High-resolution transmission electron microscope study of effects of cobalt substitution on the stability and perfection of YBa 2Cu 4O y superconductors

Abstract

A small amount of Co is doped in the starting mixtures for preparing YBa 2Cu 4O y (1-2-4) type of superconductors and the resultant materials are examined by HRTEM, in order to examine the effects of Co substitution on the structural stability and perfections of the 1-2-4 structure. Up to about 2% substitution, the 1-2-4 phase is obtained as the major phase. The stability of (CuO) 2 double-chains (D), however, decreases gradually with further substitutions and partial conversions of double-chains into (CuO) single-chains (S) are induced. With 3% substitution, domains of the 2-4-7 structure with (D S D S D) sequences are formed together with those of the 1-2-4 one with a (D D D D) sequence. Small domains of a new ordered structure (D D S D D S D D S) are also observed in Co 3% specimen, suggesting the existence of a new Y 3Ba 6 (Cu, Co) 11O y (3-6-11) phase. The stability of the 2-4-7 phase increases by further substitution and, finally, it becomes the major phase at a 5% substitution by Co.