Gd³⁺-Driven Microstructure Regulation of Tm³⁺, Yb³⁺: Gd_xBi_2-xWO₆ for High-Sensitivity Temperature Sensing and Magnetism

Tm³⁺, Yb³⁺: Gd_xBi_2-xWO₆ series upconversion luminescent materials were synthesized via a solid-state method. Calculations and XRD analysis reveal that Gd³⁺ in the lattice exhibits both substitutional and interstitial doping. Compared with the NaYF₄ matrix, the Bi₂WO₆ material doped with Gd³⁺ has a lower Debye temperature (392.45 K), and the maximum phonon energy is low; thus, the probability of nonradiative transitions decreases. SEM images and EDS results reveal that the powder particle size ranges from 0.5 to 3 μm and that the elements are evenly distributed on the surface. The diffuse reflectance absorption spectra indicate that moderate Tm³⁺, Yb³⁺ and Gd³⁺ doping shifts the absorption band of the material towards longer wavelengths and decreases the band gaps. Under 980 nm laser excitation and a Gd³⁺ doping concentration of x = 0.2, the emission intensities of Tm³⁺ at 476 nm, 683 nm, 703 nm, and 804 nm increase by 1.8, 1.24, 1.35, and 81.9 times, respectively. Calculations reveal that these emissions in the range of 450 to 850 nm originate from three-photon or two-photon absorption. Gd³⁺ codoping can prolong the lifetime of the Tm³⁺ energy levels. When this sample was excited at 980 nm (518 mW), the temperature was characterized by FIR_703nm/476nm from 298 K to 573 K, and the obtained maximum relative sensitivity was 3.4907 K^-1 (298 K). The minimum temperature resolution was 0.0083 K (298 K). Compared with that of the undoped Gd³⁺ sample, the temperature sensitivity of the Gd³⁺-codoped sample increased by 92.35 times. Under an external magnetic field of 20 kOe and a temperature of 300 K, the magnetization of the sample doped with Gd³⁺ increased fourfold to 0.08174 emu/g. This material has potential applications in temperature sensing and magnetic resonance imaging.