The Mössbauer spectra of precipitation-synthesized β-FeOOH recorded at 290, 240, 180, 160,120 and 80 K

For the preparation of β-FeOOH (sample S1), an aqueous solution of FeCl₃·6H₂O (0.3 M) was prepared and maintained under continuous stirring at 55–65 °C, after which NH₄OH (25 wt.% aqueous solution) was introduced dropwise until the pH of the reaction medium reached 6.0–6.5. The resulting brown suspension was then aged at 65 °C for 24 h to complete phase formation, followed by separation of the solid product by centrifugation and repeated washing with distilled water to remove residual ionic species. The washed precipitate was dried at 85–95 °C until constant mass, yielding the β-FeOOH material. To obtain additional information on the structural and phase composition of the synthesized β-FeOOH, low-temperature Mössbauer spectroscopy was employed. The Mössbauer spectra of sample S1 were recorded at 290, 240, 180, 160,120 and 80 K, allowing evaluation of the iron state and temperature-dependent hyperfine parameters characteristic of iron oxyhydroxide phases.