Aqueous Synthesis of Manganese Phosphate Hydrate Crystals for Creating Inorganic Pigment Materials

Manganese phosphate hydrate crystals were prepared from aqueous solutions containing MnCl₂, (NH₄)₂HPO₄, and citric acid. Switzerite [Mn₃(PO₄)₂·7H₂O] and niahite (NH₄MnPO₄·H₂O) phases were produced at room temperature, and then, hureaulite [Mn₅(PO₃OH)₂(PO₄)₂·4H₂O] particles were obtained by aging at 80 °C or by hydrothermal treatment at 100–180 °C. The lower aging temperature (80 °C) and the chelation of Mn²⁺ ions by citric acid provided a slower crystal growth, resulting in large hexagonal prism blocks of ca. 500 μm in size. These were found to be single crystals of hureaulite by powder and single-crystal X-ray diffraction analysis. Diffuse reflectance spectroscopy and evaluation using L*a*b* color parameters indicated that the large single crystals present a vivid pink color and high transparency.