High temperature structural instability of the high-Tc superconductor Pb2Sr2Y1-xCaxCu3O8+†

Abstract High-temperature X-ray analyses have been performed to study the relation between the superconducting transition and the responsible phase. It was found that the superconducting orthorhombic phase forms by partly substituting Ca 2+ for Y 3+ only under very restricted conditions of temperature and partial oxygen pressure. Structural phase instabilities occurring in air near 340 °C and 550 °C can be understood by a change of oxidation states of Cu(I) from Cu + to Cu 2+ and Pb from Pb 2+ to Pb 4+ . It is strongly suggested that ordering of oxygen atoms in the PbO CuO e PbO layers plays an important role for the formation of the superconducting state in the orthorhombic structure.