Akaganeite 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). 

Akaganeite (named after the Akagane mine in Japan) is isostructural with hollan-dite. Compounds with this structure have a tetragonal or monoclinic unit cell. Bernal et al. (1959) and Keller (1970) both concluded that the unit cell of akaganeite was tetragonal with a = 1.000 nm and c = 0.3023 nm. The structural refinement of a natural sample using XRD and Rietveld analysis indicated, however, that the unit cell is monoclinic with a = 1.060 nm, b = 0.3039 nm, c = 1.0513 nm and p = 90.24° (Post Buchwald, 1991). There are eight formula units per unit cell. 

Fig. 2.6 Structure of akaganeite. a) Arrangement of octahedral double chains in tunnels with chloride ions in the centre ofthe tunnels, b) Ball-and-stick model with unit cell outlined.

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). 

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