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Characterization of the phase composition, crystal structure and superconducting properties of Fe1.02SeyTe1−y−xSx
Physica C: Superconductivity and its applications 527 (2016) 21–27
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Characterization of the phase composition, crystal structure and superconducting properties of Fe1.02 Sey Te1−y −x Sx A.S. Abouhaswa a,∗, A.I. Merentsov a, N.V. Baranov a,b a b
Institute of Natural Sciences, Ural Federal University, 620083, Ekaterinburg, Russia M.N. Miheev Institute of Metal Physics, Ural Branch of RAS, 620990, Ekaterinburg, Russia
a r t i c l e
i n f o
Article history: Received 9 April 2016 Revised 16 May 2016 Accepted 18 May 2016 Available online 19 May 2016 Keywords: FeSe-based superconductors Solid state reactions Phase composition Crystal structure Electrical resistivity Magnetization
a b s t r a c t Two series of the Fe1.02 Se0.5 Te0.5–x Sx (I) and Fe1.02 Se0.4 Te0.6–x Sx (II) samples with the sulfur for tellurium substitution and with the invariable Se concentrations have been synthesized and studied by means of X-ray diffraction, scanning electron microscopy, electrical resistivity and magnetic susceptibility measurements. The superconducting PbO-type phase is found to persists in the first series up to x = 0.4 and in the second one up to x = 0.5. Despite the lower ionic radius of sulfur in comparison with tellurium the replacement of tellurium by sulfur does not lead to contraction of the unit cell volume of the superconducting phase in both I and II series with ternary mixture of chalcogens. Variations of the lattice parameters caused by the S for Te substitution in the Fe1.02 Se0.5 Te0.5–x Sx and Fe1.02 Se0.4 Te0.6–x Sx samples are found to be less pronounced than that reported for the Fe1.02 Te0.5 Se0.5- x Sx system and are accompanied by lowering of the critical temperature. The behavior of the lattice parameters and critical temperature of Fe(S,Se,Te) materials with the ternary mixture of chalcogens at substitutions is ascribed to the changes in the volume fraction and chemical compositions of the coexisting tetragonal and hexagonal phases. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The discovery of new iron-based superconductors has initiated advanced search for new superconducting compounds and key factors influencing their properties [1–5]. The common features of these materials are: (i) the layered crystal structure consisting of a set of square lattices of Fe atoms with pnictogen or chalcogen atoms; (ii) the similarity in the morphology of the Fermi surface; and (iii) the interplay between superconductivity and magnetism [4–6]. The FeSe (11) type compounds have a tetragonal crystal structure of the PbO-type (P4/nmm space group) with layers in which Fe cations are tetrahedrally coordinated by chalcogen and with no atoms located within the van der Waals gap. Owing to the simplicity of the crystal structure and similarities in the Fermi surface the FeSe-type compounds can be used as model systems for studying the mechanisms of superconductivity in other Fe-based superconductors having more complex crystal structures. The superconducting critical temperature (Tc ) of FeSe is found to be strongly affected by doping and by substitutions in both the Fe and chalcogen sublattices, as well as by application of hydrostatic pressure and strains (see, e.g., Ref. [4–6] for overview). The Tc values increased up to ∼ 80–100 K were recently reported for the ∗
Corresponding author. E-mail address: [email protected] (A.S. Abouhaswa).
http://dx.doi.org/10.1016/j.physc.2016.05.017 0921-4534/© 2016 Elsevier B.V. All rights reserved.
FeSe monolayer film grown on the SrTiO3 substrates [7,8]. In previous studies [9–14], the substitution effects on the crystal structure and properties of iron chalcogenide superconductors were investigated in the compounds with different binary mixtures of chalcogens (Se–Te, Se–S, Te–S). As was shown recently in [15], the presence of all three chalcogens in the Fe1.02 Te0.5 Se0.5− x Sx compounds results in unusual change of the lattice parameters with substitutions. Bearing in mind that the Fe(Se, Te) samples often exhibit an impurity hexagonal phase of the NiAs-type together with the main superconducting phase having a tetragonal PbO-type phase [16] the aim of the present work is to study how the coexistence of the tetragonal and hexagonal phases and the changes in their volume fractions, chemical composition and lattice parameters may affect the superconducting properties of the 11-type iron-chalcogenide materials. We have synthesized the Fe1.02 Se0.5 Te0.5− x Sx and Fe1.02 Se0.4 Te0.6− x Sx samples with the fixed Se content and studied the effect of the substitution of sulfur for tellurium taking into account the changes in the phase composition, lattice parameters and volume fractions of both phases bearing in mind a narrow homogeneity range of the superconducting phase and a different chalcogen solubility in both tetragonal and hexagonal phases.
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Fig. 1. Observed (symbols) and calculated (line) patterns for the samples Fe1.02 Se0.5 Te0.5− x Sx (series I) with various sulfur concentrations. Vertical bars indicate positions of Bragg reflection for the existing phases. The difference between calculated and observed intensities is shown in the bottom.
2. Experimental details Two series of polycrystalline samples with nominal compositions Fe1.02 Se0.5 Te0.5− x Sx (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) (series I) and Fe1.02 Se0.4 Te0.6− x Sx (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) (series II) were prepared by a solid state reaction method. In order to achieve a better homogeneity of final samples we used a ternary alloy of chalcogens as a precursor instead of a simple mixture of pure chalcogens. At first, the starting materials: selenium, tellurium and sulfur (all of 99.99% purity) were sealed into evacuated quartz tubes and heat treated at 200 °C for 2 h and then for 2 h at 400 °C. The resulting alloy of chalcogens was ground ground into a powder and mixed with an appropriate amount of the iron powder (of 99.98% purity). The obtained mixture was pressed into pellets. In order to reach homogeneity we used a prolongated annealing procedure: the first annealing at 500 °C for 10 h, then at 700 °C for 38 h; the second annealing at 700 °C for 120 h. The obtained samples were again ground, peletized, and then additionally heat treated third time at 700 °C for 72 h. This method of the sample preparation differs from that used in our previous work [15], where the Fe1.02 Te0.5 Se0.5− x Sx compounds were prepared from the mixture of pure chalcogens and iron powder. Beside these polycrystalline samples, two coarse-grained samples with nominal compositions Fe1.02 Se0.5 Te0.4 S0.1 and Fe1.02 Se0.4 Te0.3 S0.3 were prepared by melting at 950 °C with following cooling to room temperature for 48 h. The structure and phase purity of the samples were examined at room temperature by powder X-ray diffraction using a Bruker D8 Advance diffractometer with Cu Ka radiation. The FULLPROF program (Le Bail fit) [17] was used for analysis of diffraction patterns. The chemical composition of the melted samples
was determined by using a Carl Zeiss AURIGA Crossbeam Workstation equipped with an energy dispersive X-ray (EDX) spectrometer. The electrical resistivity of polycrystalline samples was measured by a four-probe dc method in the temperature range from 8 up to 300 K with using a closed-cycle refrigerator. For the magnetization measurements, a SQUID magnetometer MPMS XL-7 EC was used. From the magnetization data, the superconducting volume fractions were estimated taking into account the demagnetizing factor of the samples. 3. Results and discussion Figs. 1 and 2 display the X-ray diffraction patterns for the samples belonging to the series I and series II with the sulfur concentration (x = 0, 0.1, 0.3, 0.5) correspondingly along with the Rietveld refinements using the Fullprof program. For the series I (Fig. 1), the patterns at x ≤ 0.1 exhibit the Bragg reflections belonging to: (i) the dominating tetragonal PbO-type phase (P4/nmm space group), (ii) the impurity phase with a hexagonal NiAs-type structure (P63 /mmc space group), and (iii) iron oxide Fe3 O4 with cubic structure were detected as well. As can be seen from Fig. 1, the peaks of the hexagonal phase begin to grow with increasing sulfur content. Starting from x = 0.3 the peaks of a tetragonal FeS-based phase with the mackinawite structure (P4/nmm space group) appear and begin to grow as well. We noticed that the lattice parameters of the mackinawite-type phase observed in all our samples differ from those reported for FeS. Thus, for the Fe1.02 Se0.5 Te0.2 S0.3 sample, the lattice parameters of this phase were determined as: a = 3.73 A˚ and c = 5.46 A˚ which are higher than a = 3.68 A˚ and
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Fig. 2. Observed (symbols) and calculated (line) patterns for the samples Fe1.02 Se0.4 Te0.6− x Sx (series II) with various sulfur concentrations. Vertical bars indicate positions of Bragg reflection for the existing phases. The difference between calculated and observed intensities is shown in the bottom.
