Cd-substituted goethites; a structural investigation by synchrotron X-ray diffraction

The structural and physical effects of partially substituting Cd for Fe in goethite have been investigated. The solubility of Cd (super 2+) in goethite is approximately 10 mol.%, i.e. Fe (sub 0.905) Cd (sub 0.095) OOH. The structures of the substituted goethites have been refined, using the Rietveld method, from synchrotron X-ray powder diffraction data. There is a progressive increase in the size of the unit-cell parameters and unit-cell volume, upon the incorporation of much larger Cd (super 2+) ion (0.95 Aa) compared with Fe (super 3+) (0.645 Aa) in the goethite structure, together with a reduction in crystallinity. Transmission electron microscopy measurements confirm the crystallite size decreases as the Cd (super 2+) content increases in goethite structure.