Thermometry and up-conversion luminescence of Yb³⁺-Er³⁺ co-doped Na₂Ln₂Ti₃O₁₀ (Ln = Gd, La) phosphors

Yb³⁺/Er³⁺-ion co-doped Na₂Ln₂Ti₃O₁₀ (Ln = Gd, La) up-conversion (UC) phosphors were successfully synthesized by a sol-gel method, and their crystal structures were characterized by powder X-ray diffraction. Dazzling yellow-greenish light was emitted under the excitation of 980 nm near-infrared (NIR) light, composing green and red emission bands from the ²H_11/2/⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺, respectively. The optimal composition and synthesis parameters were determined according to their UC emission intensity. The photon absorption and emission processes were illustrated based on the UC mechanism, in which energy transfer (ET) from Yb³⁺ to Er³⁺ plays a pivotal role and has been proved by the variation of green emission lifetime in Er³⁺ singly and Yb³⁺/Er³⁺ co-doped Na₂Ln₂Ti₃O₁₀ samples. The temperature-dependent fluorescence intensity ratios (FIR) of the two thermal coupled energy level (TCL) emission from ²H_11/2 → ⁴I_15/2 (526 nm) and ⁴S_3/2 → ⁴I_15/2 (549 nm) were calculated in the range of 290-490 K, and their sensitivity values were approximately 0.0058 K^-1 for Na₂Gd₂Ti₃O₁₀ at 490 K and 0.0061 K^-1 for Na₂La₂Ti₃O₁₀ at 470 K, as potential optical temperature sensor.