Non-stoichiometry in Ca2Al2SiO7 enabling mixed-valent europium toward ratiometric temperature sensing

Abstract

Eu2+/Eu3+ mixed-valence couple co-doped material holds great potential for ratiometric temperature sensing owing to its different electronic configurations and electron-lattice interaction. Here, the correlation of nonstoichiometry in chemical composition, phase structures and luminescence propertis of Ca2Al2Si1−xO7:Eu is discussed, and controlled Eu2+/Eu3+ valence and tunable emission appear with decreasing Si content. It is found that the 2Ca2+ + Si4+ ↔ Eu2+ + Eu3+ + Al3+ cosubstitution accounts for the structural stability and charge balance mechanism. Benefiting from the diverse thermal dependent emission behaviors of Eu2+ and Eu3+, Ca2Al2Si1−xO7:Eu thermometer exhibits excellent temperature sensing performances with the maximum absolute and relative sensitivity being 0.024 K−1 (at 303 K) and 2.46% K−1 (at 443 K) and good signal discriminability. We propose that the emission quenching of Eu2+ is ascribed to 5d electrons depopulation through Eu2+/Eu3+ intervalence charge transfer state, while the quenching of Eu3+ comes from multi phonon relaxation. Our work demonstrates the potential of Ca2Al2Si1−xO7:Eu for noncontact optical thermometry, and also highlights mixed valence europium containing com pounds toward temperature sensing.