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.