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Effects of Fe doping on crystal structure, superconductivity and Raman spectra in SmBa2Cu3O7−δ systems
Physica C 475 (2012) 20–23
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Physica C journal homepage: www.elsevier.com/locate/physc
Effects of Fe doping on crystal structure, superconductivity and Raman spectra in SmBa2Cu3O7 d systems Renzhong Xue, Zhenping Chen ⇑, Haiyang Dai, Tao Li, Yuncai Xue, Junhong Hao Department of Technology and Physics, Zhengzhou University of Light Industry, Zhengzhou 450002, China
a r t i c l e
i n f o
Article history: Received 8 October 2011 Received in revised form 15 December 2011 Accepted 4 January 2012 Available online 3 February 2012 Keywords: SmBa2Cu3 xFexO7 d Phase transition Electrical properties Raman spectra
a b s t r a c t SmBa2Cu3 xFexO7 d (SBCFO) (x = 0.0–0.4) systems are prepared by the usual solid-state reaction technique. The effect of Fe doping on the structure, electronic transport properties and Raman spectra of SBCFO systems have been investigated. The X-ray measurement indicates that Fe ions have a significant effect on the main crystalline structure and SBCFO undergoes a structure phase transition from orthorhombic to tetragonal between x = 0.05 and 0.1. The superconducting (SC) transition temperature Tc decreases with the increase of Fe-doping content; an insulator transition appears in high doping content samples (x P 0.3). We have discussed the Raman shifts and intensity of the five normal phonon peaks and the other modes at 229 and 589 cm 1 which result from oxygen deficiency in the chain structure. These investigations reveal that the electrical transport properties and Raman spectra of SmBa2Cu3 xFexO7 d composites obviously depend on O–T transition induced by the Fe doping. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction YBa2Cu3O7 d compound system has been widely studied for improving the superconducting properties by many researchers since it was discovered. A great number of papers have been published on adding elements in Y123 system for researching the phase transition and the physical properties of YBCO samples [1–3]. By cationic substitution of different sites, REBa2Cu3O7 d (RE123, RE – rare earth elements) possesses different physical, chemical and crystallographic properties. Studies on these materials can provide some insight into the mechanism of superconductivity and improve the superconducting properties. Cationic substitutions include the replacement of Y sites by rare earth elements, the replacement of Ba sites by alkaline earth elements, and the replacement of Cu sites by metal elements. Among them, the REBa2(Cu1 xMx)3O7 d type materials (where M is a metal element like Zn, Fe, Ni, Co, Al, etc.) are of great importance. REBa2Cu3O7 d system has a unique feature; it has two non-equivalents Cu sites viz., the linear-chain Cu(1) in the O(1)–Cu(1)–O(1) units and the planar Cu(2) in the CuO2 sheets containing O(2) and O(3). The CuO2 plane is referred to as the superconducting plane: any modification in this plane strongly influences the electronic structure and the density, mobility of the charge carriers and thus suppresses the superconductivity of the systems [1,4–6]. In spite of many experimental and theoretical reports on doping YBCO with these elements, their destructive effect on superconductivity is still not completely understood. A large number of studies have therefore ⇑ Corresponding author. Tel./fax: +86 371 63556807. E-mail address: [email protected] (Z. Chen). 0921-4534/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2012.01.007
been carried out by substituting various cations for Cu as well as Y and Ba and these have yielded much useful information about the lattice and electronic structure of the systems [7,8]. Raman spectroscopy describes the active phonons and serves to determine the disorder, stoichiometry, superstructure, orientation, and impurity phases for the characterization of high-Tc cuprates such as poly- and single-crystalline samples or epitaxial films. For superconducting systems, the frequencies, the polarization properties and the resonant behavior of Raman-allowed phonons are often employed to explain the characteristic superstructures [9]. There are two merits of Raman measurements on the investigation for the crystal properties, compared to XRD. One is, in the case of YBCO, to estimate intuitively the change of the oxygen content in chain site by Raman spectra. The other is to detect impurity phases, too trivial (1–2 lm) for XRD, since the spot size of laser is comparable to the domain size of the impurity [10]. Thus, we can obtain local information on the crystal structure from the peaks in Raman spectra. In the present work, we investigate Fe additions in SBCO bulk and their influence on the transport properties. For this purpose, a series of SBCO specimens with different amounts of Fe-doping (x = 0.0–0.4) were prepared by the solid state reaction. We focus on the analysis of the structure, microstructure and the electrical properties. 2. Experimental details The polycrystalline samples of SmBa2Cu3 xFexO7 d for x = 0.0, 0.05, 0.1, 0.2, 0.3 and 0.4 were prepared by the usual solid-state reaction technique. Appropriate amounts of high purity of Sm2O3,
21
R. Xue et al. / Physica C 475 (2012) 20–23
3.94
3.92
3.90
a, b, c/3 (Å)
BaCO3, CuO, and Fe2O3 were mixed, ground and heated at 940 °C for 24 h to ensure perfect homogeneity and complete the solidstate reaction. The resulted powders were reground and pressed into pellets. The pellets were sintered in flowing oxygen at 940 °C for 12 h followed by slow cooling (1 °C/min) to 450 °C. Then they were kept at 450 °C for 72 h in flowing O2 and then gradually cooled to room temperature. The phase identification of the prepared samples was carried out using X-ray powder diffraction (XRD). Lattice parameters were calculated by the least-square method with PowderX program [11]. The electrical transport properties were performed by the standard four-probe dc resistance technique with indium contacts in conjunction. Voltages were recorded by the Aglient HP-34401A voltmeter. The Raman measurement was performed in the quasi-backscattering geometry using 50 mW of the 532 nm line of a semiconductor laser via a Renishaw inVia spectrometer with the resolution of 0.5 cm 1.
