Hematite corundum-type structure

Corundum-type Magnetic Oxide Surfaces. The substrate hematite with the corundum-type crystal structure is an antiferromagnet below 963 K. In the corundum-type structure of hematite, pairs of ferric ions are in a row spaced by single vacant sites along the <111> direction. The positions of ferric ions in each pair are shifted slightly upward or downward in the <111> direction. We denote these lattice positions as up and down sites (Au and A ), respectively. 

Figure 9. Simplified model of the (111) surface of the corundum-type structure, (a) A view of the surface from a direction slightly shifted from <111>. Only metal ions of the zeroth, first, and second layers are shown, (b) A section of the surface along the arrows depicted in part a. Hexagonally close-packed oxide ion layers are shown with lines. Surface protons are not shown, (c) A divalent Co-57 or pentavalent Sb-119 ion on the zeroth metal ion layer, (d) Aquo or hydroxyl complex of divalent Co-57 or pentavalent Sb-119 hydrogen-bonded to the surface oxide ion layers of hematite.

Corundum is one of the many polimorphs of alumina (A1203). The corundum-type structure is the structural shape of hematite (aFe203). In Figure 2.12, layers A and B of a close-packed arrangement of spheres, and the formation of the corresponding octahedral and tetrahedral sites are shown [51,52]. 

Relatively little is known about the non-stoichiometry, disorder, and transport properties of hematite Fe203 (a-corundum structure). However, it can be said with certainty that Fc203 exhibits a small excess of cations, and that this excess is dependent upon the oxygen partial pressure. Furthermore, Fc203 is a n-type conductor over a wide range of oxygen partial pressures, and the diffusion coefficients of oxygen ions and of iron ions are of the same order cm /sec at r 1200 °C) [40]. 

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