Thermally Enhanced Upconversion Luminescence by Negative Expansion in Er3+/Yb3+ Co-doped ZrScW2PO12 Phosphors

In this work, a series of Er³⁺/Yb³⁺ co-doped ZrScW₂PO₁₂ phosphors are prepared by a facile high-temperature solid-state sintering method. The influence of the Er³⁺/Yb³⁺ co-doping on the phase structure, particle morphology, and chemical composition of ZrScW₂PO₁₂ is characterized by x-ray diffraction (XRD), scanning electron microscopy, and energy-dispersive x-ray spectroscopy (EDS). It is demonstrated that the substitution of Sc³⁺ ions with Er³⁺ and Yb³⁺ ions will cause lattice expansion, but will not change the crystal structure, particle size, or morphology. Additionally, these Er³⁺/Yb³⁺ co-doped ZrScW₂PO₁₂ phosphors produce two green emission peaks and one red emission peak under laser excitation at 980 nm. Impressively, when the temperature increases, the cell volume of the Er³⁺/Yb³⁺ co-doped ZrScW₂PO₁₂ phosphors shrinks without a phase transition, leading to the reduced energy transfer distance between sensitizer Yb³⁺ ions and activator Er³⁺ ions, and greatly enhanced green emission peak of Er³⁺. The maximum relative sensitivity (Sr) and absolute sensitivity (Sa) of Er³⁺/Yb³⁺ co-doped ZrScW₂PO₁₂ reaches 1.13%/K and 1.05%/K, implying that the rationally designed Er³⁺/Yb³⁺ co-doped ZrScW₂PO₁₂ phosphors have great potential in high-performance temperature sensors.