Molten salt synthesis of barium molybdate and tungstate microcrystals

Pure microcrystalline barium molybdate BaMoO4 and barium tungstate BaWO4 materials were prepared by molten flux reaction using alkali metal nitrates as reaction media. The obtained crystals have rhombic shape and expose mostly (111) crystallographic planes. Their mean size depends on the flux temperature and the nature of the alkali metal cation. Monomeric molybdate and tungstate used as precursors yield target products already at 673 K whereas if polymerized ammonium oxosalts were used, then higher temperatures were necessary to obtain barium salts. The optimal temperature for the preparation of pure crystals with well defined shape was found to be near 773 K. UV–visible spectra have been measured to precise energy gaps in these important d0 transition metal compounds. The values of Eg for these two mixed oxides are 4.3 eV for BaMoO4 and 3.8 eV for BaWO4. Such values contradict to what can be expected from the known data on their structure and the relative electronegativity of W and Mo ions. The possible explanations of this observation are commented.