X-ray diffraction analysis of the phases forming from the decomposition of metastable double perovskites Ba 2 Cu 1+x (CO 3 ) 1−x O 2+δ or by reacting BaCO 3 , BaO 2 , and CuO does not apparently reveal, or shows only traces of, copper containing compounds. The resulting diffraction patterns are very close to aragonite-likeγ-BaCO 3 . 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(CuO x ) 1−y (CO 3 ) y ,y≤0.5, with lattice constants practically identical to those ofγ-BaCO 3 . 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 CuO x planar units, linked to form uniaxial chains. The length order in the Ba(CuO x ) 1−y (CO 3 ) y phase is extremely short reflecting the equal probability that a row of cavities is occupied by carbonate columns or CuO x 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.