X-ray and neutron-diffraction study of ${\mathrm{La}}_1{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{9-\delta }$: Influence of the Cu-O structure on $T_c$

X-ray and neutron diffraction studies on ${\mathrm{La}}_1{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{9-\delta }$ demonstrate that the structure of this compound closely resembles that of ${\mathrm{Y}}_1{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{9-\delta }$. The importance of oxygen ordering on the outer Cu planes for high-$T_c$ superconductivity is stressed. Broadening in the diffraction peaks as $T_c$ is decreased is indicative of greater disorder in the La or Ba sites and/or the oxygen configuration. Evidence for the existence of octahedral Cu-O configurations between unit cells, possibly associated with a Ba-doped ${\mathrm{La}}_{3-x}{\mathrm{Ba}}_{3+x}{\mathrm{Cu}}_6{\mathrm{O}}_{14+y}$ ($x=1\mathrm{}$) phase, is minimal.