Characterization of copper-manganese hydroxysalts and oxysalts

Copper-manganese oxysalts of various Cu:Mn atomic ratios (from Cu:Mn = 100:0 to 0:100 in steps of 10) have been prepared by coprecipitation, at constant pH, from solutions of copper and manganese nitrates added to a solution of NaHCO3; the structure and chemical nature of the Cu-Mn oxysalts have been investigated by several complementary techniques such as XRD, magnetic susceptibility, reflectance spectroscopy and XPS to obtain information both on their bulk and on their surface properties. At high Cu:Mn atomic ratios the materials consist of Mn-containing malachite while at low Cu:Mn atomic ratios the product is Cu-containing rhodochrosite. For intermediate compositions a mixture of both phases is obtained. The introduction of Mn2+ into the malachite lattice gives a smaller cell volume increase than that expected from the difference between the ionic radii of octahedral Cu2+ and Mn2+; this is probably due to less octahedral site distortion. The presence of Cu2+ (d9) in the rhodochrosite lattice shows a much bigger cell volume decrease than that expected from ionic radii difference, the plausible reason for this behaviour being a higher covalent metal-oxygen bond character and/or octahedral site distortion. The Mn-malachite phase shows a surface enrichment in Mn; in this case it is suggested that the strain-energy contribution, arising from size effects, determines the segregation of manganese. There is homogeneity between the Cu:Mn compositional ratios in the bulk and at the surface only if the Cu-rhodochrosite phase is present. The surface appears enriched in copper when the two phases are both present; in this case the manganese-malachite phase probably tends to cover the copper-rhodochrosite one.