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

摘要

A variety of metal tungstates have been synthesised in a microwave assisted oven atud100 and 150°C and corresponding autogenous water vapour pressure within a timeudframe of 30 minutes to 2 hours. The crystals formed are of submicrometer size andudshow equidimensional and needle-like crystals. Increasing the synthesis time andudtemperature results in the disappearance of the needles and the growth of theudequidimensional crystals. The Raman spectra are consistent with those reported forudthe natural equivalents of these tungstates. The lead tungstate ν1(Ag) mode is observedudat 904 cm-1, while the ν2(Ag) vibration is observed as a strong band at at 326 cm-1udaccompanied by a weak ν2(Bg) . at 356 cm-1. The ν3(Bg) vibration is located aroundud741 cm-1, whereas the ν3(Eg) is found at 751 cm-1. Finally, the ν4(Bg) vibration isudcompletely overlapping with the ν2(Ag) vibration. The band around 271 cm-1 may beudthe equivalent of the infrared active ν4(Au) activated due to strain in the crystal. Theudtwo bands at 190 and 177 cm-1 are ascribed to the ν(Pb-O) and as a translational modeudof the WO4 group in stolzite.