3.88
a b c /3
3.86
3.84
3.82 0.0
0.1
0.2
0.3
0.4
x Fig. 2. The lattice parameters a, b and c/3 versus Fe content x for SmBa2Cu3 samples.
3. Results and discussion
xFexO7 d
Fig. 1 shows the X-ray diffraction patterns of the SmBa2 Cu3 xFexO7 d samples. It is obvious that the Fe-doping has a significant effect on the main crystalline structure of the Sm-123. All samples are characterized as orthorhombic or tetragonal phase and show no observable impurity peaks. Fig. 2 reveals the changes of lattice parameters with doping content x for all the samples. With increasing Fe-doping content x from 0.0 to 0.1, the lattice parameter a increases while b decreases. When x P 0.1, the lattice parameters a and b become identical, indicating that the O–T phase transition is completed. The lattice parameter may be related to the iron preferential substitution for copper at Cu(1) or Cu(2) sites. Fe ion preferentially substitutes for copper at the Cu(1) sites as x < 0.15 and at Cu(2) sites as x > 0.25 [12]. Fe displays a valence of +3, which is more than the average Cu(1) valence of 2.33. This imbalance pulls extra oxygen occupying the vacancy positions along a direction, thereby distorting the orthorhombic configuration. For even higher doping (x > 0.2), Fe atoms will substitute at the Cu(2) sites. Fe doping brings a strong and expanded disturbance on electron (hole) distribution; this will lead to the increase of lattice parameters a, b and the decrease of parameter c. Thus it can be seen that Fe doping leads to oxygen stoichiometry modifications and correspondingly structural transformation from orthorhombic to tetragonal structure, namely the doping content affects the structure of the samples indeed. It is consistent with the results of Gd123 reported by Chen et al. [13] and Kakeshita et al. [14].
Fig. 3 shows the temperature dependence of the resistivity for SmBa2Cu3 xFexO7 d samples. It can be seen that the superconducting transition has happened for the samples with x = 0.0–0.2 above 10 K. The superconducting transition temperature Tc decreases continuously with increasing of Fe doping content. The samples with x less than 0.1 show metallic behavior in normal state. Further Fe substitution results in a semiconducting behavior. The resistivity of samples x = 0.1 and 0.2 increases as the temperature decreases from 120 K and 80 K to the onset of superconducting transition respectively. And an insulator transition appears in high doping content samples (x P 0.3). The inset of Fig. 3 shows the onset temperature Tc,onset and the zero-resistance critical temperature Tc,zero of samples as a function of Fe doping content. It can be found that Tc,zero decreases and the transition width DT = Tc,onset Tc,zero increases with increasing Fe doping content. A YBCO perovskite unit cell has five different oxygen sites: O(2, 3) in CuO2 planes, O(1) in CuO chains, and O(4) in apical chains. The O(5) sites situated between the oxygen chains are nearly empty in the orthorhombic structure [1]. Fig. 4 shows a series of typical Raman spectra for the various Fe doping content. The error bar of the Raman frequency is about 0.5 cm 1 determined from the instrumental resolution. For the undoped sample in
14
80
x =0.40
Tc,onset Tc,zero 60
.cm)
x =0.3 x =0.2 x =0.1
20
100 310
220
206
116
006
200
110 005 113
103
100
213
60
80
0.05
0.10
0.15
0.20
Fe content x
x =0.10 x =0.05 x =0.0
0 50
40
0.00
x =0.20
6
2
20
40
x =0.30
4
x =0.05 x =0
10 8
ρ 10-5 (
Intensity
x =0.4
Tc (K)
12
100
150
200
250
Temperature (K)
2θ (Degree) Fig. 1. XRD patterns for the experimental SmBa2Cu3 xFexO7
d
samples.
Fig. 3. The temperature dependence of the resistivity for the experimental SmBa2Cu3 xFexO7 d samples. The inset is the Tc,onset and Tc,zero of the samples.
R. Xue et al. / Physica C 475 (2012) 20–23
468 329
(f)
473
Intensity
476
568
618 (e) (d)
479 (c) 502 141 110
222
332 (b)
592
440 225
100
200
(a) 300
400
Raman shift
500
600
700
(cm-1)
Fig. 4. Raman spectra obtained from Fe-doped SmBa2Cu3 xFexO7 x = 0.0, (b) x = 0.05, (c) x = 0.1, (d) x = 0.2, (e) x = 0.3, (f) x = 0.4.
