EFFECTS OF STRUCTURAL PHASE-TRANSITIONS ON SUPERCONDUCTIVITY AND MAGNETISM IN SUBSTITUTED DERIVATIVES OF LA2CUO4

Structural (neutron and x-ray diffraction) and magnetic (neutron diffr action and magnetic susceptibility) measurements of materials derived from La2CuO4 by isovalent and aliovalent substitution in the La3+ site have been performed. Such substitutions permit the tilts of the CuO6 octahedra (about the (110) and (110) axes of the undistorted 14/mmm st ructure), and the hole concentration in the Cu-O layers, to be indepen dently varied. The results of these measurements, in combination with those from other research groups, demonstrate that all four tilt struc tures, HTT (high temperature tetragonal, 14/mmm), LTO1 (low temperatur e orthorhombic 1, Bmab), LTO2 (low temperature orthorhombic 2, Peen) a nd LTT (low temperature tetragonal, P4(2)/ncm), support bulk supercond uctivity, with T-c(HTT) > T-c(LTO1) > T-c(LTO2) > T-c(LTT) at the same hole concentration. Furthermore, there is a strong suppression of sup erconductivity when the doped carrier concentration is 1/8 hole per Cu 2+ in the LTT and LTO2 phases of these materials. The reason for this T, suppression is not established. Finally, the magnetic structures ad opted by the Cu2+ spins in the insulating phases, determined by neutro n diffraction, are also influenced by the tilts of the octahedra. This may reflect a structural dependence of interlayer and intralayer magn etic anisotropies.