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Improvement in structure and superconductivity of bulk FeSe0.5Te0.5 superconductors by optimizing sintering temperature
SMM-10832; No of Pages 4 Scripta Materialia xxx (2015) xxx–xxx
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Scripta Materialia journal homepage: www.elsevier.com/locate/smm
Improvement in structure and superconductivity of bulk FeSe0.5Te0.5 superconductors by optimizing sintering temperature Ning Chen a, Yongchang Liu a, Zongqing Ma a,b,⁎, Liming Yu a, Huijun Li a a b
Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science & Engineering, Tianjin University, Tianjin 300072, People's Republic of China Institute for Superconducting and Electronic Materials, University of Wollongong, NSW 2500, Australia
a r t i c l e
i n f o
Article history: Received 12 August 2015 Received in revised form 22 September 2015 Accepted 23 September 2015 Available online xxxx Keywords: Superconductivity FeSe0.5Te0.5 Sintering temperature
a b s t r a c t Sintering temperature plays a vital role in the evolution of phase structure and microstructure in polycrystalline FeSe0.5Te0.5 bulks fabricated by the two-step sintering method, and thus significantly influences their superconducting properties. Elevated sintering temperature (600–700 °C) at second step facilitates the substitution of Te into the superconducting phase, which leads to the increased lattice distortion and thus contributes to the enhancement of superconductivity (the value of Tc reaches 15.6 K). At the same time, the accelerated growth of the superconducting grains and the improved homogeneity motivated by elevated sintering temperature serve as the main reason for the sharp superconducting transition. © 2015 Elsevier B.V. All rights reserved.
The recent discovery of the iron-based superconductors [1–7] has prompted a great deal of interest in the scientific community despite of the magnetic ordering of iron ions in many compounds. Despite of their low Tc, iron-based superconductors have greater potential for application under high magnetic field in aspects of high Hc2 and small anisotropy than the copper oxides. Very recently, Tc around 8 K was reported in the newly discovered binary superconductor β-FeSe [8], owing to iron-based chalcogenides. In terms of structure, β-FeSe is composed of a stack of edge-sharing FeSe4. As reported by Mizuguchi et al., the Tc of β-FeSe significantly increased to 27 K by the application of a 1.4 GPa pressure [9], which suggests the possibility to increase Tc by the chemical pressure method. Subsequently, it has also been reported that the critical transition temperature of FeSe significantly increases to 15 K with Te substitution on Se site [10]. A chemical pressure is introduced into the structure of the Fe(Se1− xTex) through the substitution of Se by Te. As many experimental and theoretical studies have been focused on Te concentration [11–14] and the chemical addition, such as Li, Ag, Co, Ni, Sn [15–17] on the superconducting performance of FeSe1 − xTex, few investigations have been conducted to reveal the effect of processing temperature during their fabrication procedure, which, however, also play a vital role in understanding the mechanism of superconductivity, optimizing the synthesis technique as well as promoting the practical application for iron-based superconductor. Hereby, the
⁎ Corresponding author. E-mail address: [email protected] (Z. Ma).
nominal FeSe0.5Te0.5 samples in present work were fabricated at various sintering temperatures to get clues about the correlation between phase structure, microstructure and superconductivity. FeSe0.5Te0.5 polycrystalline bulk samples were synthesized through the two-step solid-state reaction route. Stoichiometrically mixed powders of Fe (99.99%), Se (99.9%) and Te (99.99%) were prepared by grinding in an agate mortar and pestle in air, and the well mixed powders were cold-pressed into pellets. The selected sintering temperatures at first step are 550 °C and 700 °C. The sintering process was performed in a tube furnace under the protection of argon gas for 12 h followed by annealing. The reacted samples were reground into fine powders and then repressed into pellets. The sintering treatment at second step was performed at 600 °C and 700 °C for 5 h respectively followed by annealing. The four fabricated samples are denoted by the values of their sintering temperatures at different steps for convenience, i.e. 550–600, 550–700, 700–600, 700–700, respectively. The phase identification of the sintered samples was determined by X-ray diffractometer (XRD, Rigaku D/max 2500) using CuKα radiation. The morphologies were characterized by Scanning Electron Microscopes (SEM, S-4800, Hitachi). The electrical resistivity measurements were carried out in a Quantum Design PPMS system recorded via a four-probe method from 300 K to 5 K. The temperature dependence of the resistivity for FeSe0.5Te0.5 samples sintered at various temperatures is presented in Fig. 1(a). All the samples exhibit semiconducting behavior above 110 K, while below this temperature all the samples exhibit metallic behavior in the normal state, which is consistent with previous research [14]. The values of the resistivity of all the samples are comparative or even smaller than
http://dx.doi.org/10.1016/j.scriptamat.2015.09.039 1359-6462/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: N. Chen, et al., Improvement in structure and superconductivity of bulk FeSe0.5Te0.5 superconductors by optimizing sintering temperature, Scripta Materialia (2015), http://dx.doi.org/10.1016/j.scriptamat.2015.09.039
2
N. Chen et al. / Scripta Materialia xxx (2015) xxx–xxx
Fig. 1. The electrical resistivity for various FeSe0.5Te0.5 samples as a function of temperature, with (a) the whole measured temperature range, (b) the magnified range of 4–24 K and (c) the differential resistivity as a function of temperature.
