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 Ca₂Al₂Si_1−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⁺, Ca₂Al₂Si_1−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 Ca₂Al₂Si_1−xO₇:Eu for noncontact optical thermometry, and also highlights mixed valence europium containing com pounds toward temperature sensing.

摘要

基于不同价态Eu²⁺/Eu³⁺的电子结构和电子晶格相互作用的差异, Eu混价共掺杂发光材料在比例型荧光温度传感领域具有应用潜力. 本文中, 我们制备了具有非化学计量的Ca₂Al₂Si_1−xO₇: Eu (x = 0.00, 0.02, 0.04)发光材料, 基于Si含量的控制, 实现了Eu²⁺和Eu³⁺的共存与调控, 并提出了2Ca²⁺ + Si⁴⁺ ←→ Eu²⁺ + Eu³⁺ + Al³⁺共取代模型, 解释了该体系的晶体结构稳定性与电荷平衡机理. 得益于Eu²⁺和Eu³⁺发光温度依赖差异大, 基于Ca₂Al₂Si_1−xO7:Eu发光材料的温度探针呈现出良好温敏性能, 其最大绝对灵敏度和相对灵敏度分别为0.024 K⁻¹(303 K)和2.46% K⁻¹(443 K), 且信号甄别度高. 通过位形坐标曲线分析, 揭示了Eu²⁺发光表现出强烈温度依赖特性的原因, 即: Eu²⁺离子5d能级上的电子易通Eu²⁺/Eu³⁺金属-金属电荷迁移带(IVCT)退激发所致. 本工作不仅表明Ca₂Al₂Si_1−xO7:Eu有望应用于非接触式温度传感, 同时也证明了具有混合价态Eu的发光材料在光学测温中的应用.