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 Eu and Eu ions, respectively. Herein, mixed-valence Eu/Eu was realized in the CaBPO built by rigidly connected BO and PO tetrahedrons by partial Eu → Eu self-reduction in the air atmosphere.