Possible New Ternary Chalcogenide Superconductors with Composite Crystal Structure

Abstract

New type of ternary sulfides with (PbS)_n(TS₂)₂(T = Ti, Nb, Ta; n≃1) formula has been prepared and characterized by using X-ray diffraction and electric resistivity measurement. The sulfides have been prepared from elemens by heat treatment in evacuated silica tube and are black-grayish microcrystalline powder with luster. Their X-ray diffraction pattern resembles that of (PbS)_nTS₂(T = Ti, V, Nb, Ta; n≃1) type of sulfides and only diffraction lines indexed to 0 0 l are strong owing to extreme preferred orientation. Considering the difference of periodic distance in the stacking direction of the both types of sulfides, it has been suggested that the structure of the (PbS)_n(TS₂)₂ type of sulfides consists of two kinds of layers alternately stacked: one two-atom-thick layer of PbS and two three-atom-thick sandwiches of TS₂. Electric resistivity of (PbS)_n(TaS₂)₂ and (PbS)_n(NbS₂)₂ measured using d.c. four-probe method in the range of 1.7–300K showed metallic temperature dependence and its drastic decrease has been found around 3K which is probably due to superconducting transition.