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 pa tterns and optical micrographs show definitely one single phase. Well- separated double superconducting transitions are, however, observed; m ost 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 transitio n 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 o xygen content external layer of the grains and to the low oxygen conte nt grain cores, respectively.