Calcination-free crystallization of rare earth double tungstate Na(La_1 − xEu_x)(WO₄)₂ by sacrificial-template hydrothermal reaction, restrained concentration quenching, and optical thermometry

A series of Na(La_1 − xEu_x)(WO₄)₂ (NLW) phosphors were directly generated without calcination and organic additive via reaction of the (La_1 − xEu_x)(OH)SO₄ template with Na₂WO₄ by hydrothermal reaction. The XRD results show that pure Na(La_1 − xEu_x)(WO₄)₂ can be obtained by the reaction at 150 °C for 24 h or at 200 °C for 2 h under the molar ratio of WO₄²⁻/ Ln³⁺ = 5. The NaLa(WO₄)₂ has a tetragonal structure in I4₁/a space group, and has lattice parameters of a = b = 5.332 Å, c = 11.731 Å. The FT-IR results show that the SO₄²⁻ and hydroxyl vibrations of the template were hardly observed in the product and confirmed that phase conversion is complete. The UV-vis analysis yield bandgap values of 4.07 and 3.94 eV for NaLa(WO₄)₂ and Na(La_0.95Eu_0.05)(WO₄)₂ respectively. Photoluminescence excitation (PLE) studies found two distinct strong excitation peaks at ~ 395 nm and ~ 465 nm, with the former slightly stronger and both significantly stronger than other excitation peaks/bands. Under 395 nm excitation, the phosphors exhibited the strongest red emission at 614 nm (⁵D₀→⁷F₂ transition of Eu³⁺). The optimal Eu³⁺ concentration was found to be as high as 25% due to the layered structure of NLW phosphor and well isolated Eu³⁺ activator by [WO₄] polyhedron. An evaluation and comparison with other phosphors were conducted to assess the performance of optical temperature sensing. It was discovered that the NLW phosphor cannot be employed as a probe for optical thermometry via fluorescence intensity ratio (FIR) mode due to similar intensity loss rates with increasing temperatures for different emission peaks. The NLW was demonstrated to be well capable of temperature sensing via fluorescence lifetime (FL) mode. Fluorescence decay analysis found that higher measurement temperature shortened the lifetime of the 614 nm main emission, and the lifetime decreased in liner mode. The maximum absolute sensitivity (S_A) and relative sensitivity (S_R) are 43.0 × 10^− 4 K^− 1 (298–498 K) and 144 × 10^− 4 K^− 1(498 K), respectively.