Abnormal Eu → Eu Reduction in CaMnEu(PO) Phosphors: Structure and Luminescent Properties.

β-Ca(PO)-type phosphors CaMnEu(PO) have been synthesized by high-temperature solid-phase reactions. The crystal structure of CaMnEu(PO) was characterized by synchrotron X-ray diffraction. The phase transitions, magnetic and photoluminescence (PL) properties were studied. The abnormal reduction Eu → Eu in air was observed in CaMnEu(PO) according to PL spectra study and confirmed by X-ray photoelectron spectroscopy (XPS). Eu shows partial reduction and coexistence of Eu/ states. It reflects in combination of a broad band from the Eu 4f5d → 4f transition and a series of sharp lines attributed to D → F transitions of Eu. Eu/Eu ions are redistributed among two crystal sites, M1 and M3, while Mn fully occupies octahedral site M5 in CaMnEu(PO). The main emission band was attributed to the D → F electric dipole transition of Eu at 395 nm excitation. The abnormal quenching of Eu emission was observed in CaMnEu(PO) phosphors with doping of the host by Mn ions. The phenomena of abnormal reduction and quenching were discussed in detail.