that of single crystals prepared by Sales et al. [18], confirming the good quality of our samples. The details of the superconducting transition are enlarged in Fig. 1(b). All samples exhibit superconducting transitions, with the results of the superconducting properties shown in Table 1. It is found that Tconset and Tc0 increases a lot when the sintering temperature at second step changes from 600 °C to 700 °C, achieving a maximum Tconset of 15.6 K and Tc0 of 12.5 K in the 700–700 sample. This Tconset value is even higher than the best one reported in the study of Yeh et al. [10]. What's more, the drawn dρ(T)/dT versus temperature curves presented in Fig. 1(c) illustrate the much smaller full width at half maximum (FWHM) in the samples sintered at higher temperature, with the specific values listed in Table 1. The results coincide with the ΔT values, all indicating the better structure in the samples sintered at 700 °C at second step on electrical level. As is known to all, there aren't any chemical reactions among the starting material between 600 °C and 700 °C at the second sintering step. However, some other changes are supposed to occur during the second step sintering process, thus contributing to the enhanced superconducting properties. To understand the enhancing mechanism of superconducting properties clearly, the detailed phase structure and microstructure of all the sintered bulks were studied. Fig. 2(a) shows the powder XRD patterns of FeSe0.5Te0.5 samples sintered at various temperatures. It is clear that the main phase represented by “T” is well indexed to the superconducting tetragonal phase. Certain peaks indexed with inevitable impurity hexagonal Fe7Se8 and Fe3O4 are also detected. All the peaks of the superconducting phase in each pattern seems the same to the ones of another. The magnified (001) and (101) peaks corresponding to the tetragonal phase in various samples are compared in Fig. 2(b) to clarify the variations of peak positions. As many researchers have reported the influence of the nominal FeSe1 − xTex concentration on the superconducting performance, few investigations have been done to reveal the effect of sintering temperature during the fabrication process, which, however, also affects the superconductivity significantly (see Fig. 1). The shift of the two peaks to lower angles indicates increased lattice parameters of the tetragonal phase, the calculated values are presented in Fig. 2(c), which are comparable to that of previous studies [11]. As few investigations have been focused on the reliance of the superconducting phase on the sintering temperature, it's
Table 1 Superconducting properties of samples sintered at various temperature. Samples
Tconset (K)
Tc0 (K)
ΔT (K)
FWHM
RRR
550–600 550–700 700–600 700–700
13.5 14.6 14.8 15.6
9.0 11.0 11.5 12.5
4.5 3.6 3.3 3.1
2.3 1.6 2.4 1.3
1.81 1.83 1.88 2.21
interesting to find that the lattice parameters show a close dependence on the sintering temperatures at second step. The lattice distortion becomes more severe in the 550–700 (or 700–700) sample than that in the 550–600 (or 700–600) one, which means the higher sintering temperature at second step facilitates substitution of Te in the lattice of the superconducting phase. According to the discussion above, the sintering temperature at second step plays a significant role in the diffusion process of Te even though there are no chemical reactions between 600 °C and 700 °C. As reported by Yeh et al., the unit cell of the tetragonal lattice was asymmetrically strained by the substitution of Se by Te in β-FeSe, with the c-axis expansion much more than that of the a-axis [10]. The Fe interlayer interaction is diminished by the larger c-axis and γ angle, which introduces large chemical pressure. Since high sintering temperature facilitates the lattice distortion, causing that the density of states increased at the Fermi level, it's not hard to understand the increased Tconset and Tc0. A deep look into the microstructure of the superconducting phase was conducted by SEM, and the images are shown in Fig. 3. As seen from the images, the superconducting phase exhibits a typical lamellar crystal structure. One may find the gradual growth of the lamellar crystals as the sintering temperature increases with cautious comparison. The thickness of the lamellar structure