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Oxygen purity effect oxygen deficiency by argon heat treatment on Y:123 superconductors
Physica B 410 (2013) 233–236
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Physica B journal homepage: www.elsevier.com/locate/physb
Oxygen purity effect oxygen deficiency by argon heat treatment on Y:123 superconductors A. Sedky a,b,n a b
Physics Department, Faculty of science, King Faisal University, Al-Hassa 31982, P.O.B 400, Saudi Arabia Physics Department, Faculty of science, Assiut University, Assiut, Egypt
a r t i c l e i n f o
abstract
Article history: Received 6 September 2012 Received in revised form 1 November 2012 Accepted 5 November 2012 Available online 19 November 2012
Argon heat treatments at 450 1C for different times of 0, 12, 24, 36, 48 and 60 h on the two Y:123 oxygenated samples with different oxygen purity (Hp 99.99% and Lp 93%) are reported. The results of X-ray diffraction, oxygen deficiency and resistivity measurements are presented in details. Furthermore, the superconducting critical temperature Tc, normal resistivity r300, residual resistivity r0, resistivity slope dr/dT and width of transition DTc are extracted from resistivity data. It is found that both c-parameter and oxygen deficiency increase by increasing annealing time, while OD and effective Cu valance decrease. But, the relative increase/decrease is more in Lp samples than that of Hp samples. Although Tc is decreased by annealing time for the two samples, the relative decrease in Tc is more in Lp samples when compared with those of Hp samples. Our results are discussed in terms of oxygen vacancies and hole carriers which are produced by annealing for the considered samples. & 2012 Elsevier B.V. All rights reserved.
Keywords: Argon annealing Oxygen purity Oxygen deficiency Normal resistivity Width of transition and critical temperatures
1. Introduction The peculiar dependence of normal and superconducting properties on oxygen stoichiometry of Y:123 high Tc superconductor has been interesting land mark in the field of superconductivity [1–5]. It is well known that argon heat treatment at a temperature above 400 1C on oxygenated Y:123 superconducting system can lose oxygen from the poor Cu–O chains and consequently the oxygen content is decreased from seven to six along with a gradual decrease in Tc [6,7], while the rich Cu–O2 planes are supposed to stay unaffected. The Tc versus oxygen deficiency plot exhibits two successive plateaus at 90 and 60 K (orthorhombic I and II). Orthorhombic I is well established and associated with an optimal doping of the CuO2 planes [8], while orthorhombic II results from an alternate ordering of oxygen rich and oxygen poor of Cu–O chains along the a-axis [9]. However, Y:123 samples should have similar mechanism of oxygen transfer through Ar heat treatment, independent on oxygen purity during sintering processes. If the mechanism of oxygen loss stays independent of the degree of oxygen purity, the normal and superconducting properties should be at least qualitatively similar in both samples, as documented for Y:123 systems before annealing
n Correspondence address: Physics Department, Faculty of science, King Faisal University, Al-Hassa 31982, P.O. Box 380, Saudi Arabia. Tel.: þ966507207331; fax: þ 96635899557. E-mail addresses: [email protected], [email protected]
0921-4526/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2012.11.014
[10–12]. However, the present study shows that the oxygen purity results in an entirely different set of properties for the two samples after Ar heat treatments, which are highlights of the present work. To clarify the above, two similar oxygenated Y:123 samples are prepared by the well known solid state reaction method. But one of them is sintered in high purity oxygen (99.99%), while the other is sintered in low purity oxygen (93%). After that Ar heat treatment at a temperature of 450 1C for five different times (0–60 h) is performed.
2. Experimental details Two similar samples of the series YBa2Cu3O7 are synthesized by the well-known solid state reaction method. The ingredients Y2O3, BaCO3 and CuO of 4 N purity are thoroughly mixed in required proportions and calcined at 900 1C in air for a period of 16 h, and then the furnace is slowly cooled to room temperature. This exercise is repeated three times with intermediate grinding at each stage. The resulting powders are ground, palletized in to two pellets. One of them is sintered in high purity oxygen (99.99% Hp) at 940 1C for a period of 24 h and then the furnace is cooled to room temperature with an intervening annealing for 24 h at 600 1C, while the other is sintered in low purity oxygen (93% Lp) at the same conditions. The 93% oxygen produced from air by the molecular sieve process contains not less than 90% and not more than 96% by volume of oxygen. The remainder consists mostly of argon and nitrogen. Also, some other gases could be
0.004 0.000 0.013 0.041 0.056 0.107 1.47 3.36 6.76 8.19 9.55 10.16 0.329 3.29 10.35 19.29 25.09 38.94 6 19 33 63 00 00 89 76 52 22 00 00 2.280 2.193 2.120 2.073 1.893 1.833 0.08 0.21 0.32 0.39 0.66 0.75 11.683 11.690 11.699 11.705 11.726 11.731 Ar0 Ar12 Ar24 Ar36 Ar48 Ar60
0.018 0.015 0.008 0.005 0.000 0.000
(dq/dT) (mO.cm/K)
q300 (mO.cm) q0 (mO.cm) DTc (K) Tc (K) P Oxy. def. Orth. dis. ˚ C (A) LP
0.003 0.003 0.005 0.006 0.017 0.037 1.07 2.86 4.98 6.13 7.33 8.92 0.09 1.94 6.38 7.67 12.04 18.84 8 11 18 25 40 85 90 84 72 60 40 00 2.273 2.226 2.200 2.147 1.993 1.907 0.09 0.16 0.20 0.28 0.51 0.64 11.683 11.689 11.695 11.702 11.713 11.717 Ar0 Ar12 Ar24 Ar36 Ar48 Ar60
0.019 0.017 0.012 0.009 0.001 0.000
q300 (mO.cm) q0 (mO.cm) DTc (K) Tc (K) P Oxy. def.
(b) C parameter, orthorhombic distortion, oxygen deficient and effective Cu valance, width