Superconducting, microstructural, and grain boundary properties of hot‐pressed PbMo₆S₈

An alternative procedure is outlined for the synthesis of high quality, fine grained PbMo₆S₈. As the grain size plays an important role on the densification process, which in turn has an influence on the magnitude of Jc, an attempt has been made to produce dense samples from such powders by "Hot Pressing". The effect of the hot‐pressing temperature on the superconducting, crystallographic, microstructural, and grain boundary characteristics of the ternary compound was evaluated. Scanning electron micrographs and ac‐susceptibility measurements indicate that hot pressing (1000–1200 °C) improves the grain connection as a consequence of better densification. However, at higher temperatures (1250–1400 °C) it also precipitates MoS₂ as an additional phase. Calorimetric data indicate a continuous broadening, as a function of hot‐pressing temperature, of the specific heat jump at Tc. Preliminary investigation on the Tc distribution of the samples shows a progressive degradation, as indicated by a smearing in Tc down at least to 8 K. The deterioration was examined using Auger electron spectroscopy and the results suggest possible compositional variations rather than oxygen defects in the phase. The origin of such behavior is examined on the basis of nonstoichiometry or chemical heterogeneity at the grain surface. In addition, grain boundary contaminants and their role on the superconducting properties are considered. Finally, the often encountered problem of transport Jc limitation in these materials is discussed in terms of interconnectivity of the grains, phases, the presence of secondary phases, impurities, inhomogeneities, and the grain boundary phases