Effects of fluorination on the 90K superconductor Ba sub 2 YCu sub 3 O sub (7-delta) (0<-delta<-1).

Substitution of fluorine for oxygen in samples of the type Ba sub 2 YCu sub 3 O sub (7-delta) (0-delta-1) is of interest because oxygen and fluorine have similar ionic radii and a variety of oxyfluorides are known to exist. In addition, there have been reports of superconductivity at 155K in the Ba-Y- Cu- O-F system. This work describes fluorination of sintered pellets of Ba sub 2 RCu sub 3 O sub (7-delta) ( R = Y, Eu; (0-delta-1) Fluorination of Ba sub 2 YCu sub 3 O sub (6.92) affords a nominal composition of Ba sub 2 YCu sub 3 O sub (6.4) F sub (1.3) and T sub c (onset) = 92K. Fluorination of Ba sub 2 YCu sub 3 O sub (6.15) gives a nominal composition of Ba sub 2 YCu sub 3 O sub (4.7) F sub (3.0), which is not superconducting to 4.2K. Fluorinated pellets appear to be single-phase as determined by X-ray powder diffraction. However, Raman, NQR, ESR, X-ray diffraction, and TEM data suggest that fluorine is not incorporated into the superconducting phase, but rather is incorporated into a secondary amorphous phase. Superconductivity above 93K is not observed in this system.