Tetrahedrally coordinated rigid crystal structure enables partial self-reduction of mixed-valence europium for optical thermometric application

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.