## 3 Results and Discussion
Those peaks are in good agreement with the (222) and (400) peaks of JCPDS file number 120393 for cubic Eu\({}_{2}\)O\({}_{3}\). The decrease in the diffraction intensities of peaks located at around 20 = 28.44\({}^{\circ}\) and 33.09\({}^{\circ}\) prove that more Eu\({}^{3+}\) ions will substitute the sites of Sr\({}^{2+}\) ions as the synthesizing temperature is raised. Even when the synthesizing temperature was 1100 \({}^{\circ}\)C, secondary or unknown phases were observed in the Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) ceramic powders. These Eu\({}_{2}\)O\({}_{3}\) and secondary or unknown phases were not observed when the synthesizing temperature was 1200 \({}^{\circ}\)C. These results suggest that when the same synthesizing temperature is used, the concentration of Eu\({}^{3+}\) ions has no apparent effect on the crystallization of Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) powders; hence, the synthesizing temperature is an important factor in determining the crystalline properties of Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) powders. Additionally, the concentration of Eu\({}^{3+}\) ions and the synthesizing temperature affect the photoluminescence properties of Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) phosphors. XRD patterns for Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) phosphors synthesized at 1200 \({}^{\circ}\)C for 4 h in the narrow range of 29-35\({}^{\circ}\) are shown in Figure 2. These results are significant. Initially, the splitting of the (220) diffraction peak was observed in the Sr\({}_{2}\)ZnMoO\({}_{6}\) and Sr\({}_{1.98}\)Eu\({}_{0.02}\)ZnMoO\({}_{6}\) phosphors, but it was not observed in other Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) phosphors. The two split peaks of the Sr\({}_{2}\)ZnMoO\({}_{6}\) phosphors were located at 20 = 31.83\({}^{\circ}\) and 31.88\({}^{\circ}\), and the two split peaks of the Sr\({}_{1.98}\)Eu\({}_{0.02}\)ZnMoO  
Figure 1: XRD patterns of Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) phosphors synthesized at (**a**) 900 \({}^{\circ}\)C; (**b**) 1000 \({}^{\circ}\)C; (**c**) 1100 \({}^{\circ}\)C; and (**d**) 1200 \({}^{\circ}\)C for 4 h, respectively. phosphors were located at 20 = 31.81\({}^{\circ}\) and 31.89\({}^{\circ}\). These results suggest that the Sr\({}_{2}\)ZnMoO\({}_{6}\) and Sr\({}_{1.98}\)Eu\({}_{0.02}\)ZnMoO\({}_{6}\) phosphors revealed a perovskite structure with the tetragonal phase. As the concentration of Eu\({}^{3+}\) was more than 0.02, the Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) phosphors would have transformed from the tetragonal phase to the (pseudo-)cubic phase, since the splitting of the (220) diffraction peak was not observed. Since the Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) phosphors were tetragonal phase or (pseudo-)cubic phase, even when the concentration of Eu\({}^{3+}\) ions was increased to 0.12, this result also proves that the Eu\({}^{3+}\) ions would have substituted into the sites of Ba\({}^{2+}\) ions. If the Eu\({}^{3+}\) ions had substituted into the sites of Zn\({}^{2+}\) (or Mo) ions, the Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) phosphors would have revealed other crystalline phases, or more secondary phases would have been revealed in the Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) phosphors rather than in the double-perovskite features. Previously, [PERSON] et al. successfully synthesized novel near-UV and blue-excited Eu\({}^{3+}\), Tb\({}^{3+}\)-co-doped one-dimensional strontium germanate full-color nanophosphors by a simple sol-hydrothermal method. They found that incorporation of the Eu\({}^{3+}\) and Tb\({}^{3+}\) ions into strontium germanate resulted in a slight shrinkage of the lattice constants and the unit cell volume because Eu\({}^{3+}\) and Tb\({}^{3+}\) had smaller radii than Sr\({}^{2+}\), indicating the Eu\({}^{3+}\) and Tb\({}^{3+}\) ions had been incorporated into the host lattice of SrGe\({}_{4}\)O\({}_{9}\) and did not change the crystal structure [24]. Those results also suggested that the Eu\({}^{3+}\) ions substituted into the sites of Sr\({}^{2+}\) ions.