SPLIT SUPERCONDUCTING TRANSITIONS IN THE SPECIFIC-HEAT AND MAGNETIC-SUSCEPTIBILITY OF YBA2CU3OX VERSUS OXYGEN-CONTENT

A homogeneous batch of YBa2Cu3Ox porous ceramics was studied in detail as a function of the annealing temperature T(a). X-ray diffraction patterns and optical micrographs show definitely one single phase. Well-separated double superconducting transitions are, however, observed; most clearly in the specific heat curves, except for T(a) almost-equal-to 460-degrees-C. Initial values are T(c1) = 87 K and T(c2) = 92 K for T(a) = 300-degrees-C. Upon increasing the annealing temperature T(a) to 500-degrees-C, T(c1) increases to 93.5 K while T(c2) remains almost constant. Beyond T(a) = 500-degrees-C, both T(c1) and T(c2) decrease. The essentially reversible splitting of the superconducting transition appears to be due to the complex kinetics of the oxygen uptake. The two critical temperatures T(c1) and T(c2) are attributed to the high oxygen content external layer of the grains and to the low oxygen content grain cores, respectively.