High-Pressure Synthesis and Electron Microscopic Study of 1212 Lead Cuprates (Pb, Cu)Sr2(Y, Ca)Cu2Oz

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Abstract
Superconducting 1212 lead cuprates, (Pb, Cu)Sr2(Y, Ca)Cu2O z (z>7.0), are synthesized under high pressures of oxygen. They crystallize in a tetragonal or orthorhombic structure, depending on temperature, oxidation state and Ca content. The orthorhombic phase is stable under higher oxidation states. Electron microscopic studies have revealed that an incommensurately modulated structure in the tetragonal phase changes to a nearly commensurate superstructure ( A=a, B=∼3b and C=2c) in the orthorhombic phase. The formation of the superstructure is due to the ordering of lead and copper atoms in the (Pb, Cu)–O sheets. High-pressure samples exhibit superconductivity below 20–60 K, depending on synthesis conditions. The superconductivity is suppressed by a slight increase in oxygen content and decrease in calcium content under the synthesis conditions used in the present study.