Phase separation and metallic conductivity in Tl _0.4 K _0.4 Fe _2−y Se _2−x S _x

Abstract

We study the effects of S-doping on the microstructure, magnetic and transport properties of the Tl0.4K0.4Fe2−ySe2−xSx superconductor. At low doping level (x ⩽ 0.8), the samples exhibit behaviors similar to those of the parent sample, i.e., the occurrence of phase separation into a Fe-vacancy phase, a charge ordering phase and an undistorted phase; the presence of a “hump”-like feature in the resistivity curves; and the development of antiferromagnetic order well above room temperature. In sharp contrast, the samples with x ⩾ 1.0 all exhibit metallic-like behavior without displaying superconductivity. And these samples are chemically phase separated into a S-poor Tl0.4K0.4Fe2−ySe2−xSx-phase and a S-rich Tl0.4K0.4Fe2−yS2-phase. Interestingly, the Fe-vacancy phase is absent in these samples, accompanied with the disappearance of semiconducting-like conductivity at high temperature and the depression of superconductivity. The charge ordering phase is responsible for the metallic behavior in heavily doped samples.

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“…This sort of phase separation was not surprising because it is often observed with inorganic materials. For example, in the single crystals of Tl 0.4 K 0.4 Fe 2– y Se 2– x S x , a phase separation into the Tl 0.4 K 0.4 Fe 2– y Se 2– x S x -lattice (the S-poor phase) and the Tl 0.4 K 0.4 Fe 2– y S 2 -lattice (the S-rich phase) at nanoscale has been observed with an increasing concentration of S doping. The powder XRD of Tl 0.4 K 0.4 Fe 2– y Se 2– x S x was completely refined with two sets of lattice constants: (1) a = 3.754 Å and c = 13.651 Å in the S-poor phase, and (2) a = 3.801 Å and c = 13.231 Å in the S-rich phase with the same space group (I4/ mmm) .…”

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