d
samples: (a)
Fig. 4a, it is clear that four peaks assigned to four Ag modes and one Blg mode of Sm123 structure can be seen in the spectra: the apical oxygen O(4) vibration along the c-axis, namely, O(4)–Ag mode (502 cm 1); two vibrations of the O(2, 3) oxygen atoms in the CuO2 planes, with one in-phase, namely, O(2, 3)–Ag mode (440 cm 1) and the other out-of-phase, namely, O(2, 3)–Blg mode (332 cm 1) respectively. Other Raman active modes are vertical vibrations along the c-axis of Cu(2) and Ba atoms, namely, Cu(2)– Ag mode (141 cm 1) and Ba–Ag mode (110 cm 1) respectively. All the frequencies are well known for phonons recorded in the orthorhombic structure [13,15,16]. Furthermore, some additional peaks are also seen in some composition. For example, the two peaks are at 229 and 589 cm 1 respectively in pure SBCO. With increasing of Fe doping level, the other peaks also gradually emerge at 568, 618 cm 1 in the samples (x > 0.1). Since Fe ions locate primarily at the Cu(1) sites at low doping level for the Fe-doped Sm123 systems, we firstly focus on the mode at 225 and 592 cm 1, which is the cation disorder mode reported by Maroni et al. [17] and Hiep et al. [18]. Doping Fe ions could destroy the symmetry of the Cu-O along the a/b-axis and then affect the vibration of Cu(1) and O(1). As a result, the intensity of 229 cm 1 mode gradually decreases and the peaks shift randomly with Fe-doping. The 592 cm 1 mode becomes broad and weak in the other compositions, in contrast to undoped sample. The results mean that the disorder of Cu site by Fe doping is enhanced in the compositions. As Fe doping content increases, some of the Fe ions randomly occupy the Cu(2) sites which could influence the Cu(2) mode directly [19]. From Fig. 4, it can be seen that the Cu(2)–Ag mode at 141 cm 1, the vibration of in-plane Cu along c-axis, is strongly suppressed with Fe doping. It is consistent with the results of Gd123 [14]. In these compositions (x P 0.1), the Cu mode is gradually broad and weak, while the Ba–Ag mode at 110 cm 1 blueshifts with increasing of Fe doping content. The result might indicate that increasing Fe doping substitutes more Cu(2) sites and that the mixing between the Ba and the in-plane Cu modes hand influences the state of CuO2 plane. If the Fe doping content is much enough, some Fe atoms will form clusters and two neighboring Fe atoms in the clusters will share one oxygen atom [20]. Because of the substitution at the Cu(2) sites and the clustering effect, no more valid oxygen vacancies are generated [21]. It is known that the vibrational mode lying at 332 cm 1 is the out-of-phase B1g of the couple O(2)–O(3). The O(2, 3)–B1g 332 cm l peak is very sharp and strong in the sample (x = 0.0) with orthorhombic structure, but the peak in the samples (x P 0.05)
gradually becomes weak with the O–T phase transition. With increasing Fe doping content, the O(2, 3)–B1g mode shows an decrease in its vibrational frequency by 3 cm 1. The frequency of O(2, 3)–B1g for x = 0.4 sample is 329 cm l. The in-phase vibrations of the plane oxygen atoms (O(2, 3)–Ag) were proved to be ideal for the detection of small variations in the buckling of the CuO2 planes [22]. The changes are seen for O(2, 3)–Ag 440 cm 1 peak in Fig. 4; the small intensity of 440 cm 1 peak disappears gradually induced by Fe doping for tetragonal structure samples. The disappearing of O(2, 3)–Ag with doping can be related with the decreased coupling of the phonon of the carriers in the CuO2 planes with increasing Fe doping content. The vibrational Raman active-phonon mode lying at 502 cm 1 originate from the apical oxygen O(4) Ag mode. Apical oxygen atom O(4) is the most sensitive to the chain ordering. It is considered that the peak corresponds to oxygen deficiency in chain structure. It is commonly accepted that the Cu–O chains play a significant role in serving as reservoirs of charge carriers. When the reservoirs are empty, the material starts displaying semiconducting transport features. The orthorhombic–tetragonal structural transition is found to correlate with a change of oxygen content, which brings the loss of superconductivity. Huong et al. [23] suggested an empirical equation of d = 13.58–0.027 m for the oxygen content, where m is the peak frequency of the O(4)–Ag mode in cm 1. Fig. 5 shows the oxygen content (7 d) versus amount of Fe-doping in terms of the equation. We can see that the oxygen content of Fe doped sample (x = 0.05) is equal to that of undoped sample; the total oxygen amount decreases with higher Fe-doping. But during the replacement of Cu2+ by the trivalent ion Fe3+, the oxygen content will increase for the charge compensation. This conflicting phenomenon might be caused by the combinational effect of the change of O(5) and O(1). Fe tends to preferentially occupy the Cu(1) chain site and leads to the localization of carriers and weakening of the Cu–O chains as carrier reservoir. Every two Fe atoms require one more oxygen atom than Cu atoms and there are six oxygen atoms around one Fe atoms for the charge compensation; most oxygen positions (O1) along the Cu–O chain (b direction) are occupied, so the extra oxygen atoms preferentially occupy the vacancy positions (O5) along the a direction [12]. The oxygen amount increases accompanying the increase of lattice parameters a. The long continuous CuO chains are thus broken into shorter CuO chain fragments. Simultaneous, as Fe doping, the hole-carriers cannot easily transfer to the CuO2 planes. It causes disorder in the CuO2 planes and forces Cu atoms to loose oxygen atom coordination to Fe [24]. The oxygen content (b direction) decreases
7.0
6.8
oxygen content
22
6.6
6.4
6.2
6.0 0.0
0.1
0.2
0.3
0.4
Fe doped content x Fig. 5. The oxygen content (7
d) versus amount of Fe-doping.
