An unexpected role of lanthanide substitution in thermally responsive phosphors NaLnTe2O7:Eu3+ (Ln = Y and Gd)

Abstract

Bifunctional applications in solid state lighting and optical thermometry are attractive in the optical field. Despite Eu³⁺ doped phosphors are widely used in white-LEDs, phosphors with high temperature sensitivity remain rare. Herein, NaLnTe₂O₇:Eu³⁺ (Ln = Y and Gd) phosphors were synthesized using a rapid microwave-assisted solid-state (MASS) method to fulfill these applications. Under 395 nm excitation, NaLnTe₂O₇:Eu³⁺ exhibit the characteristic ⁵D₀→⁷F_J (J = 1–4) transitions of Eu³⁺. Substituting Gd³⁺ for Y³⁺ enhances the luminescence by approximately 2.42 times. Structural analyses reveal that the improved luminescent properties are attributed to the more distorted and appropriate coordination environment in NaGdTe₂O₇:Eu³⁺. Finally, white-LEDs using NaGdTe₂O₇:Eu³⁺ as the red-component produce white light with high R_a of 89. Furthermore, the distinct thermal responses of the ⁵D₀ → ⁷F_J transitions enable NaLnTe₂O₇:Eu³⁺ to function as temperature sensors via fluorescence intensity ratio (FIR) strategy. NaYTe₂O₇:Eu³⁺ possesses the maximum relative/absolute sensitivity of 1.45%/15.93% K⁻¹, whereas NaGdTe₂O₇:Eu³⁺ achieves the maximum relative/absolute sensitivity of 1.53%/30.24% K⁻¹. This work highlights the significance of cationic substitution in enhancing luminescent properties for multifunctional applications.