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

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 ¹G₄ → ³H₆, ¹G₄ → ³F₄ and ³F₃ → ³H₆ transitions of Tm³⁺ ions, respectively. Non-thermally coupled levels viz.³F₃ (700 nm) and ¹G₄ (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 × 10^-3 K^-1 at 653 K for the LaVO₄: Tm³⁺/Yb³⁺ phosphor than the exponential fitting sensitivity of 19.0 × 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 × 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⁺/Yb³⁺phosphor but the latter one is too high in upconversion emission.