R. Xue et al. / Physica C 475 (2012) 20–23
accompanying the decrease of lattice parameters b. When Fe doping content x = 0.05, increasing oxygen content is equal to loose oxygen content; as Fe doped content increasing (x > 0.05), lost oxygen content is more than extra oxygen atoms of Fe atoms acquiring. That causes more oxygen depletion accompanying structure transformation. The O atoms are randomly distributed on the aand b-axes (in the tetragonal phase) [6]. It is in good agreement with the lattice parameters pattern shown in Fig. 2. The O(4) vibrating mode becomes flat and very broad and shifts to be noticeable in the SBCFO composites (x = 0.1–0.4). This flattening is attributed to disturbances and changes occurring in the interatomic distances of apical oxygen O4 to the two copper layers (Cu2). While Fe ion substitutes Cu-sites in some extent, the new chains including O(4)–Fe(1) or O(4)–Fe(2) could break the symmetry of the primeval chains and induce a transient dipole moment and polarizability. Especially, in higher doping samples, the doping Fe ions at some Cu(2) sites leading to the length of O(4)–Fe(2) chains is differ to the O(4)–Cu(2) in opposite paralleling side. And the Fe occupying randomly at part of Cu(2) sites induces different asymmetrical-chain order in different samples, which is due to the broadening and oscillate of the O(4) band between 468 cm 1and 476 cm 1. This induces the more oxygen deficiency in the SBCFO lattice structure. With increasing amounts of Fe, some weak phonons (568, 618 cm 1) appear, which might originate from the breaking of the inversion symmetry from the partially filled chains. The breaking of the chains into small segments activates another characteristic phonon at 618 cm 1 that gains intensity in the higher Fe doping (x > 0.1). Some of the Fe ions randomly occupy the Cu(2) sites in this doping content. So the mode at 618 cm 1 should be closely related with CuO2 plane. More work will be performed in future. 4. Summary We have investigated the effects of Fe-doping on structural phase transition, the electrical transport properties and Raman modes of SmBa2Cu3-xFexO7-d. The X-ray powder diffraction confirms that the crystal structure evolves from orthorhombic in x < 0.10 samples to tetragonal in x P 0.1 samples. The temperature dependence of resistivity illustrates that the Tc decreases with the increase of Fe doping, and an insulator transition appears in high doping content samples (x P 0.3). The results of Raman spectra indicate that all the spectra of the samples (x < 0.1) show four Ag modes and one Blg mode assigned to Sm-123 structure and that the relative normal modes are strongly affected by structural phase
23
transition accompanying higher doping. The changes of the modes can be explained by the asymmetrical-chain induced by Fe doping, which can induce the transient dipole moment or polarizability and hence cause the shifts and weakening or even disappearing of the bands. Acknowledgements This work was supported by NSFC (Grant Nos. 10875107 and 51002144), the Basic Research Plan on Natural Science of the Education Department of Henan Province (Grant Nos. 2011B140023 and 2009A140009). References [1] R.K. Singhal, Journal of Alloys and Compounds 1–6 (2010) 495. [2] X. Yao, A. Oka, T. Izumi, Y. Shiohara, Physica C 339 (2000) 99. [3] A.M. Saleh, M.M. Abu-Samreh, M.H. Soliman, A.A. Leghrouz, R.M.-L. Ketaneh, S. Darwish, M.I. Abu Taha, Thin Solid Films 468 (2004) 93. [4] X. Yao, T. Izumi, Y. Shiohara, Progress in Solid State Chemistry 30 (2002) 133. [5] Z.P. Chen, J. Guo, R.Z. Xue, T. Li, L. Su, C.M. Wang, H.Y. Dai, Y.C. Xue, The Journal of Superconductivity and Novel Magnetism 24 (2011) 1739, doi:10.1007/ s10948-010-1117-x. [6] Z. Trajanovic, R. Shreekala, M. Rajeswari, I. Takeuchi, T. Venkatesan, E. Bauer, F. Bridges, Physica C 289 (1997) 89. [7] N.H. Babu, K. Iida, Y. Shi, D.A. Cardwell, Physica C 468 (2008) 1340. [8] H.C. Yu, Y.G. Shi, G.C. Che, L.B. Liu, Y.H. Liu, J.Q. Li, Z.X. Zhao, Physica C 411 (2004) 94. [9] S. Hong, H. Cheong, G. Park, Physica C 470 (2010) 383. [10] T. Kakeshita, K. Hirose, S. Lee, Physica C 463–465 (2007) 96. [11] C. Dong, Applied Crystallography 838 (1999) 32. [12] X.S. Wu, S.S. Jiang, W.M. Chen, J. Lin, X. Jin, Physica C 292 (1997) 248. [13] Z.P. Chen, J. Guo, R.Z. Xue, Y.C. Xue, T. Li, K. Shi, C.M. Wang, Cryogenic and Superconductivity 38 (2010) 28. [14] T. Kakeshita, K. Hirose, S. Lee, Physica C 460–462 (2007) 1359. [15] A.A. Bolzan, G.J. Millar, A. Bhargava, I.D.R. Mackinnon, P.M. Fredericks, Materials Letters 28 (1996) 27. [16] S. Mozaffari, M. Akhavan, Physica C 468 (2008) 985. [17] V.A. Maroni, Y. Li, D.M. Feldmann, Q.X. Jia, Journal of Applied Physics 113909 (2007) 102. [18] Duong Cong Hiep, Vu Dinh Lam, Le Van Hong, Journal of Raman Spectroscopy 32 (2001) 827. [19] L.h. Liu, C. Dong, J.C. Zhang, J.Q. Li, Physica C 377 (2002) 348. [20] S. Katsuyama, Y. Ueda, K. Kosuge, Physica C: Superconductivity 165 (1990) 404. [21] L.H. Liu, C. Dong, D.M. Deng, Z.P. Chen, J.C. Zhang, Acta Physica Sinica 50 (2001) 769. [22] E. Kaldis, J. Rohler, E. Liarokapis, N. Poulakis, K. Conder, P.W. Loeffen, Physical Review Letters 79 (1997) 4894. [23] P.V. Huong, J.C. Bruyere, E. Bustarret, P. Grandchamp, Solid State Communications 72 (1989) 191. [24] S.B. Ogale, K.M. Gapchup, Pramada Lele, D.D. Choughule, R.C. Chikate, G. Marest, Physica C 257 (1996) 375.
Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Effects of Fe doping on crystal structure, superconductivity and Raman spectra in SmBa2Cu3O7 d systems Renzhong Xue, Zhenping Chen ⇑, Haiyang Dai, Tao Li, Yuncai Xue, Junhong Hao Department of Technology and Physics, Zhengzhou University of Light Industry, Zhengzhou 450002, China
a r t i c l e
i n f o
Article history: Received 8 October 2011 Received in revised form 15 December 2011 Accepted 4 January 2012 Available online 3 February 2012 Keywords: SmBa2Cu3 xFexO7 d Phase transition Electrical properties Raman spectra
a b s t r a c t SmBa2Cu3 xFexO7 d (SBCFO) (x = 0.0–0.4) systems are prepared by the usual solid-state reaction technique. The effect of Fe doping on the structure, electronic transport properties and Raman spectra of SBCFO systems have been investigated. The X-ray measurement indicates that Fe ions have a significant effect on the main crystalline structure and SBCFO undergoes a structure phase transition from orthorhombic to tetragonal between x = 0.05 and 0.1. The superconducting (SC) transition temperature Tc decreases with the increase of Fe-doping content; an insulator transition appears in high doping content samples (x P 0.3). We have discussed the Raman shifts and intensity of the five normal phonon peaks and the other modes at 229 and 589 cm 1 which result from oxygen deficiency in the chain structure. These investigations reveal that the electrical transport properties and Raman spectra of SmBa2Cu3 xFexO7 d composites obviously depend on O–T transition induced by the Fe doping. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction YBa2Cu3O7 d compound system has been widely studied for improving the superconducting properties by many researchers since it was discovered. A great number of papers have been published on adding elements in Y123 system for researching the phase transition and the physical properties of YBCO samples [1–3]. By cationic substitution of different sites, REBa2Cu3O7 d (RE123, RE – rare earth elements) possesses different physical, chemical and crystallographic properties. Studies on these materials can provide some insight into the mechanism of superconductivity and improve the superconducting properties. Cationic substitutions include the replacement of Y sites by rare earth elements, the replacement of Ba sites by alkaline earth elements, and the replacement of Cu sites by metal elements. Among them, the REBa2(Cu1 xMx)3O7 d type materials (where M is a metal element like Zn, Fe, Ni, Co, Al, etc.) are of great importance. REBa2Cu3O7 d system has a unique feature; it has two non-equivalents Cu sites viz., the linear-chain Cu(1) in the O(1)–Cu(1)–O(1) units and the planar Cu(2) in the CuO2 sheets containing O(2) and O(3). The CuO2 plane is referred to as the superconducting plane: any modification in this plane strongly influences the electronic structure and the density, mobility of the charge carriers and thus suppresses the superconductivity of the systems [1,4–6]. In spite of many experimental and theoretical reports on doping YBCO with these elements, their destructive effect on superconductivity is still not completely understood. A large number of studies have therefore ⇑ Corresponding author. Tel./fax: +86 371 63556807. E-mail address: [email protected] (Z. Chen). 0921-4534/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2012.01.007
been carried out by substituting various cations for Cu as well as Y and Ba and these have yielded much useful information about the lattice and electronic structure of the systems [7,8]. Raman spectroscopy describes the active phonons and serves to determine the disorder, stoichiometry, superstructure, orientation, and impurity phases for the characterization of high-Tc cuprates such as poly- and single-crystalline samples or epitaxial films. For superconducting systems, the frequencies, the polarization properties and the resonant behavior of Raman-allowed phonons are often employed to explain the characteristic superstructures [9]. There are two merits of Raman measurements on the investigation for the crystal properties, compared to XRD. One is, in the case of YBCO, to estimate intuitively the change of the oxygen content in chain site by Raman spectra. The other is to detect impurity phases, too trivial (1–2 lm) for XRD, since the spot size of laser is comparable to the domain size of the impurity [10]. Thus, we can obtain local information on the crystal structure from the peaks in Raman spectra. In the present work, we investigate Fe additions in SBCO bulk and their influence on the transport properties. For this purpose, a series of SBCO specimens with different amounts of Fe-doping (x = 0.0–0.4) were prepared by the solid state reaction. We focus on the analysis of the structure, microstructure and the electrical properties. 2. Experimental details The polycrystalline samples of SmBa2Cu3 xFexO7 d for x = 0.0, 0.05, 0.1, 0.2, 0.3 and 0.4 were prepared by the usual solid-state reaction technique. Appropriate amounts of high purity of Sm2O3,
21
R. Xue et al. / Physica C 475 (2012) 20–23
3.94
3.92
3.90
a, b, c/3 (Å)
BaCO3, CuO, and Fe2O3 were mixed, ground and heated at 940 °C for 24 h to ensure perfect homogeneity and complete the solidstate reaction. The resulted powders were reground and pressed into pellets. The pellets were sintered in flowing oxygen at 940 °C for 12 h followed by slow cooling (1 °C/min) to 450 °C. Then they were kept at 450 °C for 72 h in flowing O2 and then gradually cooled to room temperature. The phase identification of the prepared samples was carried out using X-ray powder diffraction (XRD). Lattice parameters were calculated by the least-square method with PowderX program [11]. The electrical transport properties were performed by the standard four-probe dc resistance technique with indium contacts in conjunction. Voltages were recorded by the Aglient HP-34401A voltmeter. The Raman measurement was performed in the quasi-backscattering geometry using 50 mW of the 532 nm line of a semiconductor laser via a Renishaw inVia spectrometer with the resolution of 0.5 cm 1.