c = 5.03 A˚ observed in FeS [18]. This means that the tetragonal FeS-based phase observed in the series I and II also contains other chalcogens. According to X-ray data in the compound with x = 0.5 (series I), the superconducting tetragonal phase is fully suppressed by the hexagonal and mackinawite-type phases. The similar behavior of the phase composition is observed in the II series (Fig. 2). However, there are some differences from the series I: (i) the reflections of the hexagonal phase with the NiAs-type structure appear at x = 0.1; (ii) the tetragonal FeS-based phase with mackinawite structure disappears at x = 0.6; and the Fe1.02 Se0.4 S0.6 sample contains only the hexagonal FeSe-based phase. The volume fractions of the hexagonal phase are observed to grow with increasing sulfur content in all of the studied series: from ∼10% at x = 0 to ∼71% at x = 0.4 in the series I and from ∼8% at x = 0 to ∼71% at x = 0.5 in the series II. It is worth to mention that in the Fe1.02 Te0.5 Se0.5– x Sx system [15] we didn’t see the disappearance of peaks from the main tetragonal phase even at x = 0.5; only successive shift of the peaks position toward small angles and reduction of the volume fraction of the main phase were observed. The concentration dependences of the a and c lattice parameters of the superconducting phase with the tetragonal PbO-type structure and hexagonal NiAS-type phase for both I and II series are shown in Fig. 3 together with the data for Fe1.02 Te0.5 Se0.5− x Sx taken from our previous work [15]. The Fe1.02 Se0.5 Te0.5 sample was obtained in the present work using the Se0.5 Te0.5 alloy as a precursor. We noticed that the duration of heat treatment at 700 °C influences the crystal structure of the main PbO-type phase. A visible difference in the lattice parameters of Fe1.02 Se0.5 Te0.5 samples (x = 0) synthesized by different methods can be seen, which apparently results from the different homogeneity of the samples. After third heat treatment, the lattice parameter c of the superconduct-
Fig. 3. Values of the lattice parameters and the unit cell volume of the superconducting tetragonal phase for the series I (filled triangles), series II (filled circles) and hexagonal phase (open triangles - series I; open squares - series II). The data for Fe1.02 Te0.5 Se0.5− x Sx samples (crosses) are taken from Ref. [15].
ing phase in Fe1.02 Se0.5 Te0.5− x Sx samples increases monotonously with increasing nominal sulfur concentration (upper panel in Fig. 3), while in the series II, the concentration dependence of the
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c lattice parameter is non-monotonous with a minimum around x = 0.2. As can be seen from Fig. 3, variations of the c parameter of the superconducting tetragonal phase in both series I and II are less pronounced than that observed in the Fe1.02 Te0.5 Se0.5− x Sx system, where the S for Se substitution is found to result in the substantial growth of the average interlayer distance (up to 3.9% [15]). Unlike the c parameter of the superconducting tetragonal phase, the lattice parameter a reflecting the changes in the intra-layer distances remains almost constant upon substitutions in all systems (middle panel). The substitution of sulfur for tellurium in the series I and II also affects not only the volume fraction of the hexagonal phase (see Figs. 1 and 2), but the lattice parameters of this phase as well. As is shown in Fig. 3, the growth of the sulfur content in the samples is accompanied by the expansion of the crystal lattice of the NiAs-type phase in the direction perpendicular to the layers, while the a lattice parameter reduces with increasing S concentration. A decrease of the unit cell volume of the hexagonal phase (see lower panel in Fig. 3) together with the growth of the Bragg peaks belonging to this phase (Figs. 1 and 2) at the substitution implies a possible variation of the chemical composition of the hexagonal phase, which obviously may affect the composition of the superconducting tetragonal phase. Thus, in both the Fe1.02 Te0.5 Se0.5− x Sx and Fe1.02 Se0.5 Te0.5− x Sx series, the replacement of Se or Te by sulfur having lower ionic radius, does not lead to reduction of the unit cell volume of the superconducting tetragonal, as it should be expected considering only the ratio of ionic radii of chalcogens. This behavior differs from that observed in pseudobinary compounds Fe(Te1− x Sex ), Fe(Se1− x Sx ) and Fe(Te1− x Sx ) in which the replacement of a chalcogen with larger ionic radius by another one with smaller ionic radius results in the lattice contraction [10– 12,19]. The anisotropy in the lattice expansion at the substitution correlates with anisotropic compressibility of FeSe-based superconductors evidenced by high-pressure experiments [20]. There are several reasons which can lead to changes in the lattice parameters of the superconducting phase in Fe(Te,Se,S) samples with the ternary mixture of chalcogens at the substitution of sulfur for tellurium or selenium: (i) the change in composition of the ternary mixture of chalcogens in the PbO-type phase; (ii) the variations in the iron/chalcogen ratio in the tetragonal and hexagonal phases due to the changes in their relative volumes; and (iii) the changes in the ionicity/covalency ratio of the iron-chalcogen bonds and van der Waals interactions [15]. It seems that all the mentioned reasons may contribute the observed changes in lattice parameters of these phases. The important point is the substitution limit of different chalcogens in the tetragonal and hexagonal phases. For the superconducting tetragonal PbO-type phase, the substitution limits of different chalgonens were determined in the pseudobinary series as xc = 0.3 for Fe1.02 Se1– x Sx [20] and xc ∼ 0.15 – 0.3 for Fe1.02 Te1– x Sx [21,22]; the solid solution Fe1.02 Te1− x Sex is observed in an extended concentration range (up to x = 0.5) [22]. As to the hexagonal NiAs-type phase, the substitution of sulfur for selenium in the pyrrhotite-type Fe7 (Se1− x Sx )8 compounds is not limited [23], however, for the Fe7 (Se1− x Tex )8 system, the solid solution is reported to exist in a rather narrow Te concentration range (0 < x < 0.15 [24]). The sulfur for selenium substitution in Fe7 (Se1− x Sx )8 leads to the contraction of the unit cell in both the a and c directions [23], as could be expected bearing in mind the difference between ionic radii of sulfur and selenium. However, in Fe7 (Se1− x Tex )8 , the Te for Se substitution was found to result in substantially different changes in the unit cell dimensions: to the contraction of the lattice along the c direction and to the growth of the a lattice parameter [24], i.e., in the flattening of the NiAs-type crystal structure. Such a difference in the lattice deformations caused by substitution in these systems apparently originates in the large polarizability of tellurium atoms in compar-
Fig. 4. Scanning electron microphotograph for the sample with the nominal composition Fe1.02 Se0.5 Te0.4 S0.1 .
