LANTHANUM-SUBSTITUTED Y-BA-CU-O SUPERCONDUCTORS SYNTHESIZED AT ELEVATED OXYGEN PRESSURES

Lanthanum substitution in the Y-Ba-Cu-O system at elevated P(O2) results in superconducting compositions which throw interesting light on the effects of nonisovalent substitution in a Y-Ba-Cu-O system. Like Ca substitution, La substitution also shifts the phase formation boundary between 123, 247, and 124 phases to higher P(O2), and stabilizes a tetragonal 123 structure at moderately elevated P(O2). However, in contrast with Ca substitution, Tc decreases continuously and rapidly with increasing La content. This can be only partly explained by a decrease of the hole concentration.