## 3 Results and Discussion
Consequently, as both of these sites are centrosymmetric, electric dipole transitions (\({}^{5}\)D\({}_{0}\)-\({}^{7}\)F\({}_{j=0,2}\), only the \({}^{5}\)D\({}_{0}\)-\({}^{7}\)F\({}_{2}\) transition is observed in this study) are forbidden and, as such, should not appear in the spectra. As the synthesizing temperature is 900 \({}^{\circ}\)C, the fact that the \({}^{5}\)D\({}_{0}\)-\({}^{7}\)F\({}_{2}\) electric dipole transition at 615 nm prevails over the \({}^{5}\)D\({}_{0}\)-\({}^{7}\)F\({}_{1}\) magnetic dipole transition at 595 nm in the case of Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) phosphors leads us to the conclusion that the Eu\({}^{3+}\) ions are generally located in a disordered manner in the Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) powders [27]. The charge-compensating oxygen vacancies surrounding the Eu\({}^{3+}\) ions will lead to the deviation from the point symmetry and relaxation of electric dipole transitions selection rules, with the appearance of the \({}^{5}\)D\({}_{0}\)-\({}^{7}\)F\({}_{2}\) transition lines in the spectra [27]. Additionally, the Eu\({}^{3+}\) luminescent centers contributing to the \({}^{5}\)D\({}_{0}\)-\({}^{7}\)F\({}_{2}\) transition line are probably located at the Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) particle surface or subsurface and will dominate the emission of Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) phosphors when the synthesizing temperature is 1200 \({}^{\circ}\)C [27]. If Eu\({}^{3+}\) ions occupy a non-centrosymmetric lattice site, both electric and magnetic transitions are possible, and the non-symmetric site occupancy will be a dominant reason for causing the emission of \({}^{5}\)D\({}_{0}\)-\({}^{7}\)F\({}_{2}\) (612 nm) transition [28]. The definition of fluorescence (or phosphorescence) lifetime is the time for the intensity of a single emission peak to decrease to 1/e (approximately 37%) of its original intensity. The decay curves of Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) phosphors were obtained at different concentrations of Eu\({}^{3+}\) (\(x\) = 0.04, 0.06, 0.08, 0.010, and 0.12), and Figure 5 shows the intensity decay of the luminescence of Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) phosphors as a function of the synthesizing temperature, which can be used to calculate the lifetime. The wavelength of the exciting light was 271 nm, and the measured wavelength for the intensity decay was 597 nm (\({}^{5}\)D\({}_{0}\)-\({}^{7}\)F\({}_{1}\)), because it had the maximum emission intensity, as the synthesizing temperature was 900-1100 \({}^{\circ}\)C. Typically, the standard form of the single exponential decay function is:  
\[A(t)=A_{0}\exp[\cdot(t/\tau)] \tag{1}\]  
where \(A_{0}\) is the initial population, \(\tau\) is constant for the decay time, and \(t\) is time. Figure 5 shows that the all curves of decay time can be fitted well by a single-exponential function. Figure 5 also shows that if the same synthesizing temperature is used, the observed decay time is almost unchanged even the \(x\) value in Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) phosphor increases from 0.04 to 0.12. Moreover, the decay time is a single exponential function for Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) phosphors with different concentrations of Eu\({}^{3+}\) ions and synthesized at different temperatures, because the activator lies in the same coordination environment [29]. Figure 5: _Cont._  
From the results shown in Figure 5, the lifetimes of Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) phosphors, which are compared in Table 1, were measured at 2.04-2.10 ms, 1.93-2.03 ms, 1.78-1.82 ms, and 1.65-1.67 ms, when the synthesizing temperature was 900 \({}^{\circ}\)C, 1000 \({}^{\circ}\)C, 1100 \({}^{\circ}\)C, and 1200 \({}^{\circ}\)C, respectively. The lifetimes of Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) phosphors apparently decreased with the increase in the synthesizing temperature. When the synthesizing temperature was 900 \({}^{\circ}\)C and 1000 \({}^{\circ}\)C, the lifetimes of the Sr\({}_{2-x}\)Eu\({}_{x}\)ZnMoO\({}_{6}\) phosphors varied more.