123 + CuO → 124 conversion slowed by lanthanum substitution

Abstract

The rate of formation of YBa₂Cu₄O₈ (124) by solid state reaction of YBa₂Cu₃O_7-δ (123) with CuO is dramatically slowed by substitution of La ³⁺ at Ba²⁺ sites. This is not an ion size effect, since substitution of Sr²⁺ for Ba²⁺ does not reduce the 123 + CuO → 124 conversion rate. The reaction kinetics have been studied as a function of La concentration and oxygen pressure. Higher oxygen pressure enhances the formation rate of 124. This is expected because conversion to 124 involves oxygen uptake since 123 is oxygen deficient at the reaction temperature. The reverse reaction 124 → 123 + CuO has been studied by thermogravimetry; the rate of this decomposition reaction remains unaffected by La substitution.