Microwave-hydrothermal preparation of alkaline-earth-metal tungstates
Microwave-hydrothermal processing was employed to synthesize nanostructured alkaline-earth-metal tungstate compounds in environmentally friendly conditions not yet found in the literature: 110 °C for times ranging from 5 to 20 min. X-ray diffraction showed that crystalline, single-phase materials were produced, while electron microscopy evidenced nanostructured particles with different morphologies. Raman spectroscopy was used to probe short-range ordering and also to obtain a reliable set of spectra containing all the Raman-active bands predicted by group-theory calculations. The Raman spectra showed no extra feature, indicating that our method gave well-ordered materials without important tetrahedral distortions. Sintered ceramics were submitted to polarized Raman scattering in parallel- and cross-polarized configurations. The Eg modes were totally evidenced in a cross-polarized configuration, while Ag and Bg modes appeared in the parallel arrangement, in complete agreement with the theoretical predictions. Photoluminescence measurements showed broad, complex emission bands, which varied from the blue to the red visible light regions. These results are related to shallow and deep defects into the band gap, through the presence of oxygen vacancies in the complex clusters with metal cations and tungsten atoms.