Thermal enhancement of the 2H11/2 → 4I15/2 up-conversion luminescence of Er3+-doped K2Yb(PO4)(MoO4) phosphors

Abstract

Er³⁺ with different concentrations doped K₂Yb(PO₄)(MoO₄) phosphors were prepared by a solid-state reaction method, and the layered orthorhombic crystal structure of the samples was confirmed by X-ray diffraction (XRD). Under the excitation of a 980 nm laser, the up-conversion luminescence (UCL) emission spectra of K₂Yb_1−x(PO₄)(MoO₄):xEr³⁺ (x = 0.05, 0.1, 0.5, 1, 2, 4, 6, and 14 mol%) phosphors were obtained, and moreover, the critical quenching concentration of Er³⁺ was found to be 1 mol%. Furthermore, the up-conversion (UC) optical temperature behaviors of the Er³⁺-doped K₂Yb(PO₄)(MoO₄) phosphors were investigated. All phosphors showed a thermal enhancement of UCL corresponding to the ²H_11/2 → ⁴I_15/2 transition of Er³⁺ because of structural distortion, eliminating the energy transfer of Yb³⁺ to Er³⁺ through Er³⁺ single-doped isostructural K₂Bi(PO₄)(MoO₄) with thermal enhancement UCL of the ²H_11/2 → ⁴I_15/2 transition. In addition, the variable temperature XRD results suggest that the K₂Yb(PO₄)(MoO₄) host is a thermal expansion material, and the variable temperature confocal Raman spectra results confirm the thermal expansion and structure distortion of PO₄ or MoO₄ tetrahedra. With the increase in the sample temperature, the UCL emission intensity of the ²H_11/2 → ⁴I_15/2 transition of Er³⁺ ions at 663 K of K₂Yb_0.999(PO₄)(MoO₄):0.001Er³⁺ is up to 5.6 times that of 303 K. Through the fluorescence intensity ratio (FIR) technique, the corresponding maximum absolute sensitivity (S_A) was derived to be 0.00866 K⁻¹ at 550 K. The results indicate that the K₂Yb(PO₄)(MoO₄) phosphors with a low Er³⁺ doping concentration can be applied at high temperatures as optical thermometers.