Enhancement of near‐infrared up‐conversion and blue down‐conversion luminescence in LuNbO4:Yb3+,Tm3+ with Ga3+ and Ta5+ substitutions

Under a 980‐nm excitation, the up‐conversion (UC) spectra of LuNbO₄:Yb³⁺,Tm³⁺ powders exhibited predominantly near‐infrared bands (~805 nm) of Tm³⁺ through an energy transfer process from Yb³⁺ to Tm³⁺. Regarding the down‐conversion (DC) luminescence of the powders, the photoluminescence excitation spectra consisted of a broad charge transfer band (270 nm) due to [NbO₄]^3? and sharp band (360 nm) of Tm³⁺, while the corresponding emission spectra exhibited a blue emission at 458 nm. Upon substitution of Ga³⁺ and Ta⁵⁺ for Lu³⁺ and Nb⁵⁺, respectively, both UC and DC luminescence properties were significantly enhanced. For the Ga³⁺ substitution, the increased emission intensity could be explained by the crystal field asymmetry surrounding the Tm³⁺ ions induced by the large difference in ionic radius between Ga³⁺ and Lu³⁺. For the Ta⁵⁺ substitution, we believe that an M′‐LuTaO₄ substructure was formed in the host, which led to the formation of a TaO₆ octahedral coordination instead of a NbO₄ tetrahedral coordination. Consequently, the crystal symmetry of the local structure was modified, and thus the UC and DC luminescence properties were enhanced. The dual‐mode (UC and DC) luminescence demonstrates that LuNbO₄:Yb³⁺,Tm³⁺ has a great potential in the fields of temperature sensing probes, anti‐counterfeiting, and bioapplications.