SUPERCONDUCTIVITY THROUGH REDOX TREATMENTS OF (Y1-ZCAZ)SR-2(CU1-XFEX)(3)O-Y

Superconductivity is developed through controlled redox treatment vari ation in a material with unusually high Fe concentration. The dependen ce of the structural, magnetic, and superconductive properties of (Y1- zCaz)Sr-2(Cu1-xFex)(3)O-y (O less than or equal to z less than or equa l to 0.08; x = 0.13 or x = 0.167) on annealing conditions is explored. Sintered bulk materials are prepared with an extended exposure at T = 1000 K. This procedure is designated OP for oxygenating preparation a nd all materials treated in this way show no signs of superconductivit y above 5 K. Subsequently, Gd getter annealing is used to control redu ctions at T = 1000 K for 4 days followed by reoxidation at 800 K for 1 h. For x = 0.167 and z = 0, a strong reduction (y = 6.3) reoxidation procedure resulted in the development of superconductivity with Tc-ons et = 30 K. As the Ca content increases, successively milder reductive conditions must be employed to obtain superconductors. A local thermod ynamic model based on the stabilities of various Fe cluster arrangemen ts and oxidative states predicts the changes expected on Ca substituti on. It is argued that Fe in a cluster with 5-fold O coordination is no t detrimental to superconductivity while 4-fold coordination quenches holes because of the uncompensated nature of this low 0 coordination.