Iron oxides (a common name for iron oxyhydroxides and oxides) are important materials which have found application in traditional and advanced technologies. These materials can be utilized as pigments, catalysts, sensors, carriers for waste metal cations, etc. Iron oxides also play an important role in natural aquatic systems, sediments and soils. Various methods of iron oxide synthesis using «wet» chemistry were reported, such as a slow or forced hydrolysis of Fe(III)-salt solutions, crystallization of ferrihydrite gel, sol-gel, microemulsion and crystallization induced by ultrasound, microwave or γ-irradiation. As a rule, these syntheses are conducted at low concentrations of Fe(III)-salts. In that case it is relatively easy to obtain monodisperse particles; these studies, however, are generally important from an academic standpoint. In the present work we have focused on precipitation from concentrated Fe³⁺ salt solutions partially neutralized with concentrated NaOH. The precipitation systems were autoclaved at 160 ^oC. The precipitation process was monitored by analysis of the precipitate by XRD, ⁵⁷Fe Mössbauer and FT-IR spectroscopies, and FE SEM. The kinetics of the reference system showed a formation of β-FeOOH (akaganéite) which was after a prolonged time transformed into β-Fe₂O₃ (hematite). The addition of Na₂HPO₄ (concentrations in the final volume 0.010, 0.025, or 0.050 M) at the beginning of the precipitation caused significant changes in the phase composition of the precipitates. An additional appearance of α‑FeOOH (goethite) was observed as a direct result of adding Na₂HPO₄ to the precipitation systems.The specific adsorption of phosphates at acidic pH values, and the aggregation effect play an important role in the formation of particles in respect of their size and morphology. The formation of the end-product α-Fe₂O₃ was controlled by the dissolution/reprecipitation mechanism.

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