A STRUCTURAL DISTORTION IN THE 123-SUPERCONDUCTOR ISOMORPH LABA2(CU2NB)O8

A low temperature structural transition in the 123-superconductor YBa2Cu3O7-δ has been proposed involving zig-zag folding of the CuO-chains, but it has not been possible to demonstrate the resulting superlattice reflections because the CuO-chains are broken after relatively short distances unless δ=0. In the 123-isomorph LaB2(Cu2Nb)O8, which is not a superconductor, Y is replaced by La, and the CuO-chains by planes of NbO-octahedra. We find by direct observation of superlattice reflections that zig-zag folding of the NbO-octahedra can indeed occur in this structure type. The tilt pattern is similar to that of the low temperature phase of SrTiO3, with the same I4/mcm space group. © 1990.