Structural characterization of Cu-rich barium copper oxides

Further investigation of Ba2Cu3O5+δ by high-resolution transmission electron microscopy and resistance measurement as a function of temperature, together with a reconsideration of earlier results, have led to an increased knowledge of the structure and properties of Cu-rich barium copper oxide. It is shown that the CuO-rich end of the BaO-CuO pseudobinary contains three closely related phases with stoichiometry Ba2Cu3O5+δ and with their structures based on a primitive hexagonal Ba atom array. (i) Low-temperature ordered Ba2Cu3O6(δ=1.0) has a double Ba-layer repeat (c=1.138 nm) and is incommensurately modulated. (ii) Low temperature disordered Ba2Cu3O5+δ has a single Ba-layer repeat (c=0.569 nm) with δ decreasing from ∼ 1.0-0.0 and a corresponding reduction of the in-plane hexagonal unit cell dimension with increasing temperature. This form is metastable with respect to the ordered phase. (iii) High-temperature ordered Ba2Cu3O5+δ (δ unknown but probably <0.5) has a single Ba-layer repeat (c = 0.568 nm) but is significantly distored from hexagonal. The three structures are eighter disordered or possess low symmetry, and none contain the 2-dimensional sheets of corner-connected square planar CuO4 units which are common to all the high Tc superconducting copper oxides. A mixture of (i) and (ii) behaved as a semi-insulator. 7 refs., 7 figs., 2 tabs