123-]124 CONVERSION SLOWED BY LANTHANUM SUBSTITUTION(CUO)

The rate of formation of YBa2Cu4O8 (124) by solid state reaction of YB a2Cu3O7-delta (123) with CuO is dramatically slowed by substitution of La3+ at Ba2+ sites. This is not an ion size effect, since substitutio n of Sr2+ for Ba2+ does not reduce the 123 + CuO --> 124 conversion ra te. The reaction kinetics have been studied as a function of La concen tration and oxygen pressure. Higher oxygen pressure enhances the forma tion rate of 124. This is expected because conversion to 124 involves oxygen uptake since 123 is oxygen deficient at the reaction temperatur e. The reverse reaction 124 --> 123 + CuO has been studied by thermogr avimetry; the rate of this decomposition reaction remains unaffected b y La substitution. (C) 1997 Elsevier Science B.V.