Fig. 5. Scanning electron microphotograph for the sample with the nominal composition Fe1.02 Se0.4 Te0.3 S0.3 .
ison with sulfur and selenium (see Ref. [25]). A higher rate of the lattice expansion of the superconducting phase observed in the Fe1.02 Te0.5 Se0.5− x Sx samples with increasing sulfur concentration in comparison with the Fe1.02 Se0.5 Te0.5− x Sx and Fe1.02 Se0.4 Te0.6− x Sx systems (Fig. 3) may be associated with the substantially lower solubility of tellurium in the hexagonal phase Fe7 (Se,Te)8 in comparison with the non-limited solubility of sulfur in Fe7 (Se,S)8 . In the Fe1.02 Te0.5 Se0.5− x Sx samples with the constant tellurium content, the growth of the volume fraction of the hexagonal phase enriched by sulfur due to the S for Se substitution may shift the ternary chalcogen mixture in the tetragonal PbO-type phase toward higher Te concentration. Such a tellurium enrichment of the tetragonal phase together with changes in covalency/ionicity ratio of iron-chalcogen bonds apparently results in the expansion of the tetragonal lattice in the c direction. Note, that a clear trend of the lattice expansion with the S for Se substitution was also recently reported for the Fe1.02 Te0.85 Se0.15− x Sx [26]. In order to reveal the change in the chemical composition of the superconducting tetragonal phase due to S for Te substitution we performed microstructure observations of the crystals taken from the melted Fe1.02 Se0.5 Te0.4 S0.1 and Fe1.02 Se0.4 Te0.3 S0.3 samples. As follows from Figs. 4 and 5 which display the scanning electron micrographs, the crystals exhibit a flat surface morphology and a layered structure. Using the EDX analysis the chemical composition
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Fig. 7. Temperature dependence of the resistivity for the Fe1.02 Se0.5 S0.5 polycrystalline sample. Inset shows the temperature dependence of the magnetization measured in a zero field cooled (ZFC) (H = 8.7 Oe) regime.
Fig. 6. Temperature dependences of the electrical resistivity for the polycrystalline samples of the series I (a) and series II (b). Insets show the resistivity behavior at low temperatures.
of the tetragonal phase in the samples with the nominal compositions Fe1.02 Se0.5 Te0.4 S0.1 and Fe1.02 Se0.4 Te0.3 S0.3 is determined to be FeSe0.38 Te0.59 S0.07 and FeSe0.18 Te0.7 S0.1, respectively. These results show that the tetragonal phase in the sample containing all three chalcogens is substantially enriched by tellurium in respect to the nominal compositions. Moreover, the larger the sulfur content in a sample, the more the deviation of the tellurium content in the superconducting tetragonal phase from the nominal value. It should be noted that in our observations, we did not detect the presence of the hexagonal NiAs-type phase on the flat surface of the crystals despite such a phase was revealed by X-ray diffraction on powder samples with the same nominal composition (see above). It can be assumed that the hexagonal phase is located between the plates of the tetragonal phase. Fig. 6 shows the temperature dependences of the electrical resistivity for the series I (Fig. 6a) and series II (Fig. 6b). All the samples of the series I demonstrate a metallic behavior, while the lower selenium concentration in the series II results in the domination of a semiconducting-like behavior at high sulfur concentrations. The resistivity values above critical temperature significantly increases with increasing sulfur content in both the I and II series. An analogous behavior of the normal state resistivity with increasing S concentration was observed in the Fe1.02 Te0.5 Se0.5- x Sx system [15]. The samples Fe1.02 Se0.5 S0.5 and Fe1.02 Se0.4 S0.6 with the maximal sulfur concentration demonstrate semiconducting behavior even at very low temperatures (see Fig. 7); the activation energy estimated from the exponential slope for the Fe1.02 Se0.5 S0.5 sample is equal to Ea = 0.032 eV. However, the magnetization measurements performed for Fe1.02 Se0.5 S0.5 have revealed a small reduction of the magnetization with decreasing temperature below 5 K, which may be attributed to a small amount of the superconducting phase existing in this sample.
Fig. 8. Critical temperatures for samples of the series I (squares), series II (triangles) and Fe1.02 Te0.5 Se0.5− x Sx (circles) as a function of the sulfur content. Tc values for Fe1.02 Te0.5 Se0.5– x Sx are taken from [15]. Tc °nset , Tc mid and Tc °ffset are shown on the upper, middle and lower panels, respectively.
