## 3 Results  
Figure 1a presents the XRD pattern of the undoped-CaMoO4 and CaMoO4 co-doped with different molar ratios of Sm3+/Tb3+. Compared to the crystallographic JCPDS card No. 290351 [9; 18], the significant diffraction peaks at 20 = 18.6\({}^{\circ}\), 28.7\({}^{\circ}\), 31.2\({}^{\circ}\), 34.3\({}^{\circ}\), 47.0\({}^{\circ}\), and 58.0\({}^{\circ}\) correspond to the (101), (112), (004), (200), (204), and (312) peak of CaMoO4  
respectively. For the undoped-CaMoO\({}_{4}\), no impurity peak of other phases is observed, indicating that the undoped-CaMoO\({}_{4}\) samples crystallized in a rather pristine tetragonal structure [9,18]. Additionally, it is noted that for the present measured XRD, after the doping of Sm\({}^{3+}\), Tb\({}^{3+}\), and the co-doping of Sm\({}^{3+}\)/Tb\({}^{3+}\) ions, no significant variations in all the major diffraction peaks were observed. Nevertheless, a careful analysis of the most significant peak of (112), as shown in Figure 1b, has indicated that the introduction of RE\({}^{3+}\) cation has induced a downshift of the (112) peak to lower angle. This observation is in line with that of Ref. [17], _viz._, increasing shift with increasing concentration. [PERSON] et al. claimed that the variations in the (112) peak are due to the structural distortions leading to polarization in the [CaOs] clusters, which are induced from the difference between electronic densities of the Ca\({}^{2+}\) in relation to the incorporated RE\({}^{3+}\)cation. The schematic structure diagram of the Sm\({}^{3+}\)/Tb\({}^{3+}\) co-doped CaMoO\({}_{4}\) is also depicted in the Figure 1b [17]. The results indicate that CaMoO\({}_{4}\) phosphors with different molar ratios of Sm\({}^{3+}\)/Tb\({}^{3+}\)-co-doping have successfully synthesized by microwave-assisted heating [18]. The 12 and 300 K PL spectra of the undoped-CaMoO\({}_{4}\) phosphors measured under 377 nm excitation are shown in Figure 2. Both spectra show a broad-band emission  
Figure 1: (**a**) The XRD patterns and (**b**) enlarged view of (112) peak of undoped-CaMoO\({}_{4}\) and Sm\({}^{3+}\)/Tb\({}^{3+}\) co-doped CaMoO\({}_{4}\) with different molar ratios of Sm\({}^{3+}\)/Tb\({}^{3+}\) ions. covering the visible electromagnetic spectrum in the wavelength region ranging from 425 to 600 nm. It is known that the profile of broad-band emission is due to multi-level and multi-phonon processes, where several paths involving the participation of multiple energy states exist within the band gap. For CaMoO\({}_{4}\) materials, the charge-transfer transitions in the intermediate levels of the band gap resulting in the broad PL spectra have been attributed to the structural and electronic distortion in the tetrahedral (MoO\({}_{4}\))\({}^{2-}\) complex ions [6; 7]. Figure 3a,b shows the 12 and 300 K PL spectra of 0.05 mol Sm\({}^{3+}\)- and 0.05 mol Tb\({}^{3+}\)-doped CaMoO\({}_{4}\) phosphors, respectively. The luminescence mechanisms of the RE\({}^{3+}\)-doped samples are presented to provide some insight to the observed emission lines. Under UV excitation, the excited electrons in (MoO\({}_{4}\))\({}^{2-}\) group would transfer directly from the (MoO\({}_{4}\))\({}^{2-}\) complex to the high-level excited states of Sm\({}^{3+}\) and Tb\({}^{3+}\) via the energy transfer process. The electrons absorb energy and are promoted from the ground state \({}^{6}\)H\({}_{5/2}\) of Sm\({}^{3+}\) (\({}^{7}\)F\({}_{6}\) of Tb\({}^{3+}\)) to the higher level excited states, and subsequently relax to the lower \({}^{4}\)G\({}_{5/2}\) of Sm\({}^{3+}\) (\({}^{5}\)D\({}_{4}\) of Tb\({}^{3+}\)) level via a non-radiative process. Finally, the electrons in the populated level would undergo transition by radiative process from \({}^{4}\)G\({}_{5/2}\) to \({}^{6}\)H\({}_{j}\) (\(j=5/2\), 7/2, 9/2) for Sm\({}^{3+}\) and from \({}^{5}\)D\({}_{4}\) to \({}^{7}\)F\({}_{j}\) (\(j=6\), 5, 4, 3) for Tb\({}^{3+}\).