Improvement of upconversion and temperature sensing properties in Ho3+/ Yb3+co-doped Ca0.5Gd(WO4)2 phosphor via incorporation of Bi3+

A series of Ca0.5Gd(WO4)2: Ho3+/Yb3+/Bi3+ phosphors prepared by solid-state reaction method were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy disperse spectroscopy (EDS), elemental mapping, upconversion (UC) spectroscopy and decay time analysis. Under 980 nm excitation, four up converted emission bands of Ho3+ were observed in all samples. The concentration quenching effect of the UC emission appears by changing the concentration of Ho3+ in Ca0.5Gd(WO4)2: x mol% Ho3+, 40 mol% Yb3+ and Yb3+ in Ca0.5Gd(WO4)2: 1 mol% Ho3+, y mol% Yb3+, and the optimal concentration of Ho3+ and Yb3+ were determined to be 1 and 40 mol%, respectively. Compared to Bi3+-absent sample, the upconversion luminescence (UCL) intensities of green emission (540 nm) and red emission (644 nm) in Ca0.5Gd(WO4)2: 1 mol% Ho3+, 40 mol % Yb3+ via 15 mol% Bi3+ doping were increased by 2.5 and 3.6 times, respectively. The involved UCL mechanism based on the power dependent UC emission spectra were analyzed in detail. Additionally, the temperature sensing performances based on non-thermally coupled levels (NTCLs) 5F4(5S2) -> 5I8, and 5F5 -> 5I8 of Ho3+ ions and thermal quenching properties of the samples were also investigated in the range of 298-573 K. The maximum relative sensitivity (SR) of Ca0.5Gd(WO4)2: 1 mol% Ho3+, 40 mol% Yb3+ and Ca0.5Gd(WO4)2: 1 mol% Ho3+, 40 mol% Yb3+, 15mol% Bi3+ reached 0.0094 K-1 and 0.0112 K-1 at 298 K, respectively. The results indicated that doping Bi3+ ions can enhance the UCL intensity and optical temperature sensing of Ca0.5Gd (WO4)2: Ho3+/Yb3+.