3.88
a b c /3
3.86
3.84
3.82 0.0
0.1
0.2
0.3
0.4
x Fig. 2. The lattice parameters a, b and c/3 versus Fe content x for SmBa2Cu3 samples.
3. Results and discussion
xFexO7 d
Fig. 1 shows the X-ray diffraction patterns of the SmBa2 Cu3 xFexO7 d samples. It is obvious that the Fe-doping has a significant effect on the main crystalline structure of the Sm-123. All samples are characterized as orthorhombic or tetragonal phase and show no observable impurity peaks. Fig. 2 reveals the changes of lattice parameters with doping content x for all the samples. With increasing Fe-doping content x from 0.0 to 0.1, the lattice parameter a increases while b decreases. When x P 0.1, the lattice parameters a and b become identical, indicating that the O–T phase transition is completed. The lattice parameter may be related to the iron preferential substitution for copper at Cu(1) or Cu(2) sites. Fe ion preferentially substitutes for copper at the Cu(1) sites as x < 0.15 and at Cu(2) sites as x > 0.25 [12]. Fe displays a valence of +3, which is more than the average Cu(1) valence of 2.33. This imbalance pulls extra oxygen occupying the vacancy positions along a direction, thereby distorting the orthorhombic configuration. For even higher doping (x > 0.2), Fe atoms will substitute at the Cu(2) sites. Fe doping brings a strong and expanded disturbance on electron (hole) distribution; this will lead to the increase of lattice parameters a, b and the decrease of parameter c. Thus it can be seen that Fe doping leads to oxygen stoichiometry modifications and correspondingly structural transformation from orthorhombic to tetragonal structure, namely the doping content affects the structure of the samples indeed. It is consistent with the results of Gd123 reported by Chen et al. [13] and Kakeshita et al. [14].
Fig. 3 shows the temperature dependence of the resistivity for SmBa2Cu3 xFexO7 d samples. It can be seen that the superconducting transition has happened for the samples with x = 0.0–0.2 above 10 K. The superconducting transition temperature Tc decreases continuously with increasing of Fe doping content. The samples with x less than 0.1 show metallic behavior in normal state. Further Fe substitution results in a semiconducting behavior. The resistivity of samples x = 0.1 and 0.2 increases as the temperature decreases from 120 K and 80 K to the onset of superconducting transition respectively. And an insulator transition appears in high doping content samples (x P 0.3). The inset of Fig. 3 shows the onset temperature Tc,onset and the zero-resistance critical temperature Tc,zero of samples as a function of Fe doping content. It can be found that Tc,zero decreases and the transition width DT = Tc,onset Tc,zero increases with increasing Fe doping content. A YBCO perovskite unit cell has five different oxygen sites: O(2, 3) in CuO2 planes, O(1) in CuO chains, and O(4) in apical chains. The O(5) sites situated between the oxygen chains are nearly empty in the orthorhombic structure [1]. Fig. 4 shows a series of typical Raman spectra for the various Fe doping content. The error bar of the Raman frequency is about 0.5 cm 1 determined from the instrumental resolution. For the undoped sample in
14
80
x =0.40
Tc,onset Tc,zero 60
.cm)
x =0.3 x =0.2 x =0.1
20
100 310
220
206
116
006
200
110 005 113
103
100
213
60
80
0.05
0.10
0.15
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Fe content x
x =0.10 x =0.05 x =0.0
0 50
40
0.00
x =0.20
6
2
20
40
x =0.30
4
x =0.05 x =0
10 8
ρ 10-5 (
Intensity
x =0.4
Tc (K)
12
100
150
200
250
Temperature (K)
2θ (Degree) Fig. 1. XRD patterns for the experimental SmBa2Cu3 xFexO7
d
samples.
Fig. 3. The temperature dependence of the resistivity for the experimental SmBa2Cu3 xFexO7 d samples. The inset is the Tc,onset and Tc,zero of the samples.
R. Xue et al. / Physica C 475 (2012) 20–23
468 329
(f)
473
Intensity
476
568
618 (e) (d)
479 (c) 502 141 110
222
332 (b)
592
440 225
100
200
(a) 300
400
Raman shift
500
600
700
(cm-1)
Fig. 4. Raman spectra obtained from Fe-doped SmBa2Cu3 xFexO7 x = 0.0, (b) x = 0.05, (c) x = 0.1, (d) x = 0.2, (e) x = 0.3, (f) x = 0.4.
