Ultra-stable Er3+/Yb3+ Co-doped scheelite single crystals for wide-range fiber-free optical thermometry

Fluorescence intensity ratio (FIR)-based optical thermometry, recognized for high reliability and rapid response, has emerged as a promising non-contact temperature sensing technology. This study focuses on enhancing the performance of Er³⁺/Yb³⁺ co-doped scheelite-type single crystals (NaY(WO₄)₂ and NaGd(WO₄)₂) for wide-range optical temperature sensing. By employing a spontaneous nucleation method, we synthesized single crystals with optimized doping concentrations (1 % Er³⁺/60 % Yb³⁺ for NYW and 3 % Er³⁺/70 % Yb³⁺ for NGW), achieving intense green upconversion (UC) emissions at 530 nm (²H_11/2 → ⁴I_15/2) and 552 nm (⁴S_3/2 → ⁴I_15/2). The single crystals exhibited high Yb³⁺→Er³⁺ energy transfer efficiencies of 11.02 % (NYW: 1 % Er³⁺, 60 % Yb³⁺) and 20.65 % (NGW: 3 % Er³⁺, 70 % Yb³⁺), enabling robust UC luminescence even under harsh conditions (pH = 1/13, 72 h) and thermal shocks (800 °C, 8 cycles). Temperature-dependent FIR analysis of thermally coupled energy levels (²H_11/2 and ⁴S_3/2) revealed linear ln(FIR) vs. 1/T relationships, with maximum relative sensitivities of 1.18 % K⁻¹ (NYW) and 1.12 % K⁻¹ (NGW) in 298–578 K. A prototype sensor utilizing NGW: 3 %Er³⁺, 70 %Yb³⁺ single crystal demonstrated practical feasibility across 80–780 K, eliminating fiber-optic requirements due to high UC efficiency. These results highlight scheelite-type single crystals as superior candidates for precision optical thermometry in extreme environments.