Enhancement of near‐infrared up‐conversion and blue down‐conversion luminescence in LuNbO 4 :Yb 3+ ,Tm 3+ with Ga 3+ and Ta 5+ substitutions

Under a 980‐nm excitation, the up‐conversion (UC) spectra of LuNbO 4 :Yb 3+ ,Tm 3+ powders exhibited predominantly near‐infrared bands (~805 nm) of Tm 3+ through an energy transfer process from Yb 3+ to Tm 3+ . Regarding the down‐conversion (DC) luminescence of the powders, the photoluminescence excitation spectra consisted of a broad charge transfer band (270 nm) due to [NbO 4 ] 3− and sharp band (360 nm) of Tm 3+ , while the corresponding emission spectra exhibited a blue emission at 458 nm. Upon substitution of Ga 3+ and Ta 5+ for Lu 3+ and Nb 5+ , respectively, both UC and DC luminescence properties were significantly enhanced. For the Ga 3+ substitution, the increased emission intensity could be explained by the crystal field asymmetry surrounding the Tm 3+ ions induced by the large difference in ionic radius between Ga 3+ and Lu 3+ . For the Ta 5+ substitution, we believe that an M′‐LuTaO 4 substructure was formed in the host, which led to the formation of a TaO 6 octahedral coordination instead of a NbO 4 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 LuNbO 4 :Yb 3+ ,Tm 3+ has a great potential in the fields of temperature sensing probes, anti‐counterfeiting, and bioapplications.