d
samples: (a)
Fig. 4a, it is clear that four peaks assigned to four Ag modes and one Blg mode of Sm123 structure can be seen in the spectra: the apical oxygen O(4) vibration along the c-axis, namely, O(4)–Ag mode (502 cm 1); two vibrations of the O(2, 3) oxygen atoms in the CuO2 planes, with one in-phase, namely, O(2, 3)–Ag mode (440 cm 1) and the other out-of-phase, namely, O(2, 3)–Blg mode (332 cm 1) respectively. Other Raman active modes are vertical vibrations along the c-axis of Cu(2) and Ba atoms, namely, Cu(2)– Ag mode (141 cm 1) and Ba–Ag mode (110 cm 1) respectively. All the frequencies are well known for phonons recorded in the orthorhombic structure [13,15,16]. Furthermore, some additional peaks are also seen in some composition. For example, the two peaks are at 229 and 589 cm 1 respectively in pure SBCO. With increasing of Fe doping level, the other peaks also gradually emerge at 568, 618 cm 1 in the samples (x > 0.1). Since Fe ions locate primarily at the Cu(1) sites at low doping level for the Fe-doped Sm123 systems, we firstly focus on the mode at 225 and 592 cm 1, which is the cation disorder mode reported by Maroni et al. [17] and Hiep et al. [18]. Doping Fe ions could destroy the symmetry of the Cu-O along the a/b-axis and then affect the vibration of Cu(1) and O(1). As a result, the intensity of 229 cm 1 mode gradually decreases and the peaks shift randomly with Fe-doping. The 592 cm 1 mode becomes broad and weak in the other compositions, in contrast to undoped sample. The results mean that the disorder of Cu site by Fe doping is enhanced in the compositions. As Fe doping content increases, some of the Fe ions randomly occupy the Cu(2) sites which could influence the Cu(2) mode directly [19]. From Fig. 4, it can be seen that the Cu(2)–Ag mode at 141 cm 1, the vibration of in-plane Cu along c-axis, is strongly suppressed with Fe doping. It is consistent with the results of Gd123 [14]. In these compositions (x P 0.1), the Cu mode is gradually broad and weak, while the Ba–Ag mode at 110 cm 1 blueshifts with increasing of Fe doping content. The result might indicate that increasing Fe doping substitutes more Cu(2) sites and that the mixing between the Ba and the in-plane Cu modes hand influences the state of CuO2 plane. If the Fe doping content is much enough, some Fe atoms will form clusters and two neighboring Fe atoms in the clusters will share one oxygen atom [20]. Because of the substitution at the Cu(2) sites and the clustering effect, no more valid oxygen vacancies are generated [21]. It is known that the vibrational mode lying at 332 cm 1 is the out-of-phase B1g of the couple O(2)–O(3). The O(2, 3)–B1g 332 cm l peak is very sharp and strong in the sample (x = 0.0) with orthorhombic structure, but the peak in the samples (x P 0.05)
gradually becomes weak with the O–T phase transition. With increasing Fe doping content, the O(2, 3)–B1g mode shows an decrease in its vibrational frequency by 3 cm 1. The frequency of O(2, 3)–B1g for x = 0.4 sample is 329 cm l. The in-phase vibrations of the plane oxygen atoms (O(2, 3)–Ag) were proved to be ideal for the detection of small variations in the buckling of the CuO2 planes [22]. The changes are seen for O(2, 3)–Ag 440 cm 1 peak in Fig. 4; the small intensity of 440 cm 1 peak disappears gradually induced by Fe doping for tetragonal structure samples. The disappearing of O(2, 3)–Ag with doping can be related with the decreased coupling of the phonon of the carriers in the CuO2 planes with increasing Fe doping content. The vibrational Raman active-phonon mode lying at 502 cm 1 originate from the apical oxygen O(4) Ag mode. Apical oxygen atom O(4) is the most sensitive to the chain ordering. It is considered that the peak corresponds to oxygen deficiency in chain structure. It is commonly accepted that the Cu–O chains play a significant role in serving as reservoirs of charge carriers. When the reservoirs are empty, the material starts displaying semiconducting transport features. The orthorhombic–tetragonal structural transition is found to correlate with a change of oxygen content, which brings the loss of superconductivity. Huong et al. [23] suggested an empirical equation of d = 13.58–0.027 m for the oxygen content, where m is the peak frequency of the O(4)–Ag mode in cm 1. Fig. 5 shows the oxygen content (7 d) versus amount of Fe-doping in terms of the equation. We can see that the oxygen content of Fe doped sample (x = 0.05) is equal to that of undoped sample; the total oxygen amount decreases with higher Fe-doping. But during the replacement of Cu2+ by the trivalent ion Fe3+, the oxygen content will increase for the charge compensation. This conflicting phenomenon might be caused by the combinational effect of the change of O(5) and O(1). Fe tends to preferentially occupy the Cu(1) chain site and leads to the localization of carriers and weakening of the Cu–O chains as carrier reservoir. Every two Fe atoms require one more oxygen atom than Cu atoms and there are six oxygen atoms around one Fe atoms for the charge compensation; most oxygen positions (O1) along the Cu–O chain (b direction) are occupied, so the extra oxygen atoms preferentially occupy the vacancy positions (O5) along the a direction [12]. The oxygen amount increases accompanying the increase of lattice parameters a. The long continuous CuO chains are thus broken into shorter CuO chain fragments. Simultaneous, as Fe doping, the hole-carriers cannot easily transfer to the CuO2 planes. It causes disorder in the CuO2 planes and forces Cu atoms to loose oxygen atom coordination to Fe [24]. The oxygen content (b direction) decreases
7.0
6.8
oxygen content
22
6.6
6.4
6.2
6.0 0.0
0.1
0.2
0.3
0.4
Fe doped content x Fig. 5. The oxygen content (7
d) versus amount of Fe-doping.
