Dependence of the structural and physical properties of Tl1-yFe2-z(Se1-xSx)2 with isovalent substitution of Se by S: decrease of TNeel with S content

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 (0Rietveld 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).