Room-temperature co-precipitation synthesis of (Ca,Sr,Ba)WO4 solid solutions: Structural refinement, morphology and band gap tuning

In this paper, a series of homogeneous Sr1-xCaxWO4, Sr1-xBaxWO4, and Sr1-2xCaxBaxWO4 solid-solutions with well-defined morphologies have been successfully prepared by the co-precipitation method at room temperature without any surfactants or toxic solvents. X-ray diffraction, Rietveld refinement data, and Raman spectroscopy analyses confirmed the formation of Sr1-xCaxWO4 and Sr1-xBaxWO4 solid-solutions in the entire range 0 <= x <= 1. In contrast, the substitution of Ca and Ba for Sr in the Sr1-2xCaxBaxWO4 leads to phase-pure materials where x = 0.0-0.2. Elemental analysis performed by energy-dispersive X-ray spectroscopy demonstrated good agreement between nominal and experimental stoichiometries. Scanning electron microscopy shows homogeneous microstructures with diverse morphologies, such as microsphere, flower, octahedron, and shuttle. These structures consist of primary crystallites having an average size less than 60 nm. The band gap energies of Sr1-xCaxWO4 and Sr1-xBaxWO4 solid-solutions were estimated to be changed from 4.90 to 4.45 eV and 4.90to 5.05 eV, respectively, with increasing nominal composition (x). For the Sr1-2xCaxBaxWO4 solid-solutions, the band gaps were in the range of 4.88-4.92eV.