Fig. 1. XRD of BEYTW∶xSi powder sintered in air and high-purity argon atmosphere

Fig. 2. SEM images. (a)-(c) BEYTW∶xSi-Air (x=0,4,6) samples sintered in air atmosphere; (d) BEYTW∶4Si-Ar sample sintered in pure argon atmosphere

Fig. 3. High resolution power XPS spectra of BEYTW∶ 4Si samples. (a) Air; (b) pure argon

Fig. 4. Spectral test results. (a) Relationship between absorbance of the sample and wavelength; (b) relationship between (αhv)² and photon energy

Fig. 5. (a) Upconversion fluorescence spectra and (b) fluorescence lifetime of samples prepared in air or high-purity argon atmosphere excited by 980 nm infrared light; (c) upconversion fluorescence spectra and (d) fluorescence lifetime of samples prepared in argon atmosphere with different oxygen biases