Characterization of the phase composition, crystal structure and superconducting properties of Fe1.02SeyTe1−y−xSx

• Тhe Fe1.02Se0.5Te0.5−xSx and Fe1.02Se0.4Te0.6−xSx samples have been synthesized.
• The S for Te substitution results in a small expansion of the crystal lattice of the PbO-type phase.
• This expansion is attributed to changes in the phase relation and chemical composition of phases.
• There is a correlation between the changes of Tc and lattice parameters of the PbO-type phase.

Two series of the Fe1.02Se0.5Te0.5–xSx (I) and Fe1.02Se0.4Te0.6–xSx (II) samples with the sulfur for tellurium substitution and with the invariable Se concentrations have been synthesized and studied by means of X-ray diffraction, scanning electron microscopy, electrical resistivity and magnetic susceptibility measurements. The superconducting PbO-type phase is found to persists in the first series up to x  = 0.4 and in the second one up to x = 0.5. Despite the lower ionic radius of sulfur in comparison with tellurium the replacement of tellurium by sulfur does not lead to contraction of the unit cell volume of the superconducting phase in both I and II series with ternary mixture of chalcogens. Variations of the lattice parameters caused by the S for Te substitution in the Fe1.02Se0.5Te0.5–xSx and Fe1.02Se0.4Te0.6–xSx samples are found to be less pronounced than that reported for the Fe1.02Te0.5Se0.5-xSx system and are accompanied by lowering of the critical temperature. The behavior of the lattice parameters and critical temperature of Fe(S,Se,Te) materials with the ternary mixture of chalcogens at substitutions is ascribed to the changes in the volume fraction and chemical compositions of the coexisting tetragonal and hexagonal phases.