Synthesis, crystal growth and structural characterisation of barium-copper oxycarbonates Ba2MxCu2-x-y(CO3)yO2+? (M ? Cu, Cd, Ca; 0.05 < x < 0.25)

Several Ba?Cu oxycarbonate compounds with general formula Ba2MxCu2-x-y(CO3)yO2+? (M?Cu, Cd, Ca; 0 < x < 0.25) have been synthesised also in the form of single crystals by using an unconventional technique that exploits the fast growth kinetics of these compounds and the high solubility of CO2 in the Ba?Cu?O melts. Extensive electron microscopy (SEM, HREM, ED, EDX) and X-ray diffraction structural studies have been carried out and structural analyses on four particularly significant crystals, with different M and x, are reported. The main structural features of these compounds are derived from the layered perovskite cuprates. Some crystal sites result indifferently occupied by Cu and C and therefore these copper oxycarbonates can be defined as carbocuprates. All the structures can be interpreted as an ordered sublattice of the barium and part of the copper atoms, the M cations and the related oxygen atoms superimposed to a partially disordered superlattice of carbonate groups and CuO2 dumbbells. The equilibrium concentration of Cu increases with the crystallisation temperature at the expense of the carbonate content. The resulting copper-rich phases show low electrical resistivity at high temperature due to the formation of a three-dimensional system of CuO4 chains