Effects of hot isostatic pressing on the lattice parameters and the transition temperature of Pb0.8Sn0.2Mo6S8

The hexagonal lattice parameters (c(h), a(h)) and the superconducting trans ition temperature (T-c) of hot isostatically pressed (HIP'ed) polycrystalli ne Pb0.8Sn0.2Mo6S8 (PSMS) have been studied. Increasing both the temperatur e and pressure of the HIP treatments decreased T-c and c(h), while a(h) was unaffected. Accompanying the decrease in T-c and c(h) with increasing HIP pressure is an increase in MoS2 content. These changes were not reversible with post-HIP processing. We have considered four possible reasons for this behavior; sulfur deficiency, metal (Pb, Sn) cation deficiency, the R (3) o ver bar-P (1) over bar low temperature structural phase transition, and oxy gen contamination. We hypothesize that the PSMS is pushed into a two-phase (PSMS + MoS2) region at higher pressures, enhancing oxygen contamination of the PSMS, and hence decreasing the c/a ratio and T-c. We conclude that the HIP temperature and pressure should be minimized for the production of hig h T-c PSMS if there is any potential for oxygen contamination. Unfortunatel y, this requirement conflicts with the desire to increase the critical curr ent density (J(c)) by using the HIP to densify the material and to improve the connectivity of polycrystalline aggregates. (C) 1998 Published by Elsev ier Science B.V. All rights reserved.