The exploratory synthesis of iron molybdates is of interest due to iron's ability to have different oxidation states and coordination environments. Also, molybdenum(VI) tends to form the tetrahedral (MoO4)2− making it a good building block for impressive magnetic structures because it is stable, versatile, magnetically silent, and can form trigonal symmetry that may cause geometric frustration. Traditionally, metal molybdates are synthesized by high-temperature methods (700 to 1200 °C). This can induce extrusion of oxygen, which causing the formation of lattice defects and lower valent metal ions, which can lead to problems in detailed measurements of physical properties. Our group has successfully growing different transition metal oxides with a variety of oxyanions building blocks using the hydrothermal technique since this relatively low-temperature method reduces the probability of decomposition of the metal oxyanions. In this work, a low-temperature hydrothermal route (200°C - 220°C) was utilized to synthesize a range of new iron molybdate structures. These reactions are carried out in aqueous solutions using various alkali/alkaline earth metal chlorides and carbonates as mineralizers in Teflon acid digestion vessel, providing an easy entry into new low-temperature products. Alkali/alkaline metal halides hydrothermal reactions can play a variety of roles. They can incorporate into the product as a salt lattice, or they can act only as a mineralizer to solubilize the initial chemicals and forming higher quality crystals. Even small changes in the halide mineralizer's identity and concentration lead to significant changes in product distribution and quality. Due to the extreme sensitivity of chemistry to the halide mineralizer, other mineralizers were examined. It was observed that carbonate plays a crucial role in forming new structures, and this role was examined in preliminary studies. It was found that a low pH environment would not allow the formation of relatively well-defined crystals. Therefore, carbonate is very useful in preparing a suitable midrange pH to enable high-quality crystal growth in these low-temperature hydrothermal reactions. Several novel iron molybdate structures are presented, disclosing new oxyanion structural chemistry and complex chemical formulas.

Role of carbonate and halides in the hydrothermal synthesis of iron molybdates

ACS Spring Meeting 2021

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