Microwave-assisted synthesis and characterisation of divalent metal tungstate nanocrystalline minerals: ferberite, hubnerite, sanmartinite, scheelite and stolzite

A variety of metal tungstates have been synthesised in a microwave-assisted oven at 100 and 150degreesC and corresponding autogenous water vapour pressure within a time frame from 30 min to 2 h. The crystals formed are of submicrometer size and show equidimensional and needle-like crystals. Increasing the synthesis time and temperature results in the disappearance of the needles and the growth of the equidimensional crystals. The Raman spectra are consistent with those reported for the natural equivalents of these tungstates. The lead tungstate v(1)(A(g)) mode is observed at 904 cm(-1), while the v(2)(A(g)) vibration is observed as a strong band at 326 cm(-1) accompanied by a weak v(2)(B-g) at 356 cm(-1). The v(3)(B-g) vibration is located around 741 cm(-1), whereas the v(3)(E-g) is found at 751 cm(-1). Finally, the v(4)(B-g) vibration is completely overlapping with the v(2)(A(g)) vibration. The band around 271 cm(-1) may be the equivalent of the infrared active v(4)(A(u)) activated due to strain in the crystal. The two bands at 190 and 177 cm(-1) are ascribed to the v(Pb-O) and as a translational mode of the WO4 group in stolzite. (C) 2004 Elsevier B.V. All rights reserved.