SURFACE AND BULK PROPERTIES OF HOT-PRESSED PBMO6S8 SUPERCONDUCTOR STUDIED BY AUGER-ELECTRON SPECTROSCOPY AND CALORIMETRY

A direct observation of the grain-boundary phases for several well-cha racterized hot-pressed PbMo6S8 samples were made by Auger electron spe ctroscopy. The surface elemental concentrations are completely differe nt when compared to its bulk composition. The thickness of this altere d composition is in the range 100-200 Angstrom gradually increasing wi th increase in hot-pressing temperature, Also, evidence of segregation of impurities, such as carbon and SiOx, to the grain-boundaries were noticed for some of the samples. Calorimetric experiments show a conti nuous broadening and a reduction in amplitude of the specific heat ano maly at T-C. This in terms of superconducting volume fraction indicate s a T-C distribution in the range 9-15 K. Such an observation can be r elated to the local inhomogeneities with respect to the ternary compos ition, i.e., a deviation from ideal stoichiometry, PbMo6S8. The result s are discussed in conjunction with X-ray diffraction, scanning electr on microscope, and energy dispersive X-ray analysis. By considering th e grain-boundary phases and the calorimetric observation of inhomogene ities, a plausible explanation is given for the low critical current d ensities in these materials.