Single-Component Luminescent Materials Activated with Mixed-Valence Eu(+2,+3): Designed Synthesis, Luminescent Properties and Mechanisms

White LEDs are considered the next-generation light source as they are environmentally friendly and have high efficiencies. Therefore, researches are being conducted to meet the performance requirements of phosphors, which are the crucial components of white LEDs. Eu2+ and Eu3+ ions have different electronic structures, which lead to distinct photoluminescence properties. The characteristic emissions of Eu2+ and Eu3+ originate from the 4f-4f and 4f-5d transitions, respectively. In order to combine their respective features, the research of mixed-valence Eu ions into single-phase phosphors has become a hot research topic in recent years. The mixed-valence Eu ion-doped phosphors have tunable luminescence properties because they possess the respective properties of Eu2+ and Eu3+. From their respective characters of Eu2+ and Eu3+, this paper mainly reviews the progress of mixed valence Eu(+2, +3) ion-activated single-component luminescent materials in recent years from three aspects: unbalanced substitution, crystal field regulation, and other systems. In addition, the respective photoluminescence properties of Eu2+ and Eu3+ and the luminescence performances and mechanisms of the mixed-valence Eu ion-activated phosphors have been summarized. The luminescence performances and mechanisms have been summarized as well. All the research works carried out in this field provide inspiration for the investigation of new phosphors.