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

A variety of metal tungstates have been synthesised in a microwave-assisted oven at 100 and 150 °C 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 ν1(Ag) mode is observed at 904 cm−1, while the ν2(Ag) vibration is observed as a strong band at 326 cm−1 accompanied by a weak ν2(Bg) at 356 cm−1. The ν3(Bg) vibration is located around 741 cm−1, whereas the ν3(Eg) is found at 751 cm−1. Finally, the ν4(Bg) vibration is completely overlapping with the ν2(Ag) vibration. The band around 271 cm−1 may be the equivalent of the infrared active ν4(Au) activated due to strain in the crystal. The two bands at 190 and 177 cm−1 are ascribed to the ν(Pb–O) and as a translational mode of the WO4 group in stolzite.