High TcSuperconducting Oxides and Solid State Chemistry

Abstract

Abstract The valences of copper, cerium and bismuth and the oxygen content of (La1−xMx)2CuO4-δ (M=Sr, Ba and Ca), Nd1.85Ce0.15CuO4-δ and Bi2.00Sr1.88Ca1.00CuO2.14Oy systems were determined by the chemical analysis and thermogravimetry. Also the relationship between the copper valence and the critical temperature Tc was revealed. The δ vs. log Po2 curves of (La1−xMx)2CuO4-δ had a sigmoid like shape which was characteristic of perovskite type oxides. The copper valence at a constant oxygen partial pressure increased proportionally with the increase in x and deviated from the linear relationship above a certain value of x. This deviation arised from the formation of oxygen deficit δ. The chemical analysis on Nd1.85Ce0.15CuO4-δ revealed that the cerium valence was 3.84, irrespective of temperature and patial pressure of oxygen. The oxygen deficit δ increased with the increase in the oxygen deficit. The copper valence was given as contour in the T - log Po2 diagram. The critical temperature appeared below the copper valence of 1.84. The measurement of oxygen deficit on Bi2.00Sr1.88Ca1.00Cu2.14Oy revealed the difference in y - log Po2 between above and below 800°C. Above 800°C, the y -log Po2 relation resembled those of other kinds of high Tc superconducting oxides. However, below 800°C y only depended on temperature, that is, y was independent of oxygen partial pressure. The tailing of resistivity - temperature curve in the vicinity of critical temperature seemd to be due to the existence of neutral oxygen species confirmed by the chemical analysis and microthermogravimetry.