Hydrothermal synthesis and structure of ferric molybdates from sodium carbonate solutions

Several new iron molybdates have been synthesized under hydrothermal conditions at relatively low temperatures (200 degrees C) to stabilize the Fe3+ oxidation state. The mineralizers are all sodium ion based and three new phases were isolated as high quality single crystals from various reaction stoichiometries, Fe(MoO4)(OH) (I), Na015Fe2Mo3O12(H2O)0.25H(2)O (IIa), and NaFe(MoO4)(2)(H2O) (III). The structures contain iron oxide octahedra arranged in chains (I), dimers (IIa), and isolated octahedra (III), that are linked by various molybdate oxyanions to form 3-D framework (I), 3-D framework with channels (IIa), and 2-D sheets in their iron molybdate motifs. Both I and III are lightly colored materials containing valence precise Fe3+, whereas IIa contains partially occupied Na (+) ions in columns within the lattice leading to concomitant valence mixing of Fe2+/Fe3+ with a corresponding darker color to the crystal. If sodium ions are not included in the synthesis and instead replaced by smaller Li (+) ions in the reaction, there is no alkali metal incorporation in the lattice and the resultant compound Fe2Mo3O12(H2O)0.25H(2)O (IIb) is formed. This is isostructural to IIa but without the partially occupied Na (+) ions in the channels which makes it another valence precise Fe3+ compound that is nearly colorless as expected since all transtions are spin forbidden in the high spin Fe3+ case. The role of carbonate as a mineralizer was examined since the reaction is highly acidic (pH similar to 1) in the absence of carbonate, and this does not lead to satisfactory product formation. The carbonate mineralizer increases the pH to 6-6.5, and this pH is maintained throughout the reaction, which does lead to high quality crystal products. Despite its absence in any of the products, the carbonate mineralizer plays an additional important (albeit unknown) role in the synthesis, because if simple hydroxide is used to increase the pH to 6-6.5, the resultant products are much lower in quality and yield.