Annealing Effects on (Ln, Ce)₂CuO₄ System

Abstract

Heat treatment effects were investigated on T'-type 214 compounds Ln_1.85Ce_0.15CuO_y with Ln=Pr, Nd, Sm or Eu. Annealing in sealed tube did not much affect on their electrical behavior of superconducting Nd_1.85Ce_0.15CuO_y. Electrical resistivitiy was reduced in the case of Ln=Sm and Eu but was enhanced on Ln=Pr with the annealing. Unit cell volume expanded with a decrease of oxygen content on Ln=Nd but shrank on Ln=Pr. These results could be understand as follows:trivalent neodymium ion has suitable ionic size in T'type structure, samarium and europium ions are too small so that oxide ion defect is easily introduced in the fluorite layer to expand its size to fit with the CuO₂ conduction plane, and praseodymium is too large so that CuO₂ conduction plane partially release its oxide ion. Surface of superconducting Nd_1.85Ce_0.15CuO_y was oxidized within 50days to the condition before the reduction to make the sample superconducing even if the sample was kept in a vacuum desiccator.