LATTICE INSTABILITIES AND THE EFFECT OF COPPER-OXYGEN-SHEET DISTORTIONS ON SUPERCONDUCTIVITY IN DOPED LA2CUO4

Synchroton x-ray and neutron-diffraction measurements demonstrate that Nd3+ substitution at the La3+ site in both metallic and insulating La2-xSrxCuO4 induces low-temperature structural instabilities which reorient the tilting of the copper-oxygen octahedra. These structural transformations strongly affect superconductivity. In addition, there is a bicritical point involving phases of Bmab, Pccn, and P4(2)/ncm space-group symmetries in the structural phase diagram of La1.6-xNd0.4SrxCuO4 at x approximately 0.15. There are also anomalies in the structural transformation temperatures and the superconducting transition temperatures when x approximately 1/8, providing evidence for an electronic instability at this doping level.