Crystal Structures and Superconductivity in the La1.5−xBa1.5+Cu7+ySystem

Members of the solid solution system La1.5−xBa1.5+Cu7+ywith x = 0, 0.125, 0.25. 0.375 and 0.5 have been prepared both in air and under one atmosphere of flowing oxygen. In general, except for x =0, the oxygen annealed compounds are superconducting while the air-annealed samples are semiconductors. Tc’s measured by Meissner effect and resistivity data range to 70K. The crystal symmetry in all cases is apparently tetragonal as verified for x = 0.375 by variable temperature Guinier x-ray experiments at temperatures down to 8K. Crystal structures have been refined from neutron powder diffraction data. A correlation is noted between Cu-0 distances which connect adjacent Cu planes - the average copper valence and the occurrence of superconductivity.