Effect of CaO Content on the Photoluminescence Excitation and Emission Properties of Bi2O3 and ZnO-Co-Doped Ca2+xGa4O8+x Phosphors

The synthesis process employed solid-state reaction methods to produce phosphors with varying CaO contents, specifically at x values of 0, 0.2, 0.4, 0.6, 0.8, and 1.0. As the CaO content (represented by the x value) increases, the crystalline structure of Ca2+xGa4O8+x + 0.01 Bi2O3 + 0.07 ZnO compositions underwent notable transformations. X-ray diffraction was used to characterize these structural changes, and we found that CaGa2O4, CaO, Ga2O3, and Ca3Ga4O9 were clearly identified. The diffraction intensities of CaGa2O4 and CaO phases increased and those of Ga2O3 and Ca3Ga4O9 decreased with the x value. Our findings revealed that the photoluminescence excitation (PLE) spectra consistently peak around 340 nm across all samples, while the photoluminescence emission (PL) spectra exhibited slight variations within the range of 474–477 nm. Most notably, the intensity of both PLE and PL spectra demonstrated a non-linear relationship with CaO content, initially increasing with higher CaO concentration, reaching maximum intensity at x = 0.4, and subsequently decreasing as the x value continued to rise. This research provides valuable insights into the relationship between composition, crystal structure, and luminescent behavior in Ca2+xGa4O8+x phosphor systems, and the theoretical mechanisms underlying these observed trends were thoroughly discussed.