Aqueous solutions of zirconium oxychloride and aluminum nitrate were coprecipitated and crystallized to form a ZrO2–Al2O3 solid solution. The upconversion (UC) emission from different Er3+-doped samples was studied. An enhancement of the green UC emission by as much as 22 times was achieved by co-doping with Yb3+ and Mo6+ ions due to an energy transfer at a higher excited-state energy, which partly avoided the non-radiative decay processes at the lower energy levels of Er3+. The UC emission of the ZrO2–Al2O3 composite system series doped with different agents was enhanced. Excess oxygen vacancies are generated by forming ZrO2–Al2O3 solid solutions, which have an energy level close to the 4F7/2 level of the Er3+ ions. The defect state promoted the energy transfer process resulting in an eight-fold increased green UC emission in ZrO2–Al2O3 solid solutions. The solid solutions have a superior color chromaticity of x = 0.25 and y = 0.71 due to the evident enhancement in the green to red emission ratio in the 8ZrO2–2Al2O3 sample.

•Long afterglow characteristics were found in Zn2GeO4:Mn2+ phosphor by co-doping Cr3+.•The doping of Cr3+ ions deepens the native defect VGe traps.•The mechanism explains the phosphorescence of the Zn2GeO4:Mn2+, Cr3+ phosphor.Zn2GeO4:Mn2+, Cr3+ phosphors were prepared by conventional solid state reaction and the photoluminescence properties were investigated. The Mn2+ activated Zn2GeO4 phosphors exhibited green emission at 533 nm due to the 4T1(4G) → 6A1(6S) transition of Mn2+ ions. With Cr3+ co-doping in Zn2GeO4 host, long afterglow characteristics were found from the same transition of Mn2+. The TL results revealed the presence of same traps in the phosphor, and the doping of Cr3+ ions deepened the VGe traps. The native defect VGe as a hole traps is responsible for the long afterglow emission in Zn2GeO4:Mn2+, Cr3+ phosphor. The possible mechanism of this phosphor has also been discussed.

•Phase transformation from monoclinic to tetragonal ZrO2 is induced due to doping cations.•The green UC emission was enhanced 20 times by optimal codoping with Mo6+ ions.•Yb3+–MoO42− dimer sensitized Er3+ resulting in the high efficiency UC process.Intense green upconversion (UC) emission has been obtained from Er–Yb–Mo tridoped ZrO2. The effect of different doping agents on the crystalline structure of the materials was investigated by XRD. Phase transformation from monoclinic to tetragonal ZrO2 occurred due to doping an appropriate amount of oversized trivalent cations and generating oxygen ion vacancies. Green UC emission in Er3+ doped ZrO2 and red UC emission in Er3+–Yb3+ codoped ZrO2 were observed upon 980 nm excitation. Remarkably, enhancement of the green UC emission as much as 20 times was achieved by tridoping with Er3+–Yb3+–Mo6+ ions in ZrO2. The high efficiency of the UC can be attributed to a novel energy transfer process in which Yb3+–MoO42− sensitized Er3+ ions at ∣2F7/2, 1T1〉 excited state.

Aqueous solutions of zirconium oxychloride and aluminum nitrate were coprecipitated and crystallized to form a ZrO2–Al2O3 solid solution. The upconversion (UC) emission from different Er3+-doped samples was studied. An enhancement of the green UC emission by as much as 22 times was achieved by co-doping with Yb3+ and Mo6+ ions due to an energy transfer at a higher excited-state energy, which partly avoided the non-radiative decay processes at the lower energy levels of Er3+. The UC emission of the ZrO2–Al2O3 composite system series doped with different agents was enhanced. Excess oxygen vacancies are generated by forming ZrO2–Al2O3 solid solutions, which have an energy level close to the 4F7/2 level of the Er3+ ions. The defect state promoted the energy transfer process resulting in an eight-fold increased green UC emission in ZrO2–Al2O3 solid solutions. The solid solutions have a superior color chromaticity of x = 0.25 and y = 0.71 due to the evident enhancement in the green to red emission ratio in the 8ZrO2–2Al2O3 sample.