Rare earth ions (Tb3+, Eu3+, Yb3+, Tm3+, Er3+, and Ho3+)-doped SrR2O4 (R = Y and Gd) were prepared by solid-state reaction, and their optical properties were investigated. Upon VUV–UV excitation, the SrY1.99O4:0.01Tb3+ exhibits a typical green emission of Tb3+ with the dominant peak located at 543 nm. In the excitation spectrum of SrY1.99O4:0.01Tb3+ monitored at 543 nm, the f–d transition of Tb3+ is observed and identified to lie in the much lower energy region compared with many other Tb3+-doped phosphors, which can be interpreted by a strong crystal field in SrY2O4 structure. Upon X-ray excitation, SrY1.99O4:0.01Eu3+ displays an intense red fluorescence, whose strongest emission peak is situated at 611 nm, ascribed to 5D0–7F2 transition of Eu3+. Under 980 nm laser excitation, blue, green, and yellow emissions have been achieved in Yb3+–Tm3+, Yb3+–Er3+, and Yb3+–Ho3+ co-doped SrGd2O4, respectively. An obvious intensity ratio enhancement of green emission to red one of Er3+ was observed in Yb3+–Tm3+–Er3+ tridoped SrGd2O4 relative to Yb3+–Er3+ co-doped samples, and the possible reason has been elucidated by a cross-relaxation process between Tm3+ and Er3+. Laser power dependence of the up-converted emissions and the energy level diagrams were studied to understand the upconversion mechanisms.
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