Structural, Morphological, Vibrational, and Photoluminescence Study of Sol–Gel-Synthesized Tm3+:NaGd(WO4)2 Blue Phosphors

Abstract

The photoluminescence properties of sol–gel-derived NaGd1−xTmx(WO4)2 sub-micron powders were studied. Formation of a crystalline material with a tetragonal structure and the I41/a space group was confirmed by powder x-ray diffraction measurements. The surface morphology and size distribution of the powders were investigated by scanning electron microscopy. Double tungstate formation was confirmed by Fourier-transform infrared and Raman spectroscopy and the corresponding vibrations were also assigned. A broad charge-transfer band (CTB) and intra-configurational 4f–4f transitions in the excitation spectra are discussed. Strong blue emission was observed at 455 nm (1D2 → 3F4) for both CTB and intra-configurational 4f–4f excitation bands, and strong emission as a result of intra-configurational 4f–4f excitation only was also observed. The concentration dependence of emission was studied, and emission was found to be maximum for x = 0.05. The corresponding critical distance was also determined and a mechanism is proposed for energy transfer by Tm3+ ions in the NaGd(WO4)2 matrix. The Commission International de l’Eclairage (CIE) coordinates were calculated and fell in the blue region of the CIE diagram. The coordinates of the blue intensity maximum were also matched with European Broadcasting Union standard values.