Preparation, resistivity, magnetic properties and specific heat of the 95 K superconductor YBa 2Cu 3.5O 7.5+x (“247”)

Abstract

Under suitable preparation conditions, Y 2Ba 4Cu 7O 15 (“247”) has the highest critical temperature of the YBCO superconductor family. We present an extensive characterization of 247 samples with T c onsets as high as 95K in bulk ceramic form. The experiments performed include metallographic examinations, X-ray diffraction, microprobe analysis, resistivity, AC susceptibility, Meissner and normal-state susceptibility, magnetic determinations of the slope of the critical field at T c and of the critical current density, and specific heat from 1 to 330 K. Several properties (e.g., normal-state susceptibility, specific heat jump at T c, slope of H c2( T) at T c, resistivity) may be considered as an approximate average between those of YBa 2Cu 4O 8 (“124”) and those of YBa 2Cu 3O 7 (“123”). The signature of fluctuations in magnetic properties is enhanced with respect to both 124 and 123. The behavior of the specific heat close to T = 0 is similar to that of the other members of the family, with the presence of a quasi linear T-term and the absence of any upturn in C/ T. The maximum T c appears to be related to optimal doping, consistent with the behavior observed in 123 as a function of the oxygen concentration.