Color tuning of single-phased Ca9KMg(PO4)7:Sr2+, Eu2+, Mn2+ phosphor based on cation substitution and energy transfer for WLED applications

Abstract

Utilizing the structural advantages of β-Ca₃(PO₄)₂ and a cation substitution strategy, the Ca_8.96-xSr_xKMg(PO₄)₇:0.04Eu²⁺ series phosphors were synthesized to achieve directional control of CIE color coordinates. Structural refinement indicated that Sr²⁺ substitution resulted in lattice expansion, while the 3.65 eV band gap of the Ca₇Sr₂KMg(PO₄)₇ matrix offered ideal sites for the activation of Eu²⁺ ions. The system demonstrates broadband emission across the entire visible spectrum under 365 nm excitation, with a full width at half maximum (FWHM) reaching 198 nm. Continuous color tunability from cold white to warm white and yellow light is achieved by modulating the concentration of Sr²⁺ doping. Crystal field theory, in conjunction with Gaussian fitting analysis, indicates that the multi-site occupation of Eu²⁺ ions at the Ca3 (437 nm), Ca2 (461 nm), Ca1 (505 nm), and K (573 nm) sites is the primary mechanism responsible for spectral broadening. The incorporation of Mn²⁺ facilitates an effective energy transfer channel between Eu²⁺ and Mn²⁺, achieving an efficiency of 81.83% and broadening the luminescence spectrum into the red light region. The evaluation of quantum efficiency indicates that the Ca_6.76Sr₂KMg(PO₄)₇:0.04Eu²⁺, 0.20Mn²⁺ co-doped system shows an increase in internal quantum efficiency (IQE) from 32.3% to 37.1% relative to the Eu²⁺ singly-doped system, as well as an 18% enhancement in thermal stability at 423 K. The encapsulated LED device exhibits superior performance (Ra = 92.9, CCT = 2496 K), providing a novel approach to full-spectrum WLED design.