Enhancing the luminescence intensity of Eu3+-activated NaYb(MoO4)2 phosphors through bismuth doping: Judd–Ofelt analysis, lighting, and temperature-sensing applications

Abstract

In this work, we investigate the impact of Bi³⁺ doping on the luminescence properties of Eu³⁺-activated NaYb(MoO₄)₂ phosphors synthesized via the conventional solid-state reaction method. Rietveld refinement of X-ray diffraction data confirmed the tetragonal crystal structure (space group I4₁/a) for all samples. UV-visible absorption spectroscopy revealed an indirect bandgap of approximately 3.25 eV for the 5% Bi³⁺-doped sample. Under UV excitation, intense red emissions originating from the ⁵D₀ → ⁷F transitions of Eu³⁺ ions were observed at 589 nm, 613 nm, 652 nm, and 700 nm, along with near-infrared emission from Yb³⁺ at 997 nm, sensitized by the MoO₄²⁻ group. Photoluminescence (PL) analysis demonstrated an enhancement in the Eu³⁺ emission intensity with increasing Bi³⁺ concentration, reaching an optimum at 5% Bi³⁺ doping. Chromaticity coordinates confirmed a significant enhancement in the red emission intensity upon Bi³⁺ incorporation. Judd–Ofelt parameters and crystal field parameters were determined, revealing that Bi³⁺ doping influences the local environment of Eu³⁺ ions, impacting the luminescence properties. Furthermore, we explored the potential of Bi³⁺/Eu³⁺ codoped NaYb(MoO₄)₂ for optical thermometry based on the fluorescence intensity ratio (FIR) technique, achieving a high relative sensitivity (S_r = 1.14% K⁻¹). This work demonstrates the influence of Bi³⁺ doping on the luminescence properties of Eu³⁺ in NaYb(MoO₄)₂ and explores its potential for applications in temperature sensing and other optoelectronic devices.