  The influence of a partial substitution with sulphur into Te sites on the
crystal, electronic and magnetic structures of FeTe is investigated by DFT
calculations. The results reveal a phase transition from the antiferromagnetic
double-stripe order for pure FeTe to the single-stripe order for S-substituted
samples, which coincides with the previously observed appearance of the
superconducting state. The magnetic transition is caused by the variations of
the average chalcogen position in the unit cell. The analyzed normal-state
properties of Fe(Te,S) and Fe(Se;S) compounds allow a detection of the well
resolved nesting-driven magnetic fluctuations only for superconducting samples,
consistent with their antiferromagnetic ground state. Thus, the role of an
S-substitution is a suppression of the double-stripe antiferromagnetic order to
give rise to the single-stripe correlations, which are associated with an
occurrence of superconductivity in Fe(Te,S) solid solutions.
