic ZnTiO 3 and hexagonal ZnTiO 3 phases. The different phases of ZnTiO 3 crystals exhibit different photoluminescence properties. The forbidden transition of 4 A 2 g → 2 T 2 g for Mn 4+ is observed in Zn 2 TiO 4 and hexagonal ZnTiO 3 (h-ZnTiO 3 ) due to the decreased symmetry. The zero photon line (ZPL) assigned to 2 E g → 4 A 2 g transition of Mn 4+ is absent in all type ZnTiO 3 phases. A sharp emission peak at 714 nm assigned to ν 6 (Stokes emission) mode of Mn 4+ in h-ZnTiO 3 is present when the temperature was below 270 K, and increases sharply in intensity with the temperature decreasing. The similar PL spectra of cubic ZnTiO 3 and Zn 2 TiO 4 co-doped with Eu 3+ and Mn 4+ show a wide emission band composed of Stokes and anti-Stokes modes. The calculated nephelauxetic parameter increases from 0.84 to 1.03 and 1.18 for Mn 4+ in h-ZnTiO 3 , cubic ZnTiO 3 and Zn 2 TiO 4 , respectively, indicating the covalency decreasing, which causes the red shift of ZPL in h-ZnTiO 3 . The Mn 4+ emissions show stronger temperature dependence than that of Eu 3+ , especially for that in h-ZnTiO 3 matrix. Based on the fluorescence intensity ratio between I Eu 3+ and I Mn 4+ , the highest relative temperature sensitivity of 2.46 %K −1 is observed in cubic ZnTiO 3 (c-ZnTiO 3 ) and Zn 2 TiO 4 . Under the excitation at 550 nm, the highest relative sensitivity of 2.9 %K −1 is achieved in c- and h-ZnTiO 3 mixed phases.
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