As follows from the data presented in Fig. 6, the maximal onset critical temperature Tc °nset ≈ 15.1 K is observed for the nonsubstituted Fe1.02 Se0.5 Te0.5 sample. The onset, middle and offset points of the critical temperature (Tc °nset , Tc mid and Tc °ffset ) are plotted in Fig. 8 for both I, II and Fe1.02 Te0.5 Se0.5− x Sx series as a function of the nominal sulfur content (x). Since the resistivity measurements were performed in the temperature range from 8 to 300 K, the zero resistivity could not be reached in samples with x = 0.3 and 0.4 for the series I and with x = 0.4 and 0.5 for the series II; therefore, the Tc °ffset values for these samples were determined by the linear extrapolation. As is seen, the increase of the sulfur concentration reduces critical temperatures in both these series. It is worth mentioning that for the series II, the reduction of critical temperature is observed even at x ≤ 0.2 where the lattice slightly contracts. However, there is a clear difference in the
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tioned above the hexagonal phase exists in the Fe7 (Se1− x Sx )8 system in the whole concentration range (0 ≤ x ≤ 1) [23] unlike in the Fe7 (Se1− x Tex )8 system [24]; therefore, one can assume that the replacement of tellurium by sulfur leads to the growth of the volume of a Fe7 (Se,S)8 -type phase. 4. Conclusion
Fig. 9. The shielding volume fractions for Fe1.02 Sey Te1− y − x Sx as a function of the nominal sulfur concentration. Insets display temperature dependences of the magnetization measured in zero field cooled (ZFC) (H = 8.7 Oe) regime for Fe1.02 Se0.4 Te0.6− x Sx and for Fe1.02 Se0.5 Te0.5− x Sx samples.
suppression rate of critical temperatures with the substitutions. In the series I, the suppression rate of Tc °ffset is higher than that of Tc °nset , while in Fe1.02 Te0.5 Se0.5− x Sx and in the series II, these rates are nearly the same. The slow suppression of Tc °nset in the series I may result from a lesser lattice expansion at the sulfur for tellurium substitution (see Fig. 3). As to a high suppression rate of Tc °ffset and growth of the difference between Tc °nset and Tc °ffset , such a behavior apparently originates in the growth of the nonhomogeneity of the samples with increasing sulfur concentration. As is seen from Fig. 3 and Fig. 8, there is a good correlation between the changes of Tc °nset and the lattice parameter c with substitution: the greater the rate of the lattice expansion along the c axis, the higher the rate of suppression of the critical temperature. It should be also noted that the Tc values for the initial Fe1.02 Se0.5 Te0.5 (x = 0) compound prepared in the present work using a binary chalcogen alloy as a precursor is found to be a bit higher than for the Fe1.02 Te0.5 Se0.5 sample synthesized from pure elements (see Fig. 8). Moreover, the superconducting transition width is observed to be reduced in the case of using the precursor. Bearing in mind the data reported in literature [27–29] in respect to the impact of the chemical inhomogeneities on the superconducting transition in FeSe-based materials one may conclude that preparation method with using an alloy of chalcogens as precursor allows preparing the samples with a better homogeneity. Fig. 9 shows the shielding volume fractions for Fe1.02 Sey Te1− y −x Sx as a function of the nominal sulfur concentration. Insets in Fig. 9 display temperature dependences of the magnetization measured in zero field cooled (H = 8.7 Oe) regime for the Fe1.02 Se0.4 Te0.6− x Sx samples with x = 0, 0.1, 0.3 and 0.5 and for the Fe1.02 Se0.5 Te0.5− x Sx samples with x = 0.1 and 0.4. The onset critical temperatures Tc mag °nset estimated from the magnetization data have approximately the same values as Tc °nset obtained from the resistivity measurements. As is seen from Fig. 9, the shielding volume fractions decreases with increasing S content in all systems with different Te and Se contents. However, these values are slightly lower than that obtained for samples Fe1.02 Te0.5 Se0.5− x Sx [15] with the same sulfur concentrations. Such a difference may be associated with the different volume fractions of hexagonal phase in these systems: in Fe1.02 Te0.5 Se0.5− x Sx the hexagonal phase volume fraction does not change with increasing sulfur content, while in Fe1.02 Se0.5 Te0.5− x Sx and Fe1.02 Se0.4 Te0.6− x Sx systems the hexagonal phase volume fraction increases with x. As was men-
An experimental study of the phase composition, crystal structure and superconducting properties of Fe1.02 Se0.5 Te0.5− x Sx (x = 0 − 0.5) (series I) and Fe1.02 Se0.4 Te0.6− x Sx (x = 0 − 0.6) (series II) compounds, which were synthesized using a ternary alloy of chalcogens as a precursor, was performed. According to the Xray powder diffraction data the superconducting tetragonal phase with the PbO-type structure exists up to nominal sulfur content x = 0.4 in Fe1.02 Se0.5 Te0.5− x Sx and x = 0.5 in Fe1.02 Se0.4 Te0.6− x Sx . The substitution of S for Te keeping the selenium content invariable slightly expands the crystal lattice of the superconducting tetragonal phase despite the smaller ionic radius of sulfur in comparison with that of tellurium. The lattice parameter c of the superconducting phase in Fe1.02 Se0.5 Te0.5− x Sx samples increases monotonously with increasing nominal sulfur concentration, while the concentration dependence of the c lattice parameter in the series II is non-monotonous with a minimum around x = 0.2. The a lattice parameter, which characterizes the in-plane intra-layer distances in the tetragonal phase, remains almost unchanged upon substitutions in all of the studied samples in both series. However, the observed lattice expansion along the c axis with increasing sulfur concentration in series I and II is substantially smaller than that previously reported for the Fe1.02 Te0.5 Se0.5− x Sx system [15]. The observed changes in the crystal lattice of the superconducting tetragonal phase in the Fe(Te,Se,S) samples at the chalcogen substitutions result from the changes in the ratio of coexisting phases (tetragonal and hexagonal) and their chemical composition. The enrichment of the superconducting tetragonal phase by tellurium with increasing sulfur content in the Fe1.02 Se0.5 Te0.5− x Sx and Fe1.02 Se0.4 Te0.6− x Sx samples was confirmed by the EDX analysis. The critical temperature reduces with increasing sulfur content in all of the studied series as well as in previously studied Fe1.02 Se0.5 Te0.5− x Sx . The correlation between the changes of Tc °nset and the c lattice parameter of the tetragonal phase was observed: the larger the rate of the lattice expansion along the c axis, the more the rate of the Tc suppression. The obtained results show that the crystal lattice and superconducting properties of FeSe-based superconductors can be tuned by the variation of S, Se and Te concentrations in the ternary mixture of chalcogens. The changes in the ratio of the volume fractions and in the composition of coexisting phases which appear due to the substitution or doping should be taken into account in further studies of the FeSe-based materials. Acknowledgements The present work was supported by RFBR (project No 16-0200480) the Ministry of Education and Science of Russia (project No 1362) and by the Ural Branch of RAS (Project No 15-17-2-22). Authors are grateful for Dr. N.V. Selezneva for the assistance in the X-Ray examination of synthesized samples. References [1] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, Iron-based layered superconductor La[O1- x Fx ]FeAs (x = 0.05−0.12) with Tc = 26 K, J. Am. Chem. Soc. 130 (2008) 3296. [2] H. Takahashi, K. Igawa, K. Arii, Y. Kamihara, M. Hirano, H. Hosono, Superconductivity at 43 K in an iron-based layered compound LaO1- x Fx FeAs, Nature 453 (2008) 376.
