SYNTHESIS, CRYSTAL-GROWTH AND STRUCTURAL CHARACTERIZATION OF BARIUM-COPPER OXYCARBONATES -X-Y)(CO3)(Y)O-2-LESS-THAN-X-LESS-THAN-0.25)(DELTA(M=CU, CD, CA, 0.05)

Several Ba-Cu oxycarbonate compounds with general formula Ba(2)M(x)Cu( 2-x-y)(CO3)(y)O-2+delta (M = Cu, Cd, Ca; 0 < x < 0.25) have been synth esised also in the form of single crystals by using an unconventional technique that exploits the fast growth kinetics of these compounds an d the high solubility of CO2 in the Ba-Cu-O melts. Extensive electron microscopy (SEM, HREM, ED, EDX) and X-ray diffraction structural studi es have been carried out and structural analyses on four particularly significant crystals, with different M and x, are reported, The main s tructural features of these compounds are derived from the layered per ovskite cuprates, Some crystal sites result indifferently occupied by Cu and C and therefore these copper oxycarbonates can be defined as ca rbocuprates, All the structures can be interpreted as an ordered subla ttice of the barium and part of the copper atoms, the M cations and th e related oxygen atoms superimposed to a partially disordered superlat tice of carbonate groups and CuO2 dumbbells, The equilibrium concentra tion of Cu increases with the crystallisation temperature at the expen se 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.