unit develops from ~ 50 nm (Fig. 3(a)) to ~300 nm (Fig. 3(d)), which is attributed to the growth of grains at relative elevated temperature. Due to the increasing sintering temperature at second step, the thicker lamellar grains surely provide a wider channel for carriers. But that is not necessary to bring about the enhanced superconductivity, only if it is combined with the overall microstructure. Special attention should be paid to the overall microstructural evolution owing to the temperature variation. The general microstructures of all the samples are presented in Fig. 4. As one can see, the distribution of Fe7Se8 and Fe3O4 around the superconducting grains which are represented by different indications is quite heterogeneous in the 550–600 sample as shown in (a). Compared to the samples with lower sintering temperatures at second step (see (a) and (c)), the samples sintered at 700 °C at second step (see (b) and (d)) exhibit more homogeneous structures with the superconducting grains stacked closely. Current transmission among superconducting grains in samples with promoted homogeneity is much easier than that in heterogeneous samples. To confirm the promoted homogeneity, one can calculate the residual resistivity ratio (RRR=ρ(300 K)/ρ(Tconset)) values of the samples in Table 1, which are acceptable according to the data obtained by K. Onar et al. The results show that the samples sintered at 700 °C at second step exhibit higher RRR values than that sintered at 600 °C at second step, in which higher RRR is a reflection of improved homogeneity on the electrical level [19]. As is evident, the higher sintering temperature at second step accelerates the growth of the superconducting grains and promotes
Please cite this article as: N. Chen, et al., Improvement in structure and superconductivity of bulk FeSe0.5Te0.5 superconductors by optimizing sintering temperature, Scripta Materialia (2015), http://dx.doi.org/10.1016/j.scriptamat.2015.09.039
N. Chen et al. / Scripta Materialia xxx (2015) xxx–xxx
3
Fig. 2. (a) XRD patterns of FeSe0.5Te0.5 samples synthesized at various temperatures. (b) The magnified (001) and (101) peaks of the tetragonal phase in various samples. (c) The temperature dependence of lattice parameters of the tetragonal phase.
the homogeneity of the samples as well. The thicker lamellar grains provide a wider channel for carriers, while the promoted homogeneity facilitates the current transmission. This is the main reason for the sharp superconducting transitions in 550–700 and 700–700 samples. In summary, FeSe0.5Te0.5 polycrystalline samples have been synthesized by the two-step sintering at different temperatures and the correlation between structure and superconductivity motivated by various sintering temperature has been investigated. The sintering temperature at second step plays a vital role in the evolution of phase structure and microstructure. The phase structure analysis indicates elevated sintering temperature at second step can increase chemical pressure by facilitating the dissolution of Te into the superconducting phase, which contributes to the increase of Tconset. Meanwhile, the accelerated
growth of the superconducting grains and the promoted homogeneity motivated by elevated sintering temperature serve as the main reason for the sharp superconducting transition. Acknowledgments The authors are grateful to China National Funds for Distinguished Young Scientists (Grant No. 51325401), the National Natural Science Foundation of China (Grant No. 51302186 and 51574178), the Natural Science Foundation of Tianjin (Grant No. 14JCQNJC03300) and The National High Technology Research and Development Program (“863”Program) of China (Granted No. 2015AA042504). This work is also supported by the Australian Research Council (Grant No. DE140101333).
Fig. 3. The SEM images of the superconducting grains in (a) 550–600, (b) 550–700, (c) 700–600 and (d) 700–700 samples.
Please cite this article as: N. Chen, et al., Improvement in structure and superconductivity of bulk FeSe0.5Te0.5 superconductors by optimizing sintering temperature, Scripta Materialia (2015), http://dx.doi.org/10.1016/j.scriptamat.2015.09.039
4
N. Chen et al. / Scripta Materialia xxx (2015) xxx–xxx
Fig. 4. The typical SEM images of (a) 550–600, (b) 550–700, (c) 700–600 and (d) 700–700 samples.
