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

Abstract: 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 330cm−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 12K, 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 78K, two sextets were resolved. The sharper one, with a hyperfine field H hf =527kOe and quadrupole shift 2ε Q =−0.12mms−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 12K, hematite has H hf =535kOe and 2ε Q =−0.03mms−1. The ferrihydrite sextet is still considerably broadened. Mössbauer spectra at RT and 78K 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 4.2K in an applied field of 6T. 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. [Copyright &y& Elsevier]