engleski

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

Low crystalline ferrihydrite (2-XRD lines) was synthesized and thermally treated between 195 and 325oC in air. The obtained products were characterized by XRD, FT-IR and 57Fe Mössbauer spectroscopies. XRD showed that alpha-Fe2O3 did not appear up to 220oC, whereas alpha-Fe2O3 crystallized as associate phase to ferrihydrite at 245 oC. Crystallization of alpha-Fe2O3 at this temperature was also proved by FT-IR spectroscopy on the basis of IR band at 330 cm-1. On the other hand, FT-IR spectrum of the sample obtained after heating at 325oC fully corresponds to that of alpha-Fe2O3. The Mössbauer spectrum recorded at room temperature (RT) of the sample prepared at 220oC 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 Mössbauer spectrum of the sample heated at 245oC 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 Hhf = 527 kOe and quadrupole shift = -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 Hhf = 535 kOe and quadrupole shift = -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 oC correspond only to alpha-Fe2O3 fraction. For the sample obtained by heating at 245oC, a Mössbauer spectrum was recorded at 4.2 K in an applied field of 60 kOe. 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.

ferrihydrite; hematite; phase transition; 57Fe Mössbauer; XRD; FT-IR

Volumes 834-836 pp. 1-580 (27 May 2007) Molecular spectroscopy and molecular structure 2006 - A Collection of Papers Presented at the XXVIIIth European Congress on Molecular Spectroscopy, Istanbul, Turkey - September 3-8, 2006