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.