References [1] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, J. Am. Chem. Soc. 130 (2008) 3296–3297. [2] G.F. Chen, Z. Li, D. Wu, G. Li, W.Z. Hu, J. Dong, P. Zheng, J.L. Luo, N.L. Wang, Phys. Rev. Lett. 100 (2008) 247002. [3] H.H. Wen, G. Mu, L. Fang, H. Yang, X. Zhu, Europhys. Lett. 82 (2008) 17009. [4] T. Wu, G. Wu, R.H. Liu, H. Chen, D.F. Fang, Nature 453 (2008) 761–762. [5] H. Takahashi, K. Igawa, K. Arii, Y. Kamihara, M. Hirano, H. Hosono, Nature 453 (2008) 376–378. [6] L. Li, Y.K. Li, Z. Ren, Y.K. Luo, X. Lin, M. He, Q. Tao, Z.W. Zhu, G.H. Cao, Z.A. Xu, Phys. Rev. B 78 (2008), 132506. [7] Z.A. Ren, W. Lu, J. Yang, W. Yi, X.L. Shen, Z.C. Li, G.C. Che, X.L. Dong, L.L. Sun, F. Zhou, Z.X. Zhao, Chin. Phys. Lett. 25 (2008) 2215–2216. [8] F.C. Hsu, J.Y. Luo, K.W. Yeh, T.K. Chen, T.W. Huang, P.M. Wu, Y.C. Lee, Y.L. Huang, Y.Y. Chu, D.C. Yan, M.K. Wu, Proc. Natl. Acad. Sci. U. S. A. 105 (2008) 14262–14264. [9] Y. Mizuguchi, F. Tomioka, S. Tsuda, T. Yamaguchi, Appl. Phys. Lett. 93 (2008) 152505. [10] K.W. Yeh, T.W. Huang, Y.L. Huang, T.K. Chen, F.C. Hsu, P.M. Wu, Y.C. Lee, Y.Y. Chu, C.L. Chen, J.Y. Luo, D.C. Yan, M.K. Wu, Europhys. Lett. 84 (2008), 37002.
[11] M.Y. Hacisalihoglu, E. Yanmaz, J. Supercond. Nov. Magn. 26 (2013) 2369–2374. [12] R.W. Gómez, V. Marquina, J.L. Pérez-Mazariego, R. Escamilla, R. Escudero, M. Quintana, J.J. Hernández-Gómez, R. Ridaura, M.L. Marquina, J. Supercond. Nov. Magn. 23 (2010) 551–557. [13] K.W. Yeh, H.C. Hsu, T.W. Huang, P.M. Wu, Y.L. Huang, T.K. Chen, J.Y. Luo, M.K. Wu, J. Phys. Soc. Jpn. 77 (2008) 19–22. [14] V.P.S. Awana, A. Pal, A. Vajpayee, M. Mudgel, H. Kishan, M. Husain, R. Zeng, S. Yu, Y.F. Guo, Y.G. Shi, K. Yamaura, E. Takayama-Muromachi, J. Appl. Phys. 107 (2010), 09E128-3. [15] C.M. Yang, P.W. Chen, I.G. Chen, X.D. Qi, D.C. Yan, M.K.Wu. Supercond, Sci. Technol. 25 (2012) 095010. [16] M. Migita, Y. Takikawa, K. Sugai, M. Takeda, M. Uehara, T. Kuramoto, Y. Takano, Y. Mizuguchi, Y. Kimishima, Physica C 484 (2013) 66–68. [17] A. Kumar, R.P. Tandon, V.P.S. Awana, IEEE Trans. Magn. 48 (2012) 4239–4241. [18] B.C. Sales, A.S. Sefat, M.A. McGuire, R.Y. Jin, D. Mandrus, Phys. Rev. B 79 (2009) 094521. [19] K. Onar, M.E. Yakinci, J. Alloys Compd. 620 (2015) 210–216.
