Thermally induced solid-state reaction of Fe2(SO4)3 with NaCl or KCl: a route to β-Fe2O3 synthesis

β-Fe2O3 is a rare crystalline polymorph of the ferric oxide family with an interesting application potential, e.g., in photocatalysis. In this study, the effect of different alkali salts addition, namely NaCl and KCl, on the preparation of β-Fe2O3 via thermally induced solid-state reaction was investigated. Two series of samples were prepared by calcining two different mixtures, Fe2(SO4)3 + NaCl (molar ratio 1:3) and Fe2(SO4)3 + KCl (molar ratio 1:3) at temperatures from 350 to 700 °C. Although the addition of either alkali salt led the preparation of β-Fe2O3 particles in wide temperature range up to 650 °C, differences in the overall phase composition and β-Fe2O3 purity were observed between the two series. The addition of KCl to Fe2(SO4)3 allowed the preparation of pure β-Fe2O3 (≥ 95%) in relatively wide temperature range of 450‒600 °C, while in the case of NaCl, pure β-Fe2O3 (≥ 95%) was found only in samples calcined at 500 °C and 550 °C. Other phases could be identified as additional ferric oxide polymorphs, γ-Fe2O3 and α-Fe2O3. The in situ XRD results suggest that, in the case of NaCl + Fe2(SO4)3 reaction, simultaneous formation of β-Fe2O3 and α-Fe2O3 may be possible between 350 and 500 °C, depending on the reaction conditions.