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 degrees C in air. The obtained products were characterized by XRD, FT-IR and Fe-57 Mossbauer spectroscopies. XRD and FT-IR did not show the formation of alpha-Fe2O3 at 220 degrees C. The IR band at 330 cm(-1) for the sample obtained after heating at 245 degrees C indicated that at this temperature some alpha-Fe2O3 has crystallized. XRD also showed the presence of alpha-Fe2O3 in the sample obtained at 245 degrees C. XRD and FT-IR spectra of the sampleobtained after heating at 325 degrees C fully correspond to that of alpha-Fe2O3. The Mossbauer spectrum recorded at RT of the sample prepared at 220 degrees 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 alpha-Fe2O3 clusters. The RT Mossbauer spectrum of the sample heated at 245 degrees C showed a superposition of a quadrupole doublet and a distributed sextet, the latter believed to be due to poorly crystalline alpha-Fe2O3. At 78 K, two sextets were resolved. The sharper one, with a hyperfine field H-hf= 527 kOe and quadrupole shift 2(epsilon Q) = -0.12 mm s(-1), and fractional area of RA = 0.22, is due to alpha-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 H-hf = 535 kOe and 2(epsilon Q) = -0.03 min s(-1). The ferrihydrite sextet is still considerably broadened. Mossbauer spectra at RT and 78 K recorded for the sample obtained upon heating at 325 degrees C correspond only to alpha-Fe2O3 fraction. For the sample obtained by heating at 245 degrees C, a Mossbauer 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 alpha-Fe2O3. (C) 2006 Elsevier B.V. All rights reserved.