Spontaneous-reduction and photoluminescence tuning in singly-doped Ba5-yCay(PO4)3Cl:Eu2+/Eu3+ phosphors

•We report a novel spontaneous reduction phenomenon in Eu3+-activated Ba5(PO4)3 phosphors prepared in air atmosphere.•The fact of mixed valence was confirmed by PL/PLE, XPS and time-resolved spectroscopy techniques.•We designed a crystal-site engineering by introducing cation Ca2+.•The developed single-doped mixed-valence phosphor system has potential applications in phosphor-converted WLEDs. An abnormal spontaneous reduction of Eu (Ⅲ) was found and the single-doped white-light emission was realized by the substitution of cation Ca2+ [Display omitted] Developing a feasible scheme for solid-state lighting with high-quality white light remains a significant challenge. Particularly, tunable luminescence with single-component white emission have been widely studied to improve the luminescence performance of phosphor-converted white light-emitting diodes (pc-WLEDs) phosphors. In this work, a novel spontaneous reduction phenomenon was first found in Eu-activated apatite-type Ba5(PO4)3Cl phosphors prepared by a solid-phase reaction in air. Under 362 nm excitation, the luminescence results showed that a broad blue Eu2+ emission band with a peak at 437 nm appears unexpectedly in addition to the usual sharp orange-red emission of Eu3+. The mixed-valence fact of Eu2+/Eu3+ was further confirmed by X-ray photoelectron spectra and time-resolved spectroscopy techniques. The underlying mechanism could be explained by a charge compensation model. Accordingly, the local crystal-site engineering control of the luminescence in Ba5-yCay(PO4)3Cl:Eu2+/Eu3+ had been studied in detail by the introduction of Ca2+. It is found that the Ca2+ substitution of Ba2+ will lead to new splitting peaks of Eu3+ emission and a red-shift followed by a blue-shift of Eu2+ emission respectively, which could eventually adjust the luminescence of phosphors to the proper white light region. These phenomena are mainly ascribed to the introduction of Ca2+ to the structure modification of activator coordination environment. Finally, based on the thermal quenching results indicate that the potential application of the single-doped mixed-valence phosphor system in pc-WLEDs.