Ratiometric optical temperature sensing properties based on up-conversion luminescence of novel NaLaTi2O6:Yb3+,Er3+/Ho3+ phosphors

To satisfy the increasing necessity of contactless optical thermometry, utilizing both thermally coupled (TCLs) or non-thermally-coupled (NTCLs) energy levels of rare earth ions in novel phosphors has significant potential for devising the optical thermometers with high temperature sensitivity. Herein, a sequence of novel Yb³⁺,Er³⁺/Ho³⁺ doped NaLaTi₂O₆ (NLTO) phosphors were sintered using a high-temperature solid-state reaction approach. Structure, phase component and luminescence performance were identified in detail. Upon 980 nm near-infrared (NIR) excitation, the obtained materials presented typical Er³⁺/Ho³⁺ characteristic emission wavelengths in visible region, which include two green emission bands around 522 and 543 nm, as well as a weaker red band around 661 nm for Er³⁺ doped NLTO, a green emission band around 545 nm and a red band around 657 nm for Ho³⁺ doped NLTO. Besides, a unusual emission band around 757 nm of Ho³⁺ in visible edge region was also clearly found. The temperature sensing properties of representative NLTO:Yb³⁺,Er³⁺/Ho³⁺ samples were assessed based on the fluorescence intensity ratio (FIR) technique of TCLs or NTCLs energy levels. The maximal absolute sensitivity (S_a) and relative sensitivity (S_r) were determined to be 0.0374 K^-1 (293 K) and 0.970% K^-1 (293 K) by taking the FIR of NTCLs ²H_11/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions in NLTO:Yb³⁺,Er³⁺. Simultaneously, the maximal S_a and S_r were 0.0306 K^-1 (573 K) and 0.776% K^-1 (373 K) using the FIR of NTCLs ⁵F₅→⁵I₈ and ⁵F₄,⁵S₂→⁵I₇ transitions in NLTO:Yb³⁺,Ho³⁺. Consequently, the temperature sensitivities above can be compared to reported results, which indicate that as-prepared materials have a promising application in optical temperature sensors field.