A novel red-emitting Ca9Gd(PO4)7:Bi3+/Eu3+ phosphor with efficient energy transfer and high thermal stability

Abstract

In this work, a new system of Ca₉Gd_0.96−xBi_0.04Eu_x(PO₄)₇ phosphor is successfully obtained via high temperature solid-state method and calcined at 1300 °C. All samples can be well indexed to the XRD pattern of Ca₉Nd(PO₄)₇ (space group: R3c (161), JCPDS No. 45-0346), indicating the addition of Bi³⁺/Eu³⁺ does not change the structure of the crystal. According to the XPS results, Bi³⁺ and Eu³⁺ are successfully dispersed and stably embedded in Ca₉Gd(PO₄)₇ material. Ca₉Gd_0.96−xBi_0.04Eu_x(PO₄)₇ phosphors show different emission peaks at 420 nm (the ³P₁ → ¹S₀ transition of Bi³⁺) and 612 nm (the ⁵D₀ → ⁷F₂ transition of Eu³⁺) under near-ultraviolet excitation at 375 nm. The intensity of Eu emission varies as the concentration of Eu³⁺, and the emission intensity is strongest when the concentration of Eu³⁺ is about 80 at%. By monitoring the Eu³⁺ emission, a Bi³⁺ excitation band is observed on the PLE spectrum, indicating the occurrence of energy transfer from Bi³⁺ to Eu³⁺, with an efficiency of 65%. Furthermore, the value of critical distance R_c is calculated to be 11.2 Å, indicating that the main cause of the quenching mechanism is the multipole interaction. Meanwhile, the energy transfer mechanism of the Ca₉Gd_0.96−xBi_0.04Eu_x(PO₄)₇ samples is primarily controlled by the dipole–dipole interaction. The temperature-dependent (in the range of 300–500 K) analysis has been obtained. The emission intensity at 500 K can maintain 63% of the room temperature, and the activation energy E_a is 240 meV. The relative high activation energy indicates that the new systems of Ca₉Gd_0.96−xBi_0.04Eu_x(PO₄)₇ phosphors have good thermal stability. The Ca₉Gd_0.96−xBi_0.04Eu_x(PO₄)₇ phosphor developed in this work is an promising candidate for application in LEDs and expected to be widely used in lighting and display applications.