Si Incorporation into Hematite by Heating Si−Ferrihydrite

Abstract

The presence of Si up to a Si/(Si + Fe) mole ratio (XSi) of 0.270, added during or after the preparation of a 2-line ferrihydrite, raised the temperature of its transformation to hematite (Tt), as determined by differential thermal analysis, from 340 °C without Si to 740 °C at XSi = 0.270. Deformation of the hematite unit cell and the greater suppression of crystal growth along a as compared to along c as XSi increased from 0 to 0.0403 were probably caused by structural incorporation of Si. Rietveld fits of X-ray diffraction (XRD) data showed a consistent Fe deficit of up to 1 Fe atom (from the ideal of 12.0) in the hematite unit cells. The magnetic hyperfine field (Bhf) at room temperature of the hematites heated to 800 °C fell regularly from 51.65 to 51.16 T as XSi increased from 0 to 0.0679. No separate SiO2 phase was detected by XRD, and the Morin transition was suppressed even at 4.2 K. At XSi ≥ 0.134, however, Bhf returned to 51.40 T, a value similar to that obtained at XSi ∼ 0.03. This increase ind...