Multifunctional NaEu(WO4)2: defect-tuned red emission and acetone sensing at room temperature†

Rare-earth double tungstate NaEu(WO₄)₂ was synthesized via a trisodium citrate (Na₃cit)-assisted hydrothermal technique, followed by calcination, to promote crystallinity and detailed investigations on their crystal structures and luminescence properties. In this study, the structural evolution of our samples synthesized with different amounts of Na₃cit was studied by employing X-ray diffraction, Rietveld refinement, Fourier transform infrared and Raman spectroscopy techniques. It was found that NaEu(WO₄)₂ belongs to the scheelite family with Na and Eu atoms occupying the same sites and antisite defects deforming EuO₈ dodecahedra. The modulation of W–O, Eu–O and angle splitting in the presence of antisite defects was identified. From in-depth X-ray photoelectron spectroscopy, we validated the deformation of the EuO₈ dodecahedron due to the presence of oxygen vacancies (V_Os), which originated from antisite defects. Herein, we show that the band gap of NaEu(WO₄)₂ is highly sensitive to defects; however, the ⁵D₀–⁷F₂ transition of Eu³⁺ at 615 nm with color coordinates (0.67, 0.33) is very robust, making NaEu(WO₄)₂ a suitable red phosphor material for near UV-type light-emitting devices (LEDs). We also identified that V_Os present in the EuO₈ dodecahedron act as active sites for acetone sensing (∼68% response to 100 ppm) with a response and recovery time of ∼3.3/10 s at room temperature, suggesting the potency of NaEu(WO₄)₂ as a multifunctional material with applications in LEDs and acetone sensors. In order to validate our experimental observations theoretically, we calculated the band structure and density of states of bare and antisite defects containing NaEu(WO₄)₂ using ab initio density functional theory and identified the sensing mechanism. We believe that our studies will be helpful in introducing new multifunctional applications of NaEu(WO₄)₂, while theoretical calculations will provide new electronic insights that may be used to understand the features of other double rare-earth tungstate materials.