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

Abstract

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 Néel 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).