Infrared study of some synthetic phases of malachite (Cu2(OH)2CO3)-hydrozincite (Zn5(OH)6(CO3)2) series

Synthetic malachite, hydrozincite and five monophasic mixed copper-zinc hydroxycarbonates have been studied by Fourier transform infrared (FTIR) spectroscopy at ambient and liquid nitrogen temperature in the region of 4000-400 cm(-1). The analysis of the spectra reveals that the samples containing up to 20% zinc retain the malachite lattice, thus forming solid solutions. The inclusion of zinc ions in malachite reflects on the positions and intensity of the bands corresponding to the internal modes of the carbonate ion, to the OH librations and to the Me-O interactions. For example, the higher and the lower frequency components of v3 shift to higher and lower frequencies, respectively. The intensity of the bands corresponding to v2 decreases with the zinc content increase. The spectrum of the sample Cu1.31Zn0.69(OH)2CO3 become diffuse and ill-resolved in the region of the Me-O interactions (region below 600 cm(-1)) and the corresponding bands are shifted to lower frequencies due to the weaker Zn-O interactions as compared with those of the copper ions. The internal modes of the carbonate ions in hydrozincite and aurichalcite are assigned and discussed taking into account the site symmetry and factor group symmetry. The OH and OD stretches (matrix-isolated HDO molecules) and the hydrogen bond strengths are interpreted in terms of Me-O interactions (synergetic effect), hydrogen bond angles and different hydrogen bond acceptor strengths of the oxygen atoms from the carbonate ions. It proves that the hydrogen bonds in hydrozincite are stronger as compared with those in malachite, irrespective of both the larger hydrogen bond lengths and the weaker Zn-O interactions in hydrozincite due to the higher hydrogen bond acceptor strength of the non-coordinated oxygen atom and the formation of bifurcated hydrogen bonds.