Mössbauer studies in the colloid system β-FeOOH–β-Fe2O3: structures and dehydration mechanism

Abstract

The Mössbauer spectrum of the recently reported β-iron(III) oxide has been measured and that of β-iron(III) oxide hydroxide re-investigated. The Néel temperature of β-iron(III) oxide lies between 300 and 380 K and the average magnetic field at 4.2 K is 49.5 tesla. Magnetic relaxation effects are observed. The spectrum above the Néel temperature is a broad asymmetric doublet, consisting of peaks having a continuous distribution of velocities, with the most probable quadrupole splitting occurring at 1.0 mm s^–1, and the most probable isomer shift, extrapolated to 295 K, occurring at 0.31 mm s^–1. While the distribution is within the known range of parameters of 6 or 5 co-ordinated Fe³⁺ in oxides, a proportion of tetrahedral Fe³⁺, with a lower isomer shift, could also exist within the broad spectral envelope, and a possible concentration range of from 0 to 40 % was estimated.

The spectrum of β-iron(III) oxide hydroxide is consistent with octahedral Fe³⁺. The high proportion of ions on the internal surfaces of the tubular structure results in a non-stoichiometric surface anion excess, and the conventional formula β-FeOOH has been reformulated as β-FeO_x(OH)_3–2x, where x≈ 0.9.

Four models of the anion vacancy distribution in β-iron(III) oxide are considered within the framework of the tubular structure retained during dehydration, and a defect structure is proposed which is consistent with the Mössbauer, X-ray, magnetic and B.E.T. data: all the internal Fe³⁺ ions are 5 co-ordinated whilst the surface Fe³⁺ ions are tetrahedrally co-ordinated.