The structure of the non-superconducting phase La₃Ba ₃Cu₆O_14+x and its relation to the high-T _c superconductor YBa₂Cu₃O_7-δ

The compound YBa₂Cu₃O_7-δ has recently received intense interest because of its high-temperature (T _c≈95K) superconducting properties¹. Shortly after its discovery, many authors independently confirmed that the structure could be regarded as an ortho-rhombically distorted tripled perovskite^2-5. We have investigated La₃Ba₃Cu₆O_14+x because it is chemically similar to YBa₂Cu₃O _7-δ and because initial work⁶ indicated a poor structure determination based on a cell not dissimilar from YBa ₂Cu₃O_7-δ. Here we report time-of-flight neutron powder diffraction results which confirm that the structure of La ₃Ba₃Cu₆O_14+x is isomorphous with the tetragonal variant of YBa₂Cu₃O_7-δ; in particular, the copper coordination and calculated valencies of both compounds agree closely. The apparent contradiction of stoichiometries between these two phases is resolved by ordering of the large cations consistent with a formulation La(La_0.25Ba_0.75)₂Cu ₃O_7+1/2x. Stoichiometries for both quenched and oxygen-annealed materials are in excellent agreement with those obtained by Er-Rakho et al.⁶, yielding more than seven oxygens per formula unit. The excess oxygen is located in the planes containing the chains of CuO ₄ planar groups in YBa₂Cu₃O _7-δ. The present results extend the upper limit of oxygen stoichiometry from RA₂Cu₃O₇ to RA ₂Cu₃O_7.2, but the materials studied here remain semiconducting down to 7 K (Fig. 1). Thus, although there is a remarkable structural stability from RA₂Cu₃O₆ (ref. 7) to RA₂Cu₃O_7.2, superconductivity appears to be confined to the range RA₂Cu₃O_6.5 to RA ₂Cu₃O₇.