THERMAL-EXPANSION, SPECIFIC-HEAT, AND PRESSURE-DEPENDENCE OF TC IN YBA2(CU1-XFEX)3O7-DELTA, YBA2(CU1-XZN)3O7-DELTA

High-resolution thermal-expansion and specific-heat measurements of YBa2(Cu(1-x)M(x))3O7-delta (M = Fe and Zn; x = 0-5%) at the superconducting transition are presented. Thermal expansivity alpha(T) provides an alternative, to the specific heat, method for examining the bulk thermodynamics of the superconducting phase transition. We show that, if \dT(c)/dp\ > 0.1 K/kbar, expansivity is a more sensitive probe of the superconducting transition than the specific heat because of a relative smaller background. The expansivity jump DELTA-alpha of the Fe doped samples increases by a factor of 4 with increasing x, in stark contrast to the specific heat jump DELTA-C0, which decreases and is unobservable for x = 5%. For Zn, a jump in alpha at T(c) was only observed for the x = 1% sample. Calculated dT(c)/dp values show a large increase (factor of 20) for Fe and little change for Zn doping. These values agree well with actual pressue data, indicating the the jumps in alpha are electronic in nature and are not due to a structural transition. The large dT(c)/dp values of the Fe doped samples are discussed in terms of a disturbed oxygen coordination of Fe occupying chain sites. Anomalies in alpha (T) at 40-90 K and almost-equal-to 250 K, indicative of structural changes, are also discussed.