Structural and optical characterization of metal tungstates (MWO(4); M=Ni, Ba, Bi) synthesized by a sucrose-templated method

BACKGROUND: Metal tungstates have attracted much attention due to their interesting structural and photoluminescence properties. Depending on the size of the bivalent cation present, the metal tungstates will adopt structures with different phases. In this work, three different phases of metal tungstates MWO(4) (M= Ba, Ni and Bi) were synthesized via the sucrose templated method. RESULTS: The powders of BaWO(4) (tetragonal), NiWO(4) (monoclinic) and Bi(2)WO(6) (orthorhombic) formed after calcination temperatures of 750, 650 and 600°C for 4 h respectively are found to be crystalline and exist in their pure phase. Based on Scherrer estimation, their crystallite size are of nanosized. BET results showed NiWO(4) has the highest surface area. BaWO(4) exhibited less Raman vibrations than the NiWO(4) because of the increased lattice symmetry but Bi(2)WO(6) showed almost the same Raman vibrations as BaWO(4). From the UV-vis spectra, the band gap transition of the metal tungstates are of the order of BaWO(4) > Bi(2)WO(6) > NiWO(4). Broad blue-green emission peaks were detected in photoluminescence spectra and the results showed the great dependence on morphology, crystallinity and size of the metal tungstates. CONCLUSION: Three different phases of metal tungstates of BaWO(4) (scheelite), NiWO(4) (wolframite) and Bi(2)WO(6) (perovskite layer) in their pure phase were successfully prepared by the simple and economical sucrose-templated method. The highest surface area is exhibited by NiWO(4) while largest band gap is shown by BaWO(4). These materials showed promising optical properties.