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Physica C: Superconductivity and its applications journal homepage: www.elsevier.com/locate/physc
Characterization of the phase composition, crystal structure and superconducting properties of Fe1.02 Sey Te1−y −x Sx A.S. Abouhaswa a,∗, A.I. Merentsov a, N.V. Baranov a,b a b
Institute of Natural Sciences, Ural Federal University, 620083, Ekaterinburg, Russia M.N. Miheev Institute of Metal Physics, Ural Branch of RAS, 620990, Ekaterinburg, Russia
a r t i c l e
i n f o
Article history: Received 9 April 2016 Revised 16 May 2016 Accepted 18 May 2016 Available online 19 May 2016 Keywords: FeSe-based superconductors Solid state reactions Phase composition Crystal structure Electrical resistivity Magnetization
a b s t r a c t Two series of the Fe1.02 Se0.5 Te0.5–x Sx (I) and Fe1.02 Se0.4 Te0.6–x Sx (II) samples with the sulfur for tellurium substitution and with the invariable Se concentrations have been synthesized and studied by means of X-ray diffraction, scanning electron microscopy, electrical resistivity and magnetic susceptibility measurements. The superconducting PbO-type phase is found to persists in the first series up to x = 0.4 and in the second one up to x = 0.5. Despite the lower ionic radius of sulfur in comparison with tellurium the replacement of tellurium by sulfur does not lead to contraction of the unit cell volume of the superconducting phase in both I and II series with ternary mixture of chalcogens. Variations of the lattice parameters caused by the S for Te substitution in the Fe1.02 Se0.5 Te0.5–x Sx and Fe1.02 Se0.4 Te0.6–x Sx samples are found to be less pronounced than that reported for the Fe1.02 Te0.5 Se0.5- x Sx system and are accompanied by lowering of the critical temperature. The behavior of the lattice parameters and critical temperature of Fe(S,Se,Te) materials with the ternary mixture of chalcogens at substitutions is ascribed to the changes in the volume fraction and chemical compositions of the coexisting tetragonal and hexagonal phases. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The discovery of new iron-based superconductors has initiated advanced search for new superconducting compounds and key factors influencing their properties [1–5]. The common features of these materials are: (i) the layered crystal structure consisting of a set of square lattices of Fe atoms with pnictogen or chalcogen atoms; (ii) the similarity in the morphology of the Fermi surface; and (iii) the interplay between superconductivity and magnetism [4–6]. The FeSe (11) type compounds have a tetragonal crystal structure of the PbO-type (P4/nmm space group) with layers in which Fe cations are tetrahedrally coordinated by chalcogen and with no atoms located within the van der Waals gap. Owing to the simplicity of the crystal structure and similarities in the Fermi surface the FeSe-type compounds can be used as model systems for studying the mechanisms of superconductivity in other Fe-based superconductors having more complex crystal structures. The superconducting critical temperature (Tc ) of FeSe is found to be strongly affected by doping and by substitutions in both the Fe and chalcogen sublattices, as well as by application of hydrostatic pressure and strains (see, e.g., Ref. [4–6] for overview). The Tc values increased up to ∼ 80–100 K were recently reported for the ∗
Corresponding author. E-mail address: [email protected] (A.S. Abouhaswa).
http://dx.doi.org/10.1016/j.physc.2016.05.017 0921-4534/© 2016 Elsevier B.V. All rights reserved.
FeSe monolayer film grown on the SrTiO3 substrates [7,8]. In previous studies [9–14], the substitution effects on the crystal structure and properties of iron chalcogenide superconductors were investigated in the compounds with different binary mixtures of chalcogens (Se–Te, Se–S, Te–S). As was shown recently in [15], the presence of all three chalcogens in the Fe1.02 Te0.5 Se0.5− x Sx compounds results in unusual change of the lattice parameters with substitutions. Bearing in mind that the Fe(Se, Te) samples often exhibit an impurity hexagonal phase of the NiAs-type together with the main superconducting phase having a tetragonal PbO-type phase [16] the aim of the present work is to study how the coexistence of the tetragonal and hexagonal phases and the changes in their volume fractions, chemical composition and lattice parameters may affect the superconducting properties of the 11-type iron-chalcogenide materials. We have synthesized the Fe1.02 Se0.5 Te0.5− x Sx and Fe1.02 Se0.4 Te0.6− x Sx samples with the fixed Se content and studied the effect of the substitution of sulfur for tellurium taking into account the changes in the phase composition, lattice parameters and volume fractions of both phases bearing in mind a narrow homogeneity range of the superconducting phase and a different chalcogen solubility in both tetragonal and hexagonal phases.
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Fig. 1. Observed (symbols) and calculated (line) patterns for the samples Fe1.02 Se0.5 Te0.5− x Sx (series I) with various sulfur concentrations. Vertical bars indicate positions of Bragg reflection for the existing phases. The difference between calculated and observed intensities is shown in the bottom.
2. Experimental details Two series of polycrystalline samples with nominal compositions Fe1.02 Se0.5 Te0.5− x Sx (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) (series I) and Fe1.02 Se0.4 Te0.6− x Sx (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) (series II) were prepared by a solid state reaction method. In order to achieve a better homogeneity of final samples we used a ternary alloy of chalcogens as a precursor instead of a simple mixture of pure chalcogens. At first, the starting materials: selenium, tellurium and sulfur (all of 99.99% purity) were sealed into evacuated quartz tubes and heat treated at 200 °C for 2 h and then for 2 h at 400 °C. The resulting alloy of chalcogens was ground ground into a powder and mixed with an appropriate amount of the iron powder (of 99.98% purity). The obtained mixture was pressed into pellets. In order to reach homogeneity we used a prolongated annealing procedure: the first annealing at 500 °C for 10 h, then at 700 °C for 38 h; the second annealing at 700 °C for 120 h. The obtained samples were again ground, peletized, and then additionally heat treated third time at 700 °C for 72 h. This method of the sample preparation differs from that used in our previous work [15], where the Fe1.02 Te0.5 Se0.5− x Sx compounds were prepared from the mixture of pure chalcogens and iron powder. Beside these polycrystalline samples, two coarse-grained samples with nominal compositions Fe1.02 Se0.5 Te0.4 S0.1 and Fe1.02 Se0.4 Te0.3 S0.3 were prepared by melting at 950 °C with following cooling to room temperature for 48 h. The structure and phase purity of the samples were examined at room temperature by powder X-ray diffraction using a Bruker D8 Advance diffractometer with Cu Ka radiation. The FULLPROF program (Le Bail fit) [17] was used for analysis of diffraction patterns. The chemical composition of the melted samples
was determined by using a Carl Zeiss AURIGA Crossbeam Workstation equipped with an energy dispersive X-ray (EDX) spectrometer. The electrical resistivity of polycrystalline samples was measured by a four-probe dc method in the temperature range from 8 up to 300 K with using a closed-cycle refrigerator. For the magnetization measurements, a SQUID magnetometer MPMS XL-7 EC was used. From the magnetization data, the superconducting volume fractions were estimated taking into account the demagnetizing factor of the samples. 3. Results and discussion Figs. 1 and 2 display the X-ray diffraction patterns for the samples belonging to the series I and series II with the sulfur concentration (x = 0, 0.1, 0.3, 0.5) correspondingly along with the Rietveld refinements using the Fullprof program. For the series I (Fig. 1), the patterns at x ≤ 0.1 exhibit the Bragg reflections belonging to: (i) the dominating tetragonal PbO-type phase (P4/nmm space group), (ii) the impurity phase with a hexagonal NiAs-type structure (P63 /mmc space group), and (iii) iron oxide Fe3 O4 with cubic structure were detected as well. As can be seen from Fig. 1, the peaks of the hexagonal phase begin to grow with increasing sulfur content. Starting from x = 0.3 the peaks of a tetragonal FeS-based phase with the mackinawite structure (P4/nmm space group) appear and begin to grow as well. We noticed that the lattice parameters of the mackinawite-type phase observed in all our samples differ from those reported for FeS. Thus, for the Fe1.02 Se0.5 Te0.2 S0.3 sample, the lattice parameters of this phase were determined as: a = 3.73 A˚ and c = 5.46 A˚ which are higher than a = 3.68 A˚ and
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Fig. 2. Observed (symbols) and calculated (line) patterns for the samples Fe1.02 Se0.4 Te0.6− x Sx (series II) with various sulfur concentrations. Vertical bars indicate positions of Bragg reflection for the existing phases. The difference between calculated and observed intensities is shown in the bottom.
