Transformation of low crystalline ferrihydrite to α-Fe2O3 in the solid state

Low crystalline ferrihydrite (2-XRD lines) was synthesized and thermally treated between 195 and 325 °C in air. The obtained products were characterized by XRD, FT-IR and 57Fe Mössbauer spectroscopies. XRD and FT-IR did not show the formation of α-Fe2O3 at 220 °C. The IR band at 330 cm−1 for the sample obtained after heating at 245 °C indicated that at this temperature some α-Fe2O3 has crystallized. XRD also showed the presence of α-Fe2O3 in the sample obtained at 245 °C. XRD and FT-IR spectra of the sample obtained after heating at 325 °C fully correspond to that of α-Fe2O3. The Mössbauer spectrum recorded at RT of the sample prepared at 220 °C showed a broadened quadrupole doublet, whereas hyperfine-field distributions are observed at 78 and 12 K, presumably due to non-uniform chemical environments for the Fe3+ and/or to the shape and size distributions of the α-Fe2O3 clusters. The RT Mössbauer spectrum of the sample heated at 245 °C showed a superposition of a quadrupole doublet and a distributed sextet, the latter believed to be due to poorly crystalline α-Fe2O3. At 78 K, two sextets were resolved. The sharper one, with a hyperfine field Hhf = 527 kOe and quadrupole shift 2εQ = −0.12 mm s−1, and fractional area of RA = 0.22, is due to α-Fe2O3, which possibly shows the coexistence of antiferromagnetic and weakly ferromagnetic spin states. The much broader magnetic component is assigned to ferrihydrite. At 12 K, hematite has Hhf = 535 kOe and 2εQ = −0.03 mm s−1. The ferrihydrite sextet is still considerably broadened. Mössbauer spectra at RT and 78 K recorded for the sample obtained upon heating at 325 °C correspond only to α-Fe2O3 fraction. For the sample obtained by heating at 245 °C, a Mössbauer spectrum was recorded at 4.2 K in an applied field of 6 T. The ferrihydrite component clearly splits into a high- and low-field contribution. The heating of 2-line ferrihydrite caused a certain crystalline ordering and in the next stage the transformation to α-Fe2O3.