Development of new Eu3+/Sm3+ co-doped CaWO4 and CaMoO4 phosphors for optical thermometry based on ratiometric approach

The Eu³⁺/Sm³⁺-co-doped CaWO₄ and CaMoO₄ scheelite-type phosphors were successfully synthesized by high-temperature solid-phase method and were investigated in detail for their promising applications in luminescence thermometry. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses were used to study the structure and morphology of the phosphors synthesized. The co-doped CaMO₄ (M = W, Mo): Eu³⁺/Sm³⁺ phosphors showed characteristic emission peaks at 534, 594, 657 and 705 nm corresponding to the intra-configurational transitions of ⁵D₁→⁷F₁, ⁵D₀→⁷F₂, ⁵D₀→⁷F₃ and ⁵D₀→⁷F₄ of Eu³⁺ ions, as well as other emissions at 564, 603 and 648 nm, which were ascribed to ⁴G_5/2 → ⁶H_5/2, ⁴G_5/2 → ⁶H_7/2, and ⁴G_5/2 → ⁶G_9/2 transitions of Sm³⁺, respectively. The temperature sensing properties of both phosphors were investigated based on different temperature evolution of several emission bands of Eu³⁺ and Sm³⁺. Specifically, using the luminescence intensity ratio (LIR) technique, we found that monitoring evolution of the LIR parameter 652/617 nm for the CaMoO₄:1%Eu³⁺/1%Sm³⁺ the maximum relative sensitivity of 1.28% K^-1 can be achieved in the cryogenic T-range (93 K). Whereas, utilizing the same LIR for the CaWO₄:1%Eu³⁺/1%Sm³⁺ results in slightly smaller sensitivity of 1.18% K^-1. The obtained results, indicate that the developed phosphors may work as effective, remote temperature sensors, having great application potential in optical thermometry.