Lattice engineering mediated negative thermal quenching in Ca₉KLa_2/3(VO₄)₇:Sm³⁺ for sensitive temperature sensing

The persistent issue of thermal quenching in phosphors has significantly limited their high-temperature applications. To address this, we designed a novel phosphor, Ca₉KLa_2/3(VO₄)₇:Sm³⁺ (CKLV:xSm³⁺), which exhibits dual thermal quenching (TQ) behavior through a defect engineering strategy. Specifically, the matrix (VO₄^3-) shows thermal quenching (TQ), while the activator Sm³⁺ exhibits negative thermal quenching (NTQ). This phenomenon stems from the synergistic effect of the phonon-assisted energy transfer from VO₄^3- to Sm³⁺ and thermally-induced energy transfer between defect levels and the excited states of Sm³⁺. The contrasting thermal quenching behaviors of Sm³⁺ and VO₄^3- form the basis for self-referenced optical thermometry, and CKLV:xSm³⁺ shows thermal sensing sensitivity with maximum Sₐ and Sᵣ values of 0.078 K^-1 (at 475 K) and 1.19% K^-1 (at 350 K), respectively. To further improve their performance, lattice engineering was employed to augment the lattice distortion and enhance the luminescence of the VO₄^3- group. This approach proved highly effective, and the optimized Ca₈MgKLa_2/3(VO₄)₇:Sm³⁺ sample achieved a Sᵣ-max of 2.01% K^-1, increased by 69% over the undoped system. Additionally, the material exhibits distinct thermochromism, with a green-to-red emission shift upon heating, enabling visual temperature displaying and high-temperature alarm functionality. These findings validate its strong potential as a high-performance luminescent thermometer for advanced sensing applications.