Enhanced deep-blue emission and quantum yield of Ca$_{0.995}$YGaO$_4$ :0.5%Bi$^{3+}$ via lithium-based flux additives

In this work, Bi$^{3+}$-doped CaYGaO$_4$ phosphors were synthesised through solid-state reaction and systematically optimised using lithium-based fluxes to enhance their photoluminescence quantum yield (PLQY) in the blue region. Comparative studies between Li$_2$O and Li$_2$CO$_3$ fluxes revealed distinct structural and optical impacts. Rietveld refinement confirmed effective Bi$^{3+}$ incorporation, with samples treated using 2% Li$_2$CO$_3$ and 1% Li$_2$O exhibiting superior crystallinity. SEM analysis showed molten-like particle morphologies and increased porosity in Li$_2$CO$_3$-treated samples features absent in those treated with Li$_2$O. XPS results highlighted a notable reduction in oxygen-related defects in Li$_2$CO$_3$-modified phosphors, correlating with improved PLQY performance. Photoluminescence measurements demonstrated strong blue emission at 430 nm under 330 nm excitation, alongside additional red (720 nm) and near-infrared (780 nm) emissions from stabilised Bi$^{2+}$ species, attributed to lithium-induced lattice rearrangements. The sample treated with Li$_2$CO$_3$ achieved a PLQY of 70%, surpassing flux-free and Li$_2$O-assisted counterparts, reinforcing the pivotal role of flux chemistry in fine-tuning emission efficiency. ... mehrIn particular, lithium carbonate flux profoundly influences Bi$^{3+}$ -doped CaYGaO4 phosphors, establishing flux chemistry as a versatile tool for engineering efficient blue emitters for photonic applications.