Influence of alkali and alkaline earth metal ion doping on the optical and thermal properties of Yb³⁺/Ho³⁺ co-doped NaBi(MoO4)₂ single crystals

The influence of alkali and alkaline earth metal ion doping on the upconversion (UC) luminescence properties of Yb^3+, Ho^3+ co-doped NaBi(MoO_4)_2 (NBM) single crystals was systematically investigated. A series of Yb^3+, Ho^3+:NBM single crystals doped with Li^+, K^+, and Ca^2+ ions were successfully grown using the Czochralski method. Their structural characteristics were analyzed through x-ray diffraction (XRD), transmission spectroscopy, and scanning electron microscopy (SEM), confirming a tetragonal crystal structure with space group I4_1/a. Under 980 nm and 450 nm laser excitation, the crystals exhibited distinct UC emission profiles, with dominant red and green emissions, respectively. Tauc plot analysis indicated that Ca^2+ doping introduced donor levels within the crystal lattice, effectively narrowing the optical bandgap. Power-dependent luminescence studies revealed that both red and green UC emissions originated from two-photon excitation processes. Temperature-dependent photoluminescence measurements under both excitation wavelengths demonstrated that co-doping with Li^+, K^+, and Ca^2+ significantly enhanced the thermal stability of the luminescent properties. Furthermore, variable-temperature fluorescence lifetime analyses showed that alkali and alkaline earth metal ion doping effectively mitigated the luminescence lifetimes. Overall, the incorporation of Li^+, K^+, and Ca^2+ ions was found to markedly improve both the UC luminescence intensity and thermal robustness of Yb^3+, Ho^3+:NBM single crystals, offering promising prospects for high-performance luminescent materials.