c = 5.03 A˚ observed in FeS [18]. This means that the tetragonal FeS-based phase observed in the series I and II also contains other chalcogens. According to X-ray data in the compound with x = 0.5 (series I), the superconducting tetragonal phase is fully suppressed by the hexagonal and mackinawite-type phases. The similar behavior of the phase composition is observed in the II series (Fig. 2). However, there are some differences from the series I: (i) the reflections of the hexagonal phase with the NiAs-type structure appear at x = 0.1; (ii) the tetragonal FeS-based phase with mackinawite structure disappears at x = 0.6; and the Fe1.02 Se0.4 S0.6 sample contains only the hexagonal FeSe-based phase. The volume fractions of the hexagonal phase are observed to grow with increasing sulfur content in all of the studied series: from ∼10% at x = 0 to ∼71% at x = 0.4 in the series I and from ∼8% at x = 0 to ∼71% at x = 0.5 in the series II. It is worth to mention that in the Fe1.02 Te0.5 Se0.5– x Sx system [15] we didn’t see the disappearance of peaks from the main tetragonal phase even at x = 0.5; only successive shift of the peaks position toward small angles and reduction of the volume fraction of the main phase were observed. The concentration dependences of the a and c lattice parameters of the superconducting phase with the tetragonal PbO-type structure and hexagonal NiAS-type phase for both I and II series are shown in Fig. 3 together with the data for Fe1.02 Te0.5 Se0.5− x Sx taken from our previous work [15]. The Fe1.02 Se0.5 Te0.5 sample was obtained in the present work using the Se0.5 Te0.5 alloy as a precursor. We noticed that the duration of heat treatment at 700 °C influences the crystal structure of the main PbO-type phase. A visible difference in the lattice parameters of Fe1.02 Se0.5 Te0.5 samples (x = 0) synthesized by different methods can be seen, which apparently results from the different homogeneity of the samples. After third heat treatment, the lattice parameter c of the superconduct-
Fig. 3. Values of the lattice parameters and the unit cell volume of the superconducting tetragonal phase for the series I (filled triangles), series II (filled circles) and hexagonal phase (open triangles - series I; open squares - series II). The data for Fe1.02 Te0.5 Se0.5− x Sx samples (crosses) are taken from Ref. [15].
ing phase in Fe1.02 Se0.5 Te0.5− x Sx samples increases monotonously with increasing nominal sulfur concentration (upper panel in Fig. 3), while in the series II, the concentration dependence of the
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c lattice parameter is non-monotonous with a minimum around x = 0.2. As can be seen from Fig. 3, variations of the c parameter of the superconducting tetragonal phase in both series I and II are less pronounced than that observed in the Fe1.02 Te0.5 Se0.5− x Sx system, where the S for Se substitution is found to result in the substantial growth of the average interlayer distance (up to 3.9% [15]). Unlike the c parameter of the superconducting tetragonal phase, the lattice parameter a reflecting the changes in the intra-layer distances remains almost constant upon substitutions in all systems (middle panel). The substitution of sulfur for tellurium in the series I and II also affects not only the volume fraction of the hexagonal phase (see Figs. 1 and 2), but the lattice parameters of this phase as well. As is shown in Fig. 3, the growth of the sulfur content in the samples is accompanied by the expansion of the crystal lattice of the NiAs-type phase in the direction perpendicular to the layers, while the a lattice parameter reduces with increasing S concentration. A decrease of the unit cell volume of the hexagonal phase (see lower panel in Fig. 3) together with the growth of the Bragg peaks belonging to this phase (Figs. 1 and 2) at the substitution implies a possible variation of the chemical composition of the hexagonal phase, which obviously may affect the composition of the superconducting tetragonal phase. Thus, in both the Fe1.02 Te0.5 Se0.5− x Sx and Fe1.02 Se0.5 Te0.5− x Sx series, the replacement of Se or Te by sulfur having lower ionic radius, does not lead to reduction of the unit cell volume of the superconducting tetragonal, as it should be expected considering only the ratio of ionic radii of chalcogens. This behavior differs from that observed in pseudobinary compounds Fe(Te1− x Sex ), Fe(Se1− x Sx ) and Fe(Te1− x Sx ) in which the replacement of a chalcogen with larger ionic radius by another one with smaller ionic radius results in the lattice contraction [10– 12,19]. The anisotropy in the lattice expansion at the substitution correlates with anisotropic compressibility of FeSe-based superconductors evidenced by high-pressure experiments [20]. There are several reasons which can lead to changes in the lattice parameters of the superconducting phase in Fe(Te,Se,S) samples with the ternary mixture of chalcogens at the substitution of sulfur for tellurium or selenium: (i) the change in composition of the ternary mixture of chalcogens in the PbO-type phase; (ii) the variations in the iron/chalcogen ratio in the tetragonal and hexagonal phases due to the changes in their relative volumes; and (iii) the changes in the ionicity/covalency ratio of the iron-chalcogen bonds and van der Waals interactions [15]. It seems that all the mentioned reasons may contribute the observed changes in lattice parameters of these phases. The important point is the substitution limit of different chalcogens in the tetragonal and hexagonal phases. For the superconducting tetragonal PbO-type phase, the substitution limits of different chalgonens were determined in the pseudobinary series as xc = 0.3 for Fe1.02 Se1– x Sx [20] and xc ∼ 0.15 – 0.3 for Fe1.02 Te1– x Sx [21,22]; the solid solution Fe1.02 Te1− x Sex is observed in an extended concentration range (up to x = 0.5) [22]. As to the hexagonal NiAs-type phase, the substitution of sulfur for selenium in the pyrrhotite-type Fe7 (Se1− x Sx )8 compounds is not limited [23], however, for the Fe7 (Se1− x Tex )8 system, the solid solution is reported to exist in a rather narrow Te concentration range (0 < x < 0.15 [24]). The sulfur for selenium substitution in Fe7 (Se1− x Sx )8 leads to the contraction of the unit cell in both the a and c directions [23], as could be expected bearing in mind the difference between ionic radii of sulfur and selenium. However, in Fe7 (Se1− x Tex )8 , the Te for Se substitution was found to result in substantially different changes in the unit cell dimensions: to the contraction of the lattice along the c direction and to the growth of the a lattice parameter [24], i.e., in the flattening of the NiAs-type crystal structure. Such a difference in the lattice deformations caused by substitution in these systems apparently originates in the large polarizability of tellurium atoms in compar-
Fig. 4. Scanning electron microphotograph for the sample with the nominal composition Fe1.02 Se0.5 Te0.4 S0.1 .
