Comparative studies of upconversion luminescence and optical temperature sensing in Tm3+/Yb3+ codoped LaVO4 and GdVO4 phosphors

Tm3+/Yb3+ codoped LaVO4 and GdVO4 phosphors are successfully synthesized using solid state reaction methods and then upconversion emission studies are performed. X-ray diffraction has confirmed a pure monoclinic phase of LaVO4 and a tetragonal phase of GdVO4. Upconversion emission through 980 nm laser diode excitation has shown a strong blue band at 475 nm and two weak red bands at 647 and 700 nm originating from (1)G(4) & RARR; H-3(6), (1)G(4) & RARR; F-3(4) and F-3(3) & RARR; H-3(6) transitions of Tm3+ ions, respectively. Non-thermally coupled levels viz.F-3(3) (700 nm) and (1)G(4) (475 nm) in both the phosphors are used for fluorescence intensity ratio based optical thermometric studies and a comparison is made. The FIR data against temperature were fitted with polynomial and exponential fittings. The results show that polynomial fitting has a higher absolute sensitivity of 21.2 x 10(-3) K-1 at 653 K for the LaVO4: Tm3+/Yb3+ phosphor than the exponential fitting sensitivity of 19.0 x 10(-3) K-1 at 653 K, while in the case of the GdVO4: Tm3+/Yb3+ phosphor both fitting functions provided the same value of absolute sensitivity, that is 13.0 x 10(-3) K-1 at 653 K. A comparison of the sensitivity values shows that the LaVO4: Tm3+/Yb3+ phosphor provides higher sensitivity than the GdVO4: Tm3+/Yb(3+)phosphor but the latter one is too high in upconversion emission.