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

A new series of Sr-based phosphates, Sr_9-xMn_xEu(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 β-Ca₃(PO₄)₂ (or β-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 Sr_9-xMn_xEu(PO₄)₇ host was investigated. The conditions for the reduction of Eu³⁺ to Eu²⁺ in air were discussed.