## 2 Results and Discussion  
The XRD patterns of Sr\({}_{3.90}\)Si\({}_{3}\)O\({}_{8}\)Cl\({}_{4}\):0.10 Eu\({}^{2+}\) and Sr\({}_{3.50}\)Si\({}_{3}\)O\({}_{8}\)Cl\({}_{4}\):0.10 Eu\({}^{2+}\), 0.40 Bi\({}^{3+}\) are shown in Figure 1. They are almost consistent with the JCPDS card 40-0074 [Sr\({}_{4}\)Si\({}_{3}\)O\({}_{8}\)Cl\({}_{4}\)]. This indicates that the phosphor Sr\({}_{4}\)Si\({}_{3}\)OsCl\({}_{4}\):Eu\({}^{2+}\) almost shares the same phase as Sr\({}_{4}\)Si\({}_{3}\)O\(8\)Cl\({}_{4}\), except for a few peaks of SrSiO\({}_{3}\) (marked by star shape). This result is in accordance with the references [15,16]. The impurity phase may be due to loss of chlorine content during the firing process. Eu\({}^{2+}\) and Bi\({}^{3+}\) maybe occupy the site of Sr\({}^{2+}\) in an octahedral site, because the ion radii of Bi\({}^{3+}\) (103 pm) and Eu\({}^{2+}\) (117 pm) are close to that of Sr\({}^{2+}\) (118 pm). Meanwhile, with the doping of Bi\({}^{3+}\)/Eu\({}^{2+}\), a slight shift in the diffraction peaks toward higher angles can be found in Figure 1. This shift is due to the difference of ionic radius between Bi\({}^{3+}\)/Eu\({}^{2+}\) and Sr\({}^{2+}\). Figure 2a is the excitation spectra of the phosphors Sr\({}_{4-4}\)Si\({}_{3}\)O\({}_{8}\)Cl\({}_{4}\):4\({}_{x}\)Eu\({}^{2+}\) (\(x\) = 0.015, 0.020, 0.025, 0.030) by monitoring emission at 490 nm. These four curves are of similar shapes. The broad excitation band from 230 nm to 430 nm is due to the 4f-5d transitions of Eu\({}^{2+}\). These broad and intense excitation bands in near UV range match well with near-UV LED chip. When the content of Eu\({}^{2+}\) is 0.10, the excitation intensity is the strongest. The emission spectra of Sr\({}_{4-4x}\)Si\({}_{3}\)OsCl\({}_{4}\):4\({}_{x}\)Eu\({}^{2+}\) (\(x\) = 0.015, 0.020, 0.025, 0.030) under 320 nm excitation are shown in Figure 2b. The blue-green emission band is due to the 4f\({}^{6}\)5d-4f\({}^{7}\) transition of Eu\({}^{2+}\). With the increase of the Eu\({}^{2+}\) content, the intensity of the blue-green emission becomes higher. When the content of Eu\({}^{2+}\) is 0.10, the emission intensity is the strongest. Since the luminescent mechanism of Eu\({}^{2+}\) is the 4f-5d allowed electric-dipole transition, the energy transfer process of Eu\({}^{2+}\) in the Sr\({}^{4}\)Si\({}^{3}\)OsCl\({}_{4}\) phosphors would be attributed to an electric multipole-mupole interaction. When the energy transfer occurs between the same sites of activators, the intensity of multipole interaction can be determined by the change of the emission intensity from the emitting level. According to the report of [PERSON] [17], the emission intensity (\(I\)) of per activator ion follows the equation [18,19]:  
Figure 1: X-ray powder diffraction (XRD) patterns of Sr\({}_{3.90}\)Si\({}_{3}\)OsCl\({}_{4}\):0.10 Eu\({}^{2+}\) and Sr\({}_{3.50}\)Si\({}_{3}\)O\({}_{8}\)Cl\({}_{4}\):0.10 Eu\({}^{2+}\), 0.40 Bi\({}^{3+}\). The Asterisk represents the peak of SrTiO\({}_{3}\). Figure 2: **(a)** Excitation spectra and **(b)** emission spectra of Sr\({}_{4-4}\)Si\({}^{3}\)OsCl\({}_{4}\):4\({}_{x}\)Eu\({}^{2+}\) (\(x=0.015\), 0.020, 0.025, 0.030). \[\frac{I}{\chi}=K[1+\beta\chi^{\frac{Q}{3}}]^{-1} \tag{1}\]  
where \(I\) is the emission intensity, \(\chi\) is the Eu\({}^{2+}\) concentration, \(K\) and \(\beta\) are constants for the same excitation condition for a given host crystal. Q is a constant of multipole interaction equals to 3, 6, 8 or 10 for energy transfer among the nearest-neighbor ions, dipole-dipole (d-d), dipole-quadruple (d-q) or quadruple-quadruple (q-q) interaction, respectively. To get a Q value for the emission center, the plot of log (\(I\)/\(\chi\)) as a function of log (\(\chi\)) for the Sr\({}^{4-4x}\)Si\({}_{3}\)OsCl\({}_{4}\):4\({}_{x}\)Eu\({}^{2+}\) phosphors is plotted, as shown in Figure 3. It can be seen that a linear relation between log (\(I\)/\(\chi\)) and log (\(\chi\)) is found and the slope is \(-0.5299\). The Q value can be obtained as 1.5897.