Effect of thermal treatment on morphology and luminescence behaviour of potassium- and sodium-yttrium double tungstate nanopowders co-doped with holmium and ytterbium

NaY(WO4)2 and KY(WO4)2 nanopowders co-doped with Ho3+ and Yb3+ ions have been synthesised by a Pechini method. Calcination of the resin as precursor at 500 °C for sodium and at 670 °C for potassium leads to the creation of suitable double tungstate powders. Calcination at higher temperature leads to increase of size of the crystalline particle. Structural properties were characterised by the X-ray diffraction (XRD), Raman and IR spectroscopy. Sodium yttrium double tungstate belonging to scheelite structure crystallises in the tetragonal space group I41/a whereas the potassium one crystallises in the monoclinic space group C2/c. Spectroscopic properties i.e. absorption, excitation, emission and luminescence kinetics were investigated at room temperature. With excitation at 457 nm both compounds exhibit a strong emission band in the green region (545 nm) and much weaker emission bands in the red region (650 and 755 nm). Emission bands in the region at about 1000 nm characteristic for 2F5/2→2F7/2 of Yb3+ electronic transition testify that the energy transfer from holmium to ytterbium ions takes place in both sodium and potassium double tungstate when Ho3+ is excited. With excitation of ytterbium at 975 nm both compounds exhibit a strong 650 nm emission band in the red region and a much weaker 550 nm emission band in the green region.