ABSTRACT
The Pechini and microwave-assisted hydrothermal syntheses of nanocrystalline Er3+ and Tm3+ co-doped MY(WO4)2, where M = Li, Na, K, double tungstates are reported. The obtained samples were characterized using standard X-ray powder diffraction (XRD) technique, Rietveld method, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and IR spectroscopy. The smallest crystallites (about 13 nm) could be obtained for the sodium samples synthesized by both the Pechini (for the resin calcined at 550 °C) and hydrothermal methods (synthesis at 230 °C). The average particle size of nanocrystalline powders increases with increasing temperature. It was found that nanocrystals retain the bulk structure with tetragonal and monoclinic symmetry for the sodium and potassium analogues, respectively. In contrast to this behaviour, LiY(WO4)2 undergoes a size-induced structural transformation from monoclinic (space group P2/n) to tetragonal (space group I41/a) symmetry. IR spectra of the synthesized sodium and potassium compounds are very similar to their bulk counterparts. IR spectra of the lithium analogues show, however, abrupt changes when the calcination temperature increases to 850 °C or higher. This behaviour is consistent with the size-induced phase transition in this compound.
ABSTRACT
Bi2WO6:Eu3+ samples were prepared by mechanically activated metathesis reaction and subsequent annealing at different temperatures of the as-prepared precursor. X-ray, TEM, Raman, IR, diffuse reflectance and luminescence studies of the prepared samples are presented. It was found that variation of the particle size have significant impact on phonon and emission properties of this material. It was observed that intensity of some Raman and IR bands significantly decreases and the bandwidth of Raman, IR and Eu3+ emission lines significantly increases with decreasing particles size. Moreover, it was observed that intensity ratios I((5)D0-(7)F2)/I((5)D0-(7)F1) and I(5D0_(7)F0)/I((5)D0-(7)F1) increase with decreasing particle size. The observed changes were attributed to phonon confinement effect, decrease in the orthorhombic distortion of the unit cell and concentration increase of surface defects.
ABSTRACT
Nanocrystals of tripotassium neodymium bis-phosphate(V) doped with ytterbium ions, K3Nd(PO4)2: Yb3+, were synthesized by Pechini method. The obtained grains, having an average size of about 40 nm, were characterised by X-ray, electron microscopic, electron absorption, luminescence and IR studies. Moreover, fluorescence decay studies were carried out at room temperature. The energy transfer from the Nd3+ to Yb3+ was described and discussed. The results were compared to those of the K3Nd(PO4)2 bulk crystal.
ABSTRACT
X-ray, electron transmission spectroscopy, vibrational and luminescence studies of LiIn(WO4)2:Cr+ nanoparticles prepared by Pechini method are reported. On annealing the sample several structural changes were observed resulting in a creation of three new, previously unknown polymorphs. It was shown that this tungstate undergoes two size-induced phase transitions from the structure similar to LiFe(WO4)2 into the structure similar to LiYb(WO4)2 and then into the structure of LiGa(WO4)2 type. These transitions occur for the critical particle size of about 100 and 30 nm, and they could be attributed mainly to some changes in the distribution of the sites occupied by Li+ and In3+ ions. Luminescence studies revealed decrease of the covalent character of chromium environment and electron-phonon coupling strength with decreasing size of the nanoparticles.
