  The effect of selenium substitution by sulfur or tellurium in the
Tl1-yFe2-zSe2 antiferromagnet was studied by x-ray and electron diffraction,
magnetization and transport measurements. Tl0.8Fe1.5(Se1-xXx)2 (nominal
composition) solid solutions were synthesized in the full x range (0<x<1) for
X=S and up to x=0.5 for X=Te, using the sealed tube technique. No
superconductivity was found down to 4.2K in the case of sulfur despite the fact
that the optimal crystallographic parameters, determined by Rietveld
refinements, are reached in the series (i.e. the Fe-(Se,S) interplane height
and (Se,S)-Fe-(Se,S) angle for which the critical superconducting transition
T$_{c}$ is usually maximal in pnictides). Quasi full Tl site (y=0.05) compared
to significant alkaline deficiency (y=0.2-0.3) in analogous A1-yFe2-zSe2 (A =
K, Rb, Cs), and the resulting differences in iron valency, density of states
and doping, are suggested to explain this absence of superconductivity.
Compounds substituted with tellurium, at least up to x=0.25, show
superconducting transitions but probably due to tetragonal Fe(Se,Te) impurity
phase. Transmission electron microscopy confirmed the existence of ordered iron
vacancies network in the samples from the
Tl$_{0.8}$Fe$_{1.5}$(Se$_{1-x}$S$_{x}$)$_{2}$ series in the form of the
tetragonal $\sqrt{5}$ a $\times \sqrt{5}$ a $\times$ c superstructure
(\textit{I4/m}) (mixed with the orthorhombic $\sqrt{2}$ a $\times 2\sqrt{2}$ a
$\times$ c form (\textit{Ibam}) if the iron vacancies level is increased). The
N\'{e}el temperature (T$_{N}$) indicating the onset of antiferromagnetism order
in the $\sqrt{5}$ a $\times \sqrt{5}$ a $\times$ c supercell decreases from
450K in the selenide (x=0) to 330K in the sulfide (x=1). We finally demonstrate
a direct linear relationship between $T_{N\acute{e}el}$ and the Fe-(Se,S) bond
length (or Fe-(Se,S) height).
