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Mixed-valence europium-activated phosphors are receiving attention in many fields, such as lighting, anti-counterfeiting, optical recording, encryption, and temperature sensing. However, it remains difficult to construct mixed-valence europium-activated compounds due to the reductive and oxidative synthesis conditions required to obtain Eu2+ and Eu3+ ions, respectively. Herein, mixed-valence Eu2+/Eu3+ was realized in the CaBPO5 built by rigidly connected BO4 and PO4 tetrahedrons by partial Eu3+ → Eu2+ self-reduction in the air atmosphere. Commendably, the CaBPO5:Eu2+/Eu3+ phosphor exhibits excellent ratiometric temperature sensing performance with the maximum absolute and relative sensitivity being as high as 0.184 K−1 and 3.444% K−1 with good signal discriminability, due to the high and low, respectively, temperature-dependence of Eu2+ and Eu3+ emissions. The rapid dropping intensity of Eu2+ in CaBPO5 with increasing temperature was due to the small energy gap (∼0.48 eV) between the Eu2+-5d state and the conduction band. Our work not only provides a novel thermometer candidate but also enlightens researchers to a method of effectively designing new mixed-valence metal-ion activated luminescent thermometers via selective tetrahedrally coordinated rigid crystal structure. [ABSTRACT FROM AUTHOR]

Fine control of the photon emission of Eu2+ activated luminescent materials is essential for advanced optoelectronic applications. However, it remains difficult to design blue light excitable Eu2+ doped oxide red phosphors due to the ionicity of the oxide ions. Herein, we have mechanistically demonstrated that an Eu2+ incorporated CaYGaO4 phosphor exhibits efficient red emission at 650 nm, as a result of the Eu2+ 4f65d1 electronic state experiencing strong crystal field splitting. DFT calculations reveal that the Eu2+ ion is energetically favored in the octahedral Ca2+ site, which is responsible for the fascinating optical properties. We have also demonstrated a strategy of post thermal annealing for improving both the emission efficiency and thermal emission quenching properties of this newly developed red phosphor. By employing the as-fabricated phosphor as a red color convertor, a warm white lighting device with a high color render index of 89.7 and low corrected color temperature of 3780 K was constructed. Our findings not only provide a promising red phosphor candidate for lighting of excellent quality, but also help to stimulate new routes for designing and improving the photon emission of rare-earth luminescent materials. [ABSTRACT FROM AUTHOR]

Mixed-valence europium-activated phosphors are receiving attention in many fields, such as lighting, anti-counterfeiting, optical recording, encryption, and temperature sensing. However, it remains difficult to construct mixed-valence europium-activated compounds due to the reductive and oxidative synthesis conditions required to obtain Eu 2+ and Eu 3+ ions, respectively. Herein, mixed-valence Eu 2+ /Eu 3+ was realized in the CaBPO 5 built by rigidly connected BO 4 and PO 4 tetrahedrons by partial Eu 3+ → Eu 2+ self-reduction in the air atmosphere. Commendably, the CaBPO 5 :Eu 2+ /Eu 3+ phosphor exhibits excellent ratiometric temperature sensing performance with the maximum absolute and relative sensitivity being as high as 0.184 K -1 and 3.444% K -1 with good signal discriminability, due to the high and low, respectively, temperature-dependence of Eu 2+ and Eu 3+ emissions. The rapid dropping intensity of Eu 2+ in CaBPO 5 with increasing temperature was due to the small energy gap (∼0.48 eV) between the Eu 2+ -5d state and the conduction band. Our work not only provides a novel thermometer candidate but also enlightens researchers to a method of effectively designing new mixed-valence metal-ion activated luminescent thermometers via selective tetrahedrally coordinated rigid crystal structure.