Alkali metal impact on structural and phonon properties of Er³⁺ and Tm³⁺ co-doped MY(WO₄)₂ (M = Li, Na, K)...

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.