Lepidocrocite unit cell

Lepidocrocite (Greek lepidos = scale, flake and krokoeis = saffron-coloured) is iso-structural with boehmite (Tab. 2.1). Unlike goethite and akaganeite which have a turmel structure, lepidocrocite is a layered compound. The orthorhombic unit cell contains four formula units and has the edge lengths, a = 1.2520(6) nm h = 0.3873(2) nm and c = 0.3071(6) nm (Ewing, 1935 Oles et al., 1970 Christensen Norlund-Christensen, 1978). 

Fig. 2.5 Structure of lepidocrocite. a) Cubic close packed anion arrangement and distribution of cations over the octahedral interstices. Projection on (001). Octahedral arrangement and unit cell outlined, b) Projection of anion close packing on (010). Octahedral arrangement and unit cell outlined, c) Projection of anion close packing on (001). Dashed circles represent Fe in the next lo A/er layer, d) Arrangement of oc-...

Oxidation of FeCl2 solutions at pH 7-8 in the presence of soluble Al led to Al substituted lepidocrocite with an Al/(Fe-i-Al) of up to 0.14 the a, h and c unit cell edge lengths fell linearly as the Al content increased (Schwertmann Wolska, 1990). The Bhf at 4.2K also decreased linearly over the same range of Al substitution, i. e. 

At room temperature, akaganeite is, like lepidocrocite, paramagnetic. It becomes antiferromagnetic below the Neel temperature of 290 K (Murad, 1988). The value of Tn and the strength of the magnetic interactions are variable and depend upon synthesis conditions (i.e. the temperature and the length of the hydrolysis period). These influence the amount of interstitial water in the compound, which in turn induces spin reduction. Tn decreases linearly to 250 K as the H20/unit cell rises to 0.02 mol mor (Chambaere De Grave, 1984 a). 

Fig. 13.04. Idealized plan of the orthorhombic structure of lepidocrocite, FeO(OH), projected on a plane perpendicular to the z axis. The heights of the atoms are indicated in fractions of the c translation. Hydrogen bonds are represented by broken lines, and crosses indicate the comers of the unit cell.

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