Investigation of Whitlockite Structured Inorganic Phosphate Phosphor for Solid-State Lighting Applications

Abstract

Phosphor converted white LEDs have been regarded as a reliable solid-state lighting source. Studies over the past few years have shown an increase in the development of phosphate-based phosphors, where whitlockite-structured phosphors have attracted intensive attention. In this research, the Ca₉MgLi(PO₄)₇-based compounds activated with Pr³⁺/ Eu²⁺/Dy³⁺ have been investigated as a potential candidate of phosphors. The phosphor materials were synthesised through the conventional solid-state reaction method. A detailed investigation of the structure was carried out using high-resolution neutron and X-ray diffraction. Results show that the compound was comprised of 5 anionic sites. The M1, M2, and M3 sites were identified to be fully occupied with the Ca²⁺ and their respective activator. The M4 site was discovered to be occupied with Li⁺ and Mg²⁺, while M5 site was occupied with Ca²⁺ and Mg²⁺. The refinement results and compositional analysis using EDS shows the synthesised phosphor has a final composition that is close to the proposed formula of Ca₉MgLi(PO₄)₇. The photoluminescence investigation on Eu²⁺ activated phosphors revealed that the specific phosphor has the emission peak of 412 nm under the excitation of 279 nm, which is not suitable for lighting applications. However, it indicates that the activator has three preferential sites in M2, M1, and M3, respectively. Further investigation has revealed that Pr³⁺ and Dy³⁺ have the potential for solid-state lighting applications with good thermal stability.