Properties of FeSe-type superconductors with ternary mixture of chalcogens

•We have synthesized the new series of superconducting iron chalcogenides.•The expansion of the unit cell was found with the increase of S content.•The decrease of the critical temperature was found with increase of S content.•The lattice expansion can be caused by difference in the iconicity of chemical bonds.

A series of Fe1.02Te0.5Se0.5−xSx (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) samples has been synthesized by solid state reaction. Keeping the tellurium content unchanged the influence of the substitution of sulfur for selenium on the crystal structure and superconducting properties of synthesized samples has been studied by means of X-ray diffraction, electrical resistivity and magnetic susceptibility measurements. It has been found that the unit cell volume of the superconducting phase increases with increasing S concentration despite the lower ionic radius of sulfur in comparison with selenium. Together with expansion of the lattice the S for Se substitution is observed to reduce the Tc value in Fe1.02(Te0.5Se0.5).