Please cite this article as: N. Chen, et al., Improvement in structure and superconductivity of bulk FeSe0.5Te0.5 superconductors by optimizing sintering temperature, Scripta Materialia (2015), http://dx.doi.org/10.1016/j.scriptamat.2015.09.039
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/smm
Improvement in structure and superconductivity of bulk FeSe0.5Te0.5 superconductors by optimizing sintering temperature Ning Chen a, Yongchang Liu a, Zongqing Ma a,b,⁎, Liming Yu a, Huijun Li a a b
Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science & Engineering, Tianjin University, Tianjin 300072, People's Republic of China Institute for Superconducting and Electronic Materials, University of Wollongong, NSW 2500, Australia
a r t i c l e
i n f o
Article history: Received 12 August 2015 Received in revised form 22 September 2015 Accepted 23 September 2015 Available online xxxx Keywords: Superconductivity FeSe0.5Te0.5 Sintering temperature
a b s t r a c t Sintering temperature plays a vital role in the evolution of phase structure and microstructure in polycrystalline FeSe0.5Te0.5 bulks fabricated by the two-step sintering method, and thus significantly influences their superconducting properties. Elevated sintering temperature (600–700 °C) at second step facilitates the substitution of Te into the superconducting phase, which leads to the increased lattice distortion and thus contributes to the enhancement of superconductivity (the value of Tc reaches 15.6 K). At the same time, the accelerated growth of the superconducting grains and the improved homogeneity motivated by elevated sintering temperature serve as the main reason for the sharp superconducting transition. © 2015 Elsevier B.V. All rights reserved.
The recent discovery of the iron-based superconductors [1–7] has prompted a great deal of interest in the scientific community despite of the magnetic ordering of iron ions in many compounds. Despite of their low Tc, iron-based superconductors have greater potential for application under high magnetic field in aspects of high Hc2 and small anisotropy than the copper oxides. Very recently, Tc around 8 K was reported in the newly discovered binary superconductor β-FeSe [8], owing to iron-based chalcogenides. In terms of structure, β-FeSe is composed of a stack of edge-sharing FeSe4. As reported by Mizuguchi et al., the Tc of β-FeSe significantly increased to 27 K by the application of a 1.4 GPa pressure [9], which suggests the possibility to increase Tc by the chemical pressure method. Subsequently, it has also been reported that the critical transition temperature of FeSe significantly increases to 15 K with Te substitution on Se site [10]. A chemical pressure is introduced into the structure of the Fe(Se1− xTex) through the substitution of Se by Te. As many experimental and theoretical studies have been focused on Te concentration [11–14] and the chemical addition, such as Li, Ag, Co, Ni, Sn [15–17] on the superconducting performance of FeSe1 − xTex, few investigations have been conducted to reveal the effect of processing temperature during their fabrication procedure, which, however, also play a vital role in understanding the mechanism of superconductivity, optimizing the synthesis technique as well as promoting the practical application for iron-based superconductor. Hereby, the
⁎ Corresponding author. E-mail address: [email protected] (Z. Ma).
nominal FeSe0.5Te0.5 samples in present work were fabricated at various sintering temperatures to get clues about the correlation between phase structure, microstructure and superconductivity. FeSe0.5Te0.5 polycrystalline bulk samples were synthesized through the two-step solid-state reaction route. Stoichiometrically mixed powders of Fe (99.99%), Se (99.9%) and Te (99.99%) were prepared by grinding in an agate mortar and pestle in air, and the well mixed powders were cold-pressed into pellets. The selected sintering temperatures at first step are 550 °C and 700 °C. The sintering process was performed in a tube furnace under the protection of argon gas for 12 h followed by annealing. The reacted samples were reground into fine powders and then repressed into pellets. The sintering treatment at second step was performed at 600 °C and 700 °C for 5 h respectively followed by annealing. The four fabricated samples are denoted by the values of their sintering temperatures at different steps for convenience, i.e. 550–600, 550–700, 700–600, 700–700, respectively. The phase identification of the sintered samples was determined by X-ray diffractometer (XRD, Rigaku D/max 2500) using CuKα radiation. The morphologies were characterized by Scanning Electron Microscopes (SEM, S-4800, Hitachi). The electrical resistivity measurements were carried out in a Quantum Design PPMS system recorded via a four-probe method from 300 K to 5 K. The temperature dependence of the resistivity for FeSe0.5Te0.5 samples sintered at various temperatures is presented in Fig. 1(a). All the samples exhibit semiconducting behavior above 110 K, while below this temperature all the samples exhibit metallic behavior in the normal state, which is consistent with previous research [14]. The values of the resistivity of all the samples are comparative or even smaller than
http://dx.doi.org/10.1016/j.scriptamat.2015.09.039 1359-6462/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: N. Chen, et al., Improvement in structure and superconductivity of bulk FeSe0.5Te0.5 superconductors by optimizing sintering temperature, Scripta Materialia (2015), http://dx.doi.org/10.1016/j.scriptamat.2015.09.039
2
N. Chen et al. / Scripta Materialia xxx (2015) xxx–xxx
Fig. 1. The electrical resistivity for various FeSe0.5Te0.5 samples as a function of temperature, with (a) the whole measured temperature range, (b) the magnified range of 4–24 K and (c) the differential resistivity as a function of temperature.
