II. EXPERIMENTAL

Polycrystalline samples with 17 compositions (listed in Table I) were prepared by solid state reaction. The pure powders of Fe (spectrum pure 99.99%), Se (99.95%), Te (99.999%), and S (99.5%), which are the products of China National Accord Medicines Corporation, were mixed and ground in an agate mortar, cold-pressed into disk (diameter is 10 mm) with 200 kg cm⁻² uniaxial stress. Afterwards, all samples were sealed in quartz tube under vacuum and heated to 900 °C for 48 h. Then samples were quickly transferred into preheated furnaces with temperature at 400 °C, kept for 48 h and quenched in ice water.

Table I.

The lattice parameters and T _c of all the samples

T represents the tetragonal doped-FeTe; T₁ represents another tetragonal phase; H represents the hexagonal phase.

Powder X-ray diffraction (XRD) data for phase identification were collected at room temperature on a PANalytical diffractometer (X'Pert PRO MRD) with CuKα radiation (40 kV, 40 mA) and a graphite monochromator in a reflection mode (2θ =10–80°, step = 0.017° 2θ). Indexation for obtaining the lattice parameters was performed using the FullProf program (Rodriguez-Carvajal, Reference Rodriguez-Carvajal2003).

The electrical resistivity was measured from 5 to 300 K via a four-probe method using a closed-cycle refrigerator. Temperature dependence of magnetization was measured from 2 to 30 K by Superconducting Quantum Interference Device (SQUID) magnetometer MPMS XL-7 EC with the field H = 40 Oe after cooling in the zero field.

III. RESULTS AND DISCUSSION

The XRD patterns of the tow typical compositions: FeTe_0.85S_0.15 and FeTe_0.85S_0.15 are shown in Figure 1. The main phase of both two samples shows tetragonal structure with space group P4/nmm. The diffraction peak of FeTe₂ could be observed as the impurity (ICS-PDF 51-1158). As mentioned before, this is because of off-stoichiometry of the iron telluride. For the determination of the limit of solid solution, the partial patterns with various Se/S content x are shown as inset. The partial patterns of Fe _y Te_1−x Se _x were also shown in the upper inset, and it can be seen that (101) peak of tetragonal phase separated into two when the x increase to 0.6 with Se doping, suggesting that the composition is already out of the doping limit with another tetragonal phase formed. For Fe _y Te_1−x S _x as shown in lower inset, the (101) peak of hexagonal phase appeared when the x increased to 0.18 with S doping, showing the sulfur is unable to enter the tetragonal structure anymore.

Figure 1.

XRD patterns of the FeTe_1−x Se _x and FeTe_1−x S _x samples. Impurity FeTe₂ is marked as “#”.

In order to determine the accurate limit of Fe _y Te_1−x Se _x and Fe _y Te_1−x S _x , the lattice parameters of the tetragonal doped-FeTe were obtained by indexing the XRD patterns and the results were listed in Table I and plotted in Figure 2. It can be seen that both the lattice parameters a and c decrease linearly with the increasing of the doping level. It is quite unexpected that the compression extent of the lattice by Se is nearly the same with that of S with the same doping content, despite the larger atomic radii of S²⁻. This could be explained by the change of length of the chemical bond. However, the doping limit of Se and S is quite different. The parameters of Fe _y Te_1−x Se _x show basically the constant values when x > 0.5, while the value is x > 0.15 in the S-doping case, indicating that the limit of the solid solution is around 0.5 and 0.15 in FeTe_1−x Se _x and FeTe_1−x S _x , respectively. It should be noted that the smaller doping limit using S as dopant could be resulted from the unmatched ionic radii of Te²⁻ and S²⁻.

Figure 2.

Lattice parameters a and c vs. the Te-site doping level in FeTe_1−x Se _x and FeTe_1−x S _x .

