ELECTRONIC SPECIFIC-HEAT OF YBA2CU3O6-K(X FROM 1.8 TO 300)

We have determined for the first time the electronic specific heat gam ma(x, T) of YBa2Cu3O6+x, for 0. 16 less-than-or-equal-to x less-than-o r-equal-to 0.97 from 1.8 to 300 K. Weakly superconducting behavior bet ween x = 0.4 and 0.8 progresses rapidly to strong coupling BCS-like su perconducting and metallic normal state behavior for x greater-than-or -equal-to 0.9. However, the continuous development of the entropy S(x, T) with x and T across the entire series suggests a progressive modif ication of the low-energy spin spectrum with hole doping rather than a simple band model. Fermi statistics and k-space pairing for all x is indicated by the magnitude and T-dependence of S(x, T). Pseudogap beha vior in S(x, T) is observed over a temperature region above T(c), whic h increases rapidly with oxygen depletion to around 200 K for x approx imately 0.7. This loss of entropy reflects normal-state correlations a pparently unrelated to the superconducting pairing.