that of single crystals prepared by Sales et al. [18], confirming the good quality of our samples. The details of the superconducting transition are enlarged in Fig. 1(b). All samples exhibit superconducting transitions, with the results of the superconducting properties shown in Table 1. It is found that Tconset and Tc0 increases a lot when the sintering temperature at second step changes from 600 °C to 700 °C, achieving a maximum Tconset of 15.6 K and Tc0 of 12.5 K in the 700–700 sample. This Tconset value is even higher than the best one reported in the study of Yeh et al. [10]. What's more, the drawn dρ(T)/dT versus temperature curves presented in Fig. 1(c) illustrate the much smaller full width at half maximum (FWHM) in the samples sintered at higher temperature, with the specific values listed in Table 1. The results coincide with the ΔT values, all indicating the better structure in the samples sintered at 700 °C at second step on electrical level. As is known to all, there aren't any chemical reactions among the starting material between 600 °C and 700 °C at the second sintering step. However, some other changes are supposed to occur during the second step sintering process, thus contributing to the enhanced superconducting properties. To understand the enhancing mechanism of superconducting properties clearly, the detailed phase structure and microstructure of all the sintered bulks were studied. Fig. 2(a) shows the powder XRD patterns of FeSe0.5Te0.5 samples sintered at various temperatures. It is clear that the main phase represented by “T” is well indexed to the superconducting tetragonal phase. Certain peaks indexed with inevitable impurity hexagonal Fe7Se8 and Fe3O4 are also detected. All the peaks of the superconducting phase in each pattern seems the same to the ones of another. The magnified (001) and (101) peaks corresponding to the tetragonal phase in various samples are compared in Fig. 2(b) to clarify the variations of peak positions. As many researchers have reported the influence of the nominal FeSe1 − xTex concentration on the superconducting performance, few investigations have been done to reveal the effect of sintering temperature during the fabrication process, which, however, also affects the superconductivity significantly (see Fig. 1). The shift of the two peaks to lower angles indicates increased lattice parameters of the tetragonal phase, the calculated values are presented in Fig. 2(c), which are comparable to that of previous studies [11]. As few investigations have been focused on the reliance of the superconducting phase on the sintering temperature, it's
Table 1 Superconducting properties of samples sintered at various temperature. Samples
Tconset (K)
Tc0 (K)
ΔT (K)
FWHM
RRR
550–600 550–700 700–600 700–700
13.5 14.6 14.8 15.6
9.0 11.0 11.5 12.5
4.5 3.6 3.3 3.1
2.3 1.6 2.4 1.3
1.81 1.83 1.88 2.21
interesting to find that the lattice parameters show a close dependence on the sintering temperatures at second step. The lattice distortion becomes more severe in the 550–700 (or 700–700) sample than that in the 550–600 (or 700–600) one, which means the higher sintering temperature at second step facilitates substitution of Te in the lattice of the superconducting phase. According to the discussion above, the sintering temperature at second step plays a significant role in the diffusion process of Te even though there are no chemical reactions between 600 °C and 700 °C. As reported by Yeh et al., the unit cell of the tetragonal lattice was asymmetrically strained by the substitution of Se by Te in β-FeSe, with the c-axis expansion much more than that of the a-axis [10]. The Fe interlayer interaction is diminished by the larger c-axis and γ angle, which introduces large chemical pressure. Since high sintering temperature facilitates the lattice distortion, causing that the density of states increased at the Fermi level, it's not hard to understand the increased Tconset and Tc0. A deep look into the microstructure of the superconducting phase was conducted by SEM, and the images are shown in Fig. 3. As seen from the images, the superconducting phase exhibits a typical lamellar crystal structure. One may find the gradual growth of the lamellar crystals as the sintering temperature increases with cautious comparison. The thickness of the lamellar structure unit develops from ~ 50 nm (Fig. 3(a)) to ~300 nm (Fig. 3(d)), which is attributed to the growth of grains at relative elevated temperature. Due