Hematite band structure

Fig. 6.4 Band structure of hematite (Zhang et al., 1993, with permission).

Galvez et al. (1999) demonstrated that phosphorus up to a P/Fe mol ratio of 0.03 mol mol , can be incorporated into the hematite structure by heating P-con-taining 2-line ferrihydrite. Support for structural incorporation comes from a higher unit cell c (1.3776 => 1.3824 nm), IR-stretching bands of P-OH, a lowered intensity ratio of the XRD 104/113 lines and congruent release of Fe and P upon dissolution. 

The band positions of Fe oxides are also influenced by the substitution for Fe by other cations in the structure, as indicated partly by their colour. Scheinost et al. (1999) noticed a linear shift in the position of the Ai " Ti transition from 943 to 985 nm and that of the Ai " T2 transition from 653 to 671 nm for 47 synthetic goethites whose Al-substitution (Al/(Al-i-Fe) ranged between 0 and 0.33 mol mol (R = 0.92 for both). Mn "-substituted goethites showed bands arising from Mn " near 454 and 596 nm. The overall reflectivity in the visible range decreased as structural Mn increased from 0 to 0.20 mol mol (Vempati et al., 1995). The same effect has been observed for V "-substituted goethites (Schwertmann Pfab, 1994). The position of the EPT band of Mn "-substituted hematite shifted to 545 nm and that of the Ai " T2 transition to 700 nm (Vempati et al., 1995). The position of the same transition shifted from ca. 600 to 592 nm as the Al-substitution in hematite rose from 0 to 0.125 mol mol (Kosmas et al., 1986). Crystal size and crystal shape also have an effect on diffuse reflectance, as shown for hematite (see Fig. 6.12). As the crystals become smaller, reflectance increases and needles also reflect more than cubes, i. e. the colour becomes more vivid. 

Matsumoto [98] tried to summarize the work done on the electronic structure of iron oxides and concluded that the about 2 eV in the bandgap of the hematite semiconductor measured in photoelectrochemistry indeed is based on the 3d band transition between the Fe3+ ions, which supports a Mott-Hubbard insulator. 

Table 3.2. shows that data collected from the literature for onset and flatband potentials for various pH generally are related. However, there are some discrepancies, especially for the onset potential of nanosized hematite in pH 13 solution. One may conclude that the onset and flat band potential are dependent on the crystal structure, surface morphology and electrolyte medium. The data listed in Table 3.2. are plotted and shown in Fig. 3.4. 

Fig. 4.30. Molecular-orbital/band models to illustrate the electronic structures of hematite and ilmenite and based on MS-SCF-Za calculations on FeO , FeOs", and TiO/ clusters. The double arrows labeled (a), (b), (c) refer to electronic transitions giving rise to optical properties (after Vaughan and Tossell, 1978).

The smaller spheroidal and ellipsoidal bodies of type E, 4-8 jttm in size and preserved due to impregnation with carbonaceous material, fine-grained hematite, or greenalite, differ from the structures that have been described. These structures are found in black cherts, red jasper bands, and granule-containing greenalite cherts (see Fig. 34). The algal structures at the base of the Biwabik formation also contain unique filiform structures of type F their diameter is 1-2, sometimes up to 6 jam. 

For Fe oxides, FTIR spectroscopy provides a rapid means of identification. It can detect traces (1-2%) of goethite in a sample of hematite. In addition, low levels of impurities arising from insufficient washing, for example nitrate in ferrihydrite (band at 1384 cm ) and carbonate in goethite (ca. 1300 and 1500 cm ) can be detected. Broadening of the absorption bands reflects a decrease in crystal perfection (Cambier, 1986). Structural substitution of Fe by A1 (Schulze Schwertmann, 1984), Mn (Stiers Schwertmann 1985), and Cr (Schwertmann et al. 1989) causes a shift in the positions of the bands. 

The crystallinity of the Al hematite changes with the level of Al in the structure a maximum is often obtained at low to medium substitution (5-10 mol%). This corresponds to a minimum in XRD peak broadening and a maximum in y-ray absorption (Schwertmann et al., 1979 DeGrave et al., 1982 Murad and Schwertmann, 1986). Both X-ray diffraction peaks (Fig. 10-2) and IR absorption bands (Fig. 10-3) show a shift due to Al-for-Fe substitution. The surface area of the hematites is around 40 to 50 m /g. 

Since the reduction of adsorbed molecular oxygen competes with detachment of the reduced surface iron from the crystal lattice, it is the efficiency of detachment that decides to what extent oxygen inhibits the photochemical reductive dissolution of hydrous iron(III) oxides. The efficiency of detachment depends above all on the crystallinity of the iron(III) hydroxide phase and is expected to be much higher with iron(IIl) hydroxide phases less crystalline and thus less stable than hematite. Not only does the efficiency of the light-induced dissolution of iron(III) hydroxides depend on their crystal and surface structure, but so does the efficiency of photoxidation of electron donors. Leland and Bard (1987) have reported that the rate constants of photooxidation of oxalate and sulfite varies by about two orders of magnitude with different iron(III) oxides. From their data they concluded that this appears to be due to differences in crystal and surface structure rather than to difference in surface area, hydro-dynamic diameter, or band gap. ... 

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