Fig. 5. Scanning electron microphotograph for the sample with the nominal composition Fe1.02 Se0.4 Te0.3 S0.3 .
ison with sulfur and selenium (see Ref. [25]). A higher rate of the lattice expansion of the superconducting phase observed in the Fe1.02 Te0.5 Se0.5− x Sx samples with increasing sulfur concentration in comparison with the Fe1.02 Se0.5 Te0.5− x Sx and Fe1.02 Se0.4 Te0.6− x Sx systems (Fig. 3) may be associated with the substantially lower solubility of tellurium in the hexagonal phase Fe7 (Se,Te)8 in comparison with the non-limited solubility of sulfur in Fe7 (Se,S)8 . In the Fe1.02 Te0.5 Se0.5− x Sx samples with the constant tellurium content, the growth of the volume fraction of the hexagonal phase enriched by sulfur due to the S for Se substitution may shift the ternary chalcogen mixture in the tetragonal PbO-type phase toward higher Te concentration. Such a tellurium enrichment of the tetragonal phase together with changes in covalency/ionicity ratio of iron-chalcogen bonds apparently results in the expansion of the tetragonal lattice in the c direction. Note, that a clear trend of the lattice expansion with the S for Se substitution was also recently reported for the Fe1.02 Te0.85 Se0.15− x Sx [26]. In order to reveal the change in the chemical composition of the superconducting tetragonal phase due to S for Te substitution we performed microstructure observations of the crystals taken from the melted Fe1.02 Se0.5 Te0.4 S0.1 and Fe1.02 Se0.4 Te0.3 S0.3 samples. As follows from Figs. 4 and 5 which display the scanning electron micrographs, the crystals exhibit a flat surface morphology and a layered structure. Using the EDX analysis the chemical composition
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Fig. 7. Temperature dependence of the resistivity for the Fe1.02 Se0.5 S0.5 polycrystalline sample. Inset shows the temperature dependence of the magnetization measured in a zero field cooled (ZFC) (H = 8.7 Oe) regime.
Fig. 6. Temperature dependences of the electrical resistivity for the polycrystalline samples of the series I (a) and series II (b). Insets show the resistivity behavior at low temperatures.
of the tetragonal phase in the samples with the nominal compositions Fe1.02 Se0.5 Te0.4 S0.1 and Fe1.02 Se0.4 Te0.3 S0.3 is determined to be FeSe0.38 Te0.59 S0.07 and FeSe0.18 Te0.7 S0.1, respectively. These results show that the tetragonal phase in the sample containing all three chalcogens is substantially enriched by tellurium in respect to the nominal compositions. Moreover, the larger the sulfur content in a sample, the more the deviation of the tellurium content in the superconducting tetragonal phase from the nominal value. It should be noted that in our observations, we did not detect the presence of the hexagonal NiAs-type phase on the flat surface of the crystals despite such a phase was revealed by X-ray diffraction on powder samples with the same nominal composition (see above). It can be assumed that the hexagonal phase is located between the plates of the tetragonal phase. Fig. 6 shows the temperature dependences of the electrical resistivity for the series I (Fig. 6a) and series II (Fig. 6b). All the samples of the series I demonstrate a metallic behavior, while the lower selenium concentration in the series II results in the domination of a semiconducting-like behavior at high sulfur concentrations. The resistivity values above critical temperature significantly increases with increasing sulfur content in both the I and II series. An analogous behavior of the normal state resistivity with increasing S concentration was observed in the Fe1.02 Te0.5 Se0.5- x Sx system [15]. The samples Fe1.02 Se0.5 S0.5 and Fe1.02 Se0.4 S0.6 with the maximal sulfur concentration demonstrate semiconducting behavior even at very low temperatures (see Fig. 7); the activation energy estimated from the exponential slope for the Fe1.02 Se0.5 S0.5 sample is equal to Ea = 0.032 eV. However, the magnetization measurements performed for Fe1.02 Se0.5 S0.5 have revealed a small reduction of the magnetization with decreasing temperature below 5 K, which may be attributed to a small amount of the superconducting phase existing in this sample.
Fig. 8. Critical temperatures for samples of the series I (squares), series II (triangles) and Fe1.02 Te0.5 Se0.5− x Sx (circles) as a function of the sulfur content. Tc values for Fe1.02 Te0.5 Se0.5– x Sx are taken from [15]. Tc °nset , Tc mid and Tc °ffset are shown on the upper, middle and lower panels, respectively.
