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 decomposition of metastable double perovskites Ba2Cu1+x(CO3)1-xO2+δ or by reacting BaCO3, BaO2, and CuO does not apparently reveal, or shows only traces of, copper containing compounds. The resulting diffraction patterns are very close to aragonite-like γ-BaCO3. However, infrared and Raman spectroscopy data, electron diffraction, energy dispersion X-ray analysis performed on single crystal micrograins, and high-resolution electron microscopy reveal the existence of a mimetic compound corresponding to the formula Ba(CuOx)1-y(CO3)y, y ≤ 0.5, with lattice constants practically identical to those of γ-BaCO3. The new structure is characterized by the same hexagonal close packing of the Ba atoms, but the octahedral cavities are indifferently occupied by carbonate groups or CuOx planar units, linked to form uniaxial chains. The length order in the Ba(CuOx)1-y(CO3)y phase is extremely short reflecting the equal probability that a row of cavities is occupied by carbonate columns or CuOx chains. Under suitable conditions, not completely identified as yet, the double perovskite and the aragonite-like structures transform one into the other on thermal cycling.