Superconductivity through redox treatments of

Abstract

Superconductivity is developed through controlled redox treatment variation in a material with unusually high Fe concentration. The dependence of the structural, magnetic, and superconductive properties of ; 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 and all materials treated in this way show no signs of superconductivity above 5 K. Subsequently, Gd getter annealing is used to control reductions 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 . As the Ca content increases, successively milder reductive conditions must be employed to obtain superconductors. A local thermodynamic model based on the stabilities of various Fe cluster arrangements and oxidative states predicts the changes expected on Ca substitution. It is argued that Fe in a cluster with 5-fold O coordination is not detrimental to superconductivity while 4-fold coordination quenches holes because of the uncompensated nature of this low O coordination.