Abnormal Eu 3+ → Eu 2+ Reduction in Ca 9- x Mn x Eu(PO 4 ) 7 Phosphors: Structure and Luminescent Properties.

β-Ca 3 (PO 4 ) 2 -type phosphors Ca 9- x Mn x Eu(PO 4 ) 7 have been synthesized by high-temperature solid-phase reactions. The crystal structure of Ca 8 MnEu(PO 4 ) 7 was characterized by synchrotron X-ray diffraction. The phase transitions, magnetic and photoluminescence (PL) properties were studied. The abnormal reduction Eu 3+ → Eu 2+ in air was observed in Ca 9- x Mn x Eu(PO 4 ) 7 according to PL spectra study and confirmed by X-ray photoelectron spectroscopy (XPS). Eu 3+ shows partial reduction and coexistence of Eu 2+ / 3+ states. It reflects in combination of a broad band from the Eu 2+ 4f 6 5d 1 → 4f 7 transition and a series of sharp lines attributed to 5 D 0 → 7 F J transitions of Eu 3+ . Eu 2+ /Eu 3+ ions are redistributed among two crystal sites, M1 and M3, while Mn 2+ fully occupies octahedral site M5 in Ca 8 MnEu(PO 4 ) 7 . The main emission band was attributed to the 5 D 0 → 7 F 2 electric dipole transition of Eu 3+ at 395 nm excitation. The abnormal quenching of Eu 3+ emission was observed in Ca 9- x Mn x Eu(PO 4 ) 7 phosphors with doping of the host by Mn 2+ ions. The phenomena of abnormal reduction and quenching were discussed in detail.