Co-Doping Effect of Mn²⁺ and Eu³⁺ on Luminescence in Strontiowhitlockite Phosphors

A new series of Sr-based phosphates, Sr9-xMnxEu(PO4)(7), were synthesized using the high-temperature solid-state method in air. It was found that these compounds have the same structure as strontiowhitlockite, which is a beta-Ca-3(PO4)(2) (or beta-TCP) structure. The concentration of Mn2+ ions required to form a pure strontiowhitlockite phase was determined. An unusual partial reduction of Eu3+ to Eu2+ in air was observed and confirmed by photoluminescence (PL) and electron spin resonance (ESR) spectra measurements. The PL spectra recorded under 370 nm excitation showed transitions of both 4f5d-4f Eu2+ and 4f-4f Eu3+. The total integral intensity of the PL spectra, monitored at 395 nm, decreased with increasing Mn2+ concentration due to quenching effect of Eu3+ by the Mn2+ levels. The temperature dependence of Eu2+ photoluminescence in a Sr9-xMnxEu(PO4)(7) host was investigated. The conditions for the reduction of Eu3+ to Eu2+ in air were discussed.