  We have investigated the effect of atomic substitutions in the FeSe system,
which exhibits the simplest crystal structure among the iron-based
superconductors. An enhancement of the superconducting transition temperature
Tc was observed with the substitution of S or Te for Se; the Tc increased with
S substitution by up to 20 %, and also increased with Te substitution up to 75
%. In contrast, Co or Ni substitutions for the Fe site significantly suppressed
superconductivity. In this work we present a detailed description of the
substitution technique employed to determine Tc in the FeSe system.
