Formation and characterization of the solid solutions (CrxFe1−x)2O3, 0⩽x⩽1

The solid solutions (Cr_xFe_1–x)₂O₃, 0 x 1, were prepared by traditional ceramic procedures. The samples were characterized using X-ray diffraction, Mössbauer, Fourier transform infra-red (FT-IR) and optical spectroscopic measurements. In the whole concentration range two phases exist phase F, -(Cr_xFe_1–x)₂O₃, which is isostructural with -Fe₂O₃ and phase C, which is closely related to Cr₂O₃. Phase F exists in samples heated up to 900°C, for 0 x 0.95. Phase C exists from x0.27 to x=1 for samples heated up to 900°C and from x0.65 to x=1 for samples heated up to 1200 °C. For samples heated up to 900 °C, the solubility limits were 27.5 ± 0.5 mol% of Cr₂O₃ in -Fe₂O₃ and 4.0 ± 0.5 mol % of -Fe₂O₃ in Cr₂O₃. For the samples heated at 1200 °C the diffraction peaks for the F and C phases in the two phase region were severely overlapped and thus the solubility limits could not be determined accurately as for previous samples. ⁵⁷Fe Mössbauer spectra of the samples heated up to 1200 °C showed significant broadening of spectral lines and a gradual decrease of the hyperfine magnetic field with increase of x up to 0.50. For x0.7, a paramagnetic doublet with collapsing sextet was observed. The spectra were interpreted in terms of an electronic relaxation effect; however, an agglomeration of iron ions which would contribute to the superparamagnetic effect could not be excluded. The FT-IR spectra showed transition effects in accordance with the X-ray diffraction results. The most intense absorption bands, observed for the samples heated up to 1200 °C, were located at 460 and 370 nm (22 000 and 27 000cm^–1) for x 0.5, 500 and 360 nm for x < 0.3, and might be correlated with the strong enhancement of the pair transitions through antiferromagnetic interactions. The intensification of the ⁶A₁ 4T₁ Fe³⁺ ions in all spectra and the development of the absorption at 13000 cm^–1 due to a metal-metal charge transfer (Cr³⁺ Fe³⁺) transition, might be explained by exchange coupling which has been observed in some spinel compounds.     

X-ray diffraction and57Fe Mössbauer spectra of the system Fe2O3−Ga2O3

The influence of gallium substitution on the chemical and structural properties of haematite, α-Fe₂O₃, has been studied using X-ray diffraction and⁵⁷Fe Mössbauer spectroscopy. The presence of only α-(Ga _x Fe_1?x )₂O₃ phase is detected for the compositions withx between 0.01 and 0.90. A gradual decrease of the unit-cell parameters of α-(Ga _x Fe_1?x )₂O₃ with the increase of gallium substitution is measured.⁵⁷Fe Mössbauer spectra showed that the value of the magnetic hyperfine field of pure α-Fe₂O₃ decreases with increasing gallium for iron substitution. The hyperfine magnetic structure, which is observed for α-(Ga _x Fe_1?x )₂O₃ at room temperature, collapsed for the composition withx?0.50. The changes in the⁵⁷Fe Mössbauer spectra of the α-(Ga _x Fe_1?x )₂O₃ phase are discussed in the sense of the electronic relaxation and the superparamagnetic effects.