Synthesis, crystal growth and structural characterisation of barium-copper oxycarbonates Ba 2M xCu 2− x− y(CO 3) yO 2+δ (M  Cu, Cd, Ca; 0.05 < x < 0.25)

Abstract

Several BaCu oxycarbonate compounds with general formula Ba 2M x Cu 2− x − y(CO 3) y O 2+δ (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 CO 2 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 CuO 2 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 CuO 4 chains.