Evidence for superconductivity in SrTa2S5 and metallic characteristics of SrNb2S5

A strontium niobium sulfide, SrNb 2 S 5 , and a strontium tantalum sulfide, SrTa 2 S 5 , have been successfully synthesized by a sulfurization method using CS 2 .SrNb 2 S 5 and SrTa 2 S 5 have hexagonal structures with lattice constants of a = 3.361, c = 24.06 A and a = 3.306, c = 24.26 A, respectively. SrNb 2 S 5 shows metallic conductivity and temperature-independent susceptibility with an extremely small Curie-Weiss contribution at low temperatures. SrTa 2 S 5 exhibits a superconducting transition at 2.75 K (midpoint). The superconducting transition temperature of SrTa 2 S 5 depends on the purity of specimens. We have verified that the high-purity specimen shows the superconducting transition with a much lower T c than that of low-purity specimen.