PREPARATION OF NEW STRONTIUM NIOBIUM SULFIDE AND STRONTIUM TANTALUM SULFIDE

New strontium ternary sulfides, SrM2S5 (M=Nb, Ta) , have been prepared by the reaction of CS2 with a mixture of SrCO3 and Nb2O5, or SrCO3 and Ta2O5. The powder X-ray diffraction patterns were indexable on the basis of hexagonal cells with lattice parameters of. a=3.3592(7)angstrom c=23.995(6)angstrom SrNb2S5, and a=3.3063(5)angstrom, c=24.276(4)angstrom for SrTa2S5. The electron diffraction measurements, however, revealed the existence of the supercells of square-root 7a x square-root 7a for SrNb2S5, and square-root 28a x square-root 28a for SrTa2S5. Superconductivities with Tc's around 3K were observed in SrTa2S5 and SrNb2S5.