Preparation of new strontium niobium sulfide and strontium tantalum sulfide

New strontium ternary sulfides, SrM[sub 2]S[sub 5] (M=NB, Ta), have been prepared by the reaction of CS[sub 2] with a mixture of SrCO[sub 3] and Nb[sub 2]O[sub 5], or SrCO[sub 3] and Ta[sub 2]O[sub 5]. 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] for SrNb[sub 2]S[sub 5], and a=3.3063 (5) [angstrom], c=24.276 (4) [angstrom] for SrTa[sub 2]S[sub 5]. The electron diffraction measurements, however, revealed the existence of the supercells of [radical]28a x [radical]28a for SrTa[sub 2]S[sub 5]. Superconductivities with Tc's around 3K were observed in SrTa[sub 2]S[sub 5] and SrNb[sub 2]S[sub 5].