to the increasing sintering temperature at second step, the thicker lamellar grains surely provide a wider channel for carriers. But that is not necessary to bring about the enhanced superconductivity, only if it is combined with the overall microstructure. Special attention should be paid to the overall microstructural evolution owing to the temperature variation. The general microstructures of all the samples are presented in Fig. 4. As one can see, the distribution of Fe7Se8 and Fe3O4 around the superconducting grains which are represented by different indications is quite heterogeneous in the 550–600 sample as shown in (a). Compared to the samples with lower sintering temperatures at second step (see (a) and (c)), the samples sintered at 700 °C at second step (see (b) and (d)) exhibit more homogeneous structures with the superconducting grains stacked closely. Current transmission among superconducting grains in samples with promoted homogeneity is much easier than that in heterogeneous samples. To confirm the promoted homogeneity, one can calculate the residual resistivity ratio (RRR=ρ(300 K)/ρ(Tconset)) values of the samples in Table 1, which are acceptable according to the data obtained by K. Onar et al. The results show that the samples sintered at 700 °C at second step exhibit higher RRR values than that sintered at 600 °C at second step, in which higher RRR is a reflection of improved homogeneity on the electrical level [19]. As is evident, the higher sintering temperature at second step accelerates the growth of the superconducting grains and promotes
Please cite this article as: N. Chen, et al., Improvement in structure and superconductivity of bulk FeSe0.5Te0.5 superconductors by optimizing sintering temperature, Scripta Materialia (2015), http://dx.doi.org/10.1016/j.scriptamat.2015.09.039
N. Chen et al. / Scripta Materialia xxx (2015) xxx–xxx
3
Fig. 2. (a) XRD patterns of FeSe0.5Te0.5 samples synthesized at various temperatures. (b) The magnified (001) and (101) peaks of the tetragonal phase in various samples. (c) The temperature dependence of lattice parameters of the tetragonal phase.
the homogeneity of the samples as well. The thicker lamellar grains provide a wider channel for carriers, while the promoted homogeneity facilitates the current transmission. This is the main reason for the sharp superconducting transitions in 550–700 and 700–700 samples. In summary, FeSe0.5Te0.5 polycrystalline samples have been synthesized by the two-step sintering at different temperatures and the correlation between structure and superconductivity motivated by various sintering temperature has been investigated. The sintering temperature at second step plays a vital role in the evolution of phase structure and microstructure. The phase structure analysis indicates elevated sintering temperature at second step can increase chemical pressure by facilitating the dissolution of Te into the superconducting phase, which contributes to the increase of Tconset. Meanwhile, the accelerated
growth of the superconducting grains and the promoted homogeneity motivated by elevated sintering temperature serve as the main reason for the sharp superconducting transition. Acknowledgments The authors are grateful to China National Funds for Distinguished Young Scientists (Grant No. 51325401), the National Natural Science Foundation of China (Grant No. 51302186 and 51574178), the Natural Science Foundation of Tianjin (Grant No. 14JCQNJC03300) and The National High Technology Research and Development Program (“863”Program) of China (Granted No. 2015AA042504). This work is also supported by the Australian Research Council (Grant No. DE140101333).
Fig. 3. The SEM images of the superconducting grains in (a) 550–600, (b) 550–700, (c) 700–600 and (d) 700–700 samples.
Please cite this article as: N. Chen, et al., Improvement in structure and superconductivity of bulk FeSe0.5Te0.5 superconductors by optimizing sintering temperature, Scripta Materialia (2015), http://dx.doi.org/10.1016/j.scriptamat.2015.09.039
4
N. Chen et al. / Scripta Materialia xxx (2015) xxx–xxx
Fig. 4. The typical SEM images of (a) 550–600, (b) 550–700, (c) 700–600 and (d) 700–700 samples.
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Please cite this article as: N. Chen, et al., Improvement in structure and superconductivity of bulk FeSe0.5Te0.5 superconductors by optimizing sintering temperature, Scripta Materialia (2015), http://dx.doi.org/10.1016/j.scriptamat.2015.09.039