Ln^sup 3+^ (Er^sup 3+^, Tm^sup 3+^ and Ho^sup 3+^)-doped NaYb(MoO^sub 4^)^sub 2^ upconversion phosphors as wide range temperature sensors with high sensitivity

Well-crystallized NaYb(MoO4)2 doped with lanthanide ions (Ln3+: Er3+, Tm3+ and Ho3+) are successfully synthesized by hydrothermal method with further calcination. And the samples with different morphologies are obtained by changing the pH values and the molar ratio of Ln(NO3)3/Na2MoO4 of the resultant solutions. The upconversion (UC) luminescence spectra of NaYb(MoO4)2: Ln3+ between 323 K and 573 K under 980 nm excitation exhibit temperature-dependent property. The fluorescence intensity ratio (FIR) techniques with thermally coupled states (TCS) (Er3+: 2H11/2, 4S3/2 → 4I15/2, Tm3+: 1G4(1),(2) → 3H6, Ho3+: 5F5(1),(2) → 5I8) have high sensitivity (S) values (Er3+: 0.0122 K-1 at 548 K, Tm3+: 0.0025 K-1 at 323 K and Ho3+: 0.00035 K-1 at 323 K). The non-TCS (Ho3+: 5F5, 5S2/5F4 → 5I8) is used to extend the range of S, and the slope of FIR (non-TCS)-T is the S value as high as 0.02455 K-1 in the wide temperature range from 323 K to 573 K.