FROM CARBONATE-CUPRATES TO CUPRATE-CARBONATES - THE STRUCTURAL EQUIVALENCE OF CO3 AND CUOX GROUPS IN THE BA-CU-C-O SYSTEM

X-ray diffraction analysis of the phases forming from the decompositio n of metastable double perovskites Ba2Cu1+x(CO3)(1-x)O2+delta or by re acting BaCO3, BaO2, and CuO does not apparently reveal, or shows only traces of, copper containing compounds. The resulting diffraction patt erns are very close to aragonite-like gamma-BaCO3. However, infrared a nd Raman spectroscopy data, electron diffraction, energy dispersion X- ray analysis performed on single crystal micrograins, and high-resolut ion electron microscopy reveal the existence of a mimetic compound cor responding to the formula Ba(CUOx)(1-y)(CO3)(y), y less than or equal to 0.5, with lattice constants practically identical to those of gamma -BaCO3. The new structure is characterized by the same hexagonal close packing of the Ba atoms, but the octahedral cavities are indifferentl y occupied by carbonate groups or CuOx planar units, linked to form un iaxial chains. The length order in the Ba(CuOx)(1-y)(CO3)(y) phase is extremely short reflecting the equal probability that a row of cavitie s is occupied by carbonate columns or CuOx chains. Under suitable cond itions, not completely identified as yet, the double perovskite and th e aragonite-like structures transform one into the other on thermal cy cling. (C) 1997 Academic Press.