PREPARATION, RESISTIVITY, MAGNETIC-PROPERTIES AND SPECIFIC-HEAT OF THE 95-K SUPERCONDUCTOR YBA2CU3.5O7.5+X (247)

Under suitable preparation conditions, Y2BaCu7O15 ("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 95 K 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 YBa2Cu4O8 ("124") and those of YBa2Cu3O7 ("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.