Ferrihydrite unit cell

The unit cell parameters given by Towe and Bradley (1967) for 6-line ferrihydrite are a = 0.508 nm and c = 0.94 nm. There are four formula units per unit cell. A con-... 

The structure derived from a Rietveld fit of a neutron diffraction pattern of a 6-line ferrihydrite which showed more and sharper lines (Fig. 2.9, lower) than an XRD pattern, was in agreement with the structure proposed by Drits et al. (1993) except that it was not necessary to assume the presence of hematite in order to produce a satisfactory fit (Jansen et al. 2002). The unit cell of the defect free phase had a = 0.29514(9) nm and c = 0.9414(9) nm and the average domain size derived from line broadening was 2.7(0.8) nm. Since forced hydrolysis of an Fe solution at elevated temperatures will ultimately lead to hematite, it is likely that incipient hematite formation may occur under certain synthesis conditions. Neither these studies nor Mbssbauer spectroscopy, which showed only a singular isomer shift at 4.2 K characteristic of Fe, supported the presence of " Fe (Cardile, 1988 Pankhurst Pollard, 1992). However, the presence, at the surface, of some Fe with lower (<6) coordination, perhaps as tetrahedra (Eggleton and Fitzpatrick, 1988) which may have become unsaturated on heating, has been suggested on the basis of XAFS results (Zhao et al. 1994). 

A common method of synthesizing M-substituted oxides, particularly goethite and hematite is to add base to mixed M-Fe salt solutions to precipitate M-associated ferrihydrite. Most ions do not change their oxidation state, but incorporation of Mn and Co in goethite is preceded by oxidation of these ions to the trivalent state (Giovanoli Cornell, 1992). An indication of whether isomorphous substitution has occurred can be obtained from changes in the unit cell dimensions of the Fe oxides... 

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. 

A 6-line ferrihydrite whose water content was reduced from 146 to 26 g kg by heating for 3000 hr at 123 °C, while the oxalate soluble proportion decreased from 100% to 12% and the unit cell volume from 0.3091 to 0.3079 nm ... 

---