Linking Transition Metal Molybdate Formation to Polyoxometalate and Chloride Complex Speciation: An In Situ X‐Ray Total Scattering Study

A profound mechanistic understanding of material formation is imperative for developing tailored syntheses. However, insights into solution-based oxide syntheses remain limited. Hydrothermal MMoO₄ (M = Fe, Ni, Co) formation is governed by the initial pH, implying the involvement of pH-dependent polyoxomolybdate cluster chemistry. Nevertheless, the exact role of clusters in forming molybdates remains poorly understood. Herein, in situ X-ray total scattering is used to follow the hydrothermal syntheses of MMoO4 and related hydrates at various pH values, using Co-based reaction mixtures as our primary model system. Focusing on the structural investigation of noncrystalline species, no heteropolyoxometalates are found under the probed conditions. This contrasts our results from reported MWO4 formation pathways and thereby reiterates the element-specific nature of nucleation chemistry. Moreover, pH- and temperature-dependent formation of bimetallic phases that cannot be explained by polyoxomolybdate chemistry alone is observed. These results therefore point toward a product-directing role of the 3d metal complex speciation under hydrothermal conditions, emphasizing the mechanistic complexity of ternary oxide formation.