The solid solution with the variation of the Fe content has also been measured in these two systems, with the limit content x: Fe _y Te_0.85Se_0.5 and Fe _y Te_0.5S_0.15. Samples with different y were measure by XRD, and the partial patterns are shown in Figure 3. Figure 3(a) shows the patterns of Fe _y Te_0.85Se_0.5, where it can be seen that the diffraction peak of FeTe₂ disappeared with the Fe adding into the samples with y = 1.04. As the y increased, the diffraction peak of iron appeared (Jade PDF#65-2574) in Fe _y Te_0.5Se_0.5 with y = 1.13, indicating that the limit of the solid solution region has been crossed. For Fe _y Te_0.85Se_0.15 in Figure 3(b), reflections of FeTe₂ disappeared with a larger amount of excess Fe added. The diffraction peak of pure iron was not observed unit y increased to 1.16.

Figure 3.

Partial XRD of (a) Fe _y Te_0.5Se_0.5, and (b) XRD of Fe _y Te_0.85Se_0.15.

Based on the limit of the solid solution obtained in this work, the composition for preparing the polycrystalline samples on Fe _y Te_1−x X _x (X = Se, S) with single phase was recommended in Figure 4, where it can be seen that both the solid solution are off-stoichiometry, toward to the Fe-rich side. It is noted that the solution limit of binary FeTe in Figure 4 was carried from the literature (Okamoto et al., Reference Okamoto and Tanner1990).

Figure 4.

The solid solution region of the Fe _y Te_1−x X _x (X=Se, S) tetragonal phase (the solution limit of binary FeTe was carried from the literature).

Temperature dependence of the magnetization was measured on the samples with x _Se ≤ 0.5 and x _S ≤ 0.15. T _c values of all measured samples were listed in Table II. The maximum T _c was obtained on the solid solution limit in both doping system with the Fe nominal composition y = 1. The typical curves of FeTe_0.5Se_0.5 and FeTe_0.85S_0.15 were plotted in Figure 5. It can be seen that T _c ^mag of these two compositions is 13.5 and 10 K. The temperature dependence of electrical resistivity of these two samples was also measured and shown in Figure 6. FeTe_0.5Se_0.5 shows the metallic behavior while FeTe_0.85S_0.15 exhibits a semiconductor-like behavior. From the inset of Figure 6, T _c ^onset was obtained at 15 and 10.5 K for these two samples. These values are in good agreement with the previous works (Fang et al., Reference Fang, Pham, Qian, Liu, Vehstedt, Liu, Spinu and Mao2008; Mizuguchi et al., Reference Mizuguchi, Tomioka, Tsuda, Yamaguchi and Takano2009a).

Figure 5.

Temperature dependence of the magnetization of FeTe_0.85S_0.15 and FeTe_0.5S_0.5.

Figure 6.

Temperature dependence of the resistivity of FeTe_0.85S_0.15 and FeTe_0.5S_0.5.

Table II.

Critical temperature T _c obtained from the magnetic susceptibility.

“–” means that T _c was not observed with temperature down to 2 K.

Using the superconductivity obtained from the magnetization, we presented a 3D-graph for showing T _c as a function of both x and y in Fe _y Te_1−x X _x (X = Se, S) (shown in Figure 7). It is clearly show that the superconductivity was improved by the smaller anion Se/S doping on Te-site, but suppressed by adding Fe into the system.

Figure 7.

3D-graph of T _c as a function of both x and y in (a) Fe _y Te_1−x Se _x and (b) Fe _y Te_1−x S _x system.

It seems that T _c in the Te-site doped FeTe would be increased by simultaneously increasing the doping level of Se/S and minimizing the excess Fe in the structure. This suggests that the superconductivity in iron chalcogenides is truly enhanced by compressing the lattice, but the interstitial Fe also plays an important role. Although intercalated excess Fe ion is found to be harmful to the superconductivity in this work, T _c of FeSe was increased to ~31 K with potassium intercalation (Guo et al., Reference Guo, Jin, Wang, Wang, Zhu, Zhou, He and Chen2010), and this value could be further raised to ~46 K with decreasing the K content (Ying et al., Reference Ying, Chen, Wang, Jin, Lai, Zhou, Zhang, Shen and Wang2013). It is suggested that the ion intercalation could be another effective route to adjust the superconductivity of iron chalcogenides, besides changing the lattice parameters.