As follows from the data presented in Fig. 6, the maximal onset critical temperature Tc °nset ≈ 15.1 K is observed for the nonsubstituted Fe1.02 Se0.5 Te0.5 sample. The onset, middle and offset points of the critical temperature (Tc °nset , Tc mid and Tc °ffset ) are plotted in Fig. 8 for both I, II and Fe1.02 Te0.5 Se0.5− x Sx series as a function of the nominal sulfur content (x). Since the resistivity measurements were performed in the temperature range from 8 to 300 K, the zero resistivity could not be reached in samples with x = 0.3 and 0.4 for the series I and with x = 0.4 and 0.5 for the series II; therefore, the Tc °ffset values for these samples were determined by the linear extrapolation. As is seen, the increase of the sulfur concentration reduces critical temperatures in both these series. It is worth mentioning that for the series II, the reduction of critical temperature is observed even at x ≤ 0.2 where the lattice slightly contracts. However, there is a clear difference in the
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A.S. Abouhaswa et al. / Physica C: Superconductivity and its applications 527 (2016) 21–27
tioned above the hexagonal phase exists in the Fe7 (Se1− x Sx )8 system in the whole concentration range (0 ≤ x ≤ 1) [23] unlike in the Fe7 (Se1− x Tex )8 system [24]; therefore, one can assume that the replacement of tellurium by sulfur leads to the growth of the volume of a Fe7 (Se,S)8 -type phase. 4. Conclusion
Fig. 9. The shielding volume fractions for Fe1.02 Sey Te1− y − x Sx as a function of the nominal sulfur concentration. Insets display temperature dependences of the magnetization measured in zero field cooled (ZFC) (H = 8.7 Oe) regime for Fe1.02 Se0.4 Te0.6− x Sx and for Fe1.02 Se0.5 Te0.5− x Sx samples.
suppression rate of critical temperatures with the substitutions. In the series I, the suppression rate of Tc °ffset is higher than that of Tc °nset , while in Fe1.02 Te0.5 Se0.5− x Sx and in the series II, these rates are nearly the same. The slow suppression of Tc °nset in the series I may result from a lesser lattice expansion at the sulfur for tellurium substitution (see Fig. 3). As to a high suppression rate of Tc °ffset and growth of the difference between Tc °nset and Tc °ffset , such a behavior apparently originates in the growth of the nonhomogeneity of the samples with increasing sulfur concentration. As is seen from Fig. 3 and Fig. 8, there is a good correlation between the changes of Tc °nset and the lattice parameter c with substitution: the greater the rate of the lattice expansion along the c axis, the higher the rate of suppression of the critical temperature. It should be also noted that the Tc values for the initial Fe1.02 Se0.5 Te0.5 (x = 0) compound prepared in the present work using a binary chalcogen alloy as a precursor is found to be a bit higher than for the Fe1.02 Te0.5 Se0.5 sample synthesized from pure elements (see Fig. 8). Moreover, the superconducting transition width is observed to be reduced in the case of using the precursor. Bearing in mind the data reported in literature [27–29] in respect to the impact of the chemical inhomogeneities on the superconducting transition in FeSe-based materials one may conclude that preparation method with using an alloy of chalcogens as precursor allows preparing the samples with a better homogeneity. Fig. 9 shows the shielding volume fractions for Fe1.02 Sey Te1− y −x Sx as a function of the nominal sulfur concentration. Insets in Fig. 9 display temperature dependences of the magnetization measured in zero field cooled (H = 8.7 Oe) regime for the Fe1.02 Se0.4 Te0.6− x Sx samples with x = 0, 0.1, 0.3 and 0.5 and for the Fe1.02 Se0.5 Te0.5− x Sx samples with x = 0.1 and 0.4. The onset critical temperatures Tc mag °nset estimated from the magnetization data have approximately the same values as Tc °nset obtained from the resistivity measurements. As is seen from Fig. 9, the shielding volume fractions decreases with increasing S content in all systems with different Te and Se contents. However, these values are slightly lower than that obtained for samples Fe1.02 Te0.5 Se0.5− x Sx [15] with the same sulfur concentrations. Such a difference may be associated with the different volume fractions of hexagonal phase in these systems: in Fe1.02 Te0.5 Se0.5− x Sx the hexagonal phase volume fraction does not change with increasing sulfur content, while in Fe1.02 Se0.5 Te0.5− x Sx and Fe1.02 Se0.4 Te0.6− x Sx systems the hexagonal phase volume fraction increases with x. As was men-
An experimental study of the phase composition, crystal structure and superconducting properties of Fe1.02 Se0.5 Te0.5− x Sx (x = 0 − 0.5) (series I) and Fe1.02 Se0.4 Te0.6− x Sx (x = 0 − 0.6) (series II) compounds, which were synthesized using a ternary alloy of chalcogens as a precursor, was performed. According to the Xray powder diffraction data the superconducting tetragonal phase with the PbO-type structure exists up to nominal sulfur content x = 0.4 in Fe1.02 Se0.5 Te0.5− x Sx and x = 0.5 in Fe1.02 Se0.4 Te0.6− x Sx . The substitution of S for Te keeping the selenium content invariable slightly expands the crystal lattice of the superconducting tetragonal phase despite the smaller ionic radius of sulfur in comparison with that of tellurium. The lattice parameter c of the superconducting phase in Fe1.02 Se0.5 Te0.5− x Sx samples increases monotonously with increasing nominal sulfur concentration, while the concentration dependence of the c lattice parameter in the series II is non-monotonous with a minimum around x = 0.2. The a lattice parameter, which characterizes the in-plane intra-layer distances in the tetragonal phase, remains almost unchanged upon substitutions in all of the studied samples in both series. However, the observed lattice expansion along the c axis with increasing sulfur concentration in series I and II is substantially smaller than that previously reported for the Fe1.02 Te0.5 Se0.5− x Sx system [15]. The observed changes in the crystal lattice of the superconducting tetragonal phase in the Fe(Te,Se,S) samples at the chalcogen substitutions result from the changes in the ratio of coexisting phases (tetragonal and hexagonal) and their chemical composition. The enrichment of the superconducting tetragonal phase by tellurium with increasing sulfur content in the Fe1.02 Se0.5 Te0.5− x Sx and Fe1.02 Se0.4 Te0.6− x Sx samples was confirmed by the EDX analysis. The critical temperature reduces with increasing sulfur content in all of the studied series as well as in previously studied Fe1.02 Se0.5 Te0.5− x Sx . The correlation between the changes of Tc °nset and the c lattice parameter of the tetragonal phase was observed: the larger the rate of the lattice expansion along the c axis, the more the rate of the Tc suppression. The obtained results show that the crystal lattice and superconducting properties of FeSe-based superconductors can be tuned by the variation of S, Se and Te concentrations in the ternary mixture of chalcogens. The changes in the ratio of the volume fractions and in the composition of coexisting phases which appear due to the substitution or doping should be taken into account in further studies of the FeSe-based materials. Acknowledgements The present work was supported by RFBR (project No 16-0200480) the Ministry of Education and Science of Russia (project No 1362) and by the Ural Branch of RAS (Project No 15-17-2-22). Authors are grateful for Dr. N.V. Selezneva for the assistance in the X-Ray examination of synthesized samples. References [1] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, Iron-based layered superconductor La[O1- x Fx ]FeAs (x = 0.05−0.12) with Tc = 26 K, J. Am. Chem. Soc. 130 (2008) 3296. [2] H. Takahashi, K. Igawa, K. Arii, Y. Kamihara, M. Hirano, H. Hosono, Superconductivity at 43 K in an iron-based layered compound LaO1- x Fx FeAs, Nature 453 (2008) 376.
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