Phase relations in the dioxide-trioxide region of some 3d transition Metal-WO ternary systems

The phases occurring in the MnWO, FeWO, CoWO, and NiWO systems at 1373°K have been determined using X-ray diffraction and electron and optical microscopy. Experimentally most attention was given to the MnWO system, where it was found that Mn entered as the Mn²⁺ ion into the WO₃ host matrix and formed a perovskite-related bronze Mn_xWO₃. The highest observed x-value in the bronze is about 0.027. In addition a metastable θ_w(Mn) oxide with the Mo₅O₁₄ structure and a disordered oxide of overall composition approximately (Mn, W)O_2.82 were found. The FeWO system was similar to the MnWO system but significant differences occurred in the CoWO and NiWO systems where M_xWO₃ bronze phases were not observed to form at 1373°K. The stability of the M_xWO₃ and the θ_w(M) oxides formed are discussed in terms of the ionic size of the M ions involved. It is suggested that M_xWO₃ bronzes are metastable if these M ions are small.