R. Xue et al. / Physica C 475 (2012) 20–23
accompanying the decrease of lattice parameters b. When Fe doping content x = 0.05, increasing oxygen content is equal to loose oxygen content; as Fe doped content increasing (x > 0.05), lost oxygen content is more than extra oxygen atoms of Fe atoms acquiring. That causes more oxygen depletion accompanying structure transformation. The O atoms are randomly distributed on the aand b-axes (in the tetragonal phase) [6]. It is in good agreement with the lattice parameters pattern shown in Fig. 2. The O(4) vibrating mode becomes flat and very broad and shifts to be noticeable in the SBCFO composites (x = 0.1–0.4). This flattening is attributed to disturbances and changes occurring in the interatomic distances of apical oxygen O4 to the two copper layers (Cu2). While Fe ion substitutes Cu-sites in some extent, the new chains including O(4)–Fe(1) or O(4)–Fe(2) could break the symmetry of the primeval chains and induce a transient dipole moment and polarizability. Especially, in higher doping samples, the doping Fe ions at some Cu(2) sites leading to the length of O(4)–Fe(2) chains is differ to the O(4)–Cu(2) in opposite paralleling side. And the Fe occupying randomly at part of Cu(2) sites induces different asymmetrical-chain order in different samples, which is due to the broadening and oscillate of the O(4) band between 468 cm 1and 476 cm 1. This induces the more oxygen deficiency in the SBCFO lattice structure. With increasing amounts of Fe, some weak phonons (568, 618 cm 1) appear, which might originate from the breaking of the inversion symmetry from the partially filled chains. The breaking of the chains into small segments activates another characteristic phonon at 618 cm 1 that gains intensity in the higher Fe doping (x > 0.1). Some of the Fe ions randomly occupy the Cu(2) sites in this doping content. So the mode at 618 cm 1 should be closely related with CuO2 plane. More work will be performed in future. 4. Summary We have investigated the effects of Fe-doping on structural phase transition, the electrical transport properties and Raman modes of SmBa2Cu3-xFexO7-d. The X-ray powder diffraction confirms that the crystal structure evolves from orthorhombic in x < 0.10 samples to tetragonal in x P 0.1 samples. The temperature dependence of resistivity illustrates that the Tc decreases with the increase of Fe doping, and an insulator transition appears in high doping content samples (x P 0.3). The results of Raman spectra indicate that all the spectra of the samples (x < 0.1) show four Ag modes and one Blg mode assigned to Sm-123 structure and that the relative normal modes are strongly affected by structural phase
23
transition accompanying higher doping. The changes of the modes can be explained by the asymmetrical-chain induced by Fe doping, which can induce the transient dipole moment or polarizability and hence cause the shifts and weakening or even disappearing of the bands. Acknowledgements This work was supported by NSFC (Grant Nos. 10875107 and 51002144), the Basic Research Plan on Natural Science of the Education Department of Henan Province (Grant Nos. 2011B140023 and 2009A140009). References [1] R.K. Singhal, Journal of Alloys and Compounds 1–6 (2010) 495. [2] X. Yao, A. Oka, T. Izumi, Y. Shiohara, Physica C 339 (2000) 99. [3] A.M. Saleh, M.M. Abu-Samreh, M.H. Soliman, A.A. Leghrouz, R.M.-L. Ketaneh, S. Darwish, M.I. Abu Taha, Thin Solid Films 468 (2004) 93. [4] X. Yao, T. Izumi, Y. Shiohara, Progress in Solid State Chemistry 30 (2002) 133. [5] Z.P. Chen, J. Guo, R.Z. Xue, T. Li, L. Su, C.M. Wang, H.Y. Dai, Y.C. Xue, The Journal of Superconductivity and Novel Magnetism 24 (2011) 1739, doi:10.1007/ s10948-010-1117-x. [6] Z. Trajanovic, R. Shreekala, M. Rajeswari, I. Takeuchi, T. Venkatesan, E. Bauer, F. Bridges, Physica C 289 (1997) 89. [7] N.H. Babu, K. Iida, Y. Shi, D.A. Cardwell, Physica C 468 (2008) 1340. [8] H.C. Yu, Y.G. Shi, G.C. Che, L.B. Liu, Y.H. Liu, J.Q. Li, Z.X. Zhao, Physica C 411 (2004) 94. [9] S. Hong, H. Cheong, G. Park, Physica C 470 (2010) 383. [10] T. Kakeshita, K. Hirose, S. Lee, Physica C 463–465 (2007) 96. [11] C. Dong, Applied Crystallography 838 (1999) 32. [12] X.S. Wu, S.S. Jiang, W.M. Chen, J. Lin, X. Jin, Physica C 292 (1997) 248. [13] Z.P. Chen, J. Guo, R.Z. Xue, Y.C. Xue, T. Li, K. Shi, C.M. Wang, Cryogenic and Superconductivity 38 (2010) 28. [14] T. Kakeshita, K. Hirose, S. Lee, Physica C 460–462 (2007) 1359. [15] A.A. Bolzan, G.J. Millar, A. Bhargava, I.D.R. Mackinnon, P.M. Fredericks, Materials Letters 28 (1996) 27. [16] S. Mozaffari, M. Akhavan, Physica C 468 (2008) 985. [17] V.A. Maroni, Y. Li, D.M. Feldmann, Q.X. Jia, Journal of Applied Physics 113909 (2007) 102. [18] Duong Cong Hiep, Vu Dinh Lam, Le Van Hong, Journal of Raman Spectroscopy 32 (2001) 827. [19] L.h. Liu, C. Dong, J.C. Zhang, J.Q. Li, Physica C 377 (2002) 348. [20] S. Katsuyama, Y. Ueda, K. Kosuge, Physica C: Superconductivity 165 (1990) 404. [21] L.H. Liu, C. Dong, D.M. Deng, Z.P. Chen, J.C. Zhang, Acta Physica Sinica 50 (2001) 769. [22] E. Kaldis, J. Rohler, E. Liarokapis, N. Poulakis, K. Conder, P.W. Loeffen, Physical Review Letters 79 (1997) 4894. [23] P.V. Huong, J.C. Bruyere, E. Bustarret, P. Grandchamp, Solid State Communications 72 (1989) 191. [24] S.B. Ogale, K.M. Gapchup, Pramada Lele, D.D. Choughule, R.C. Chikate, G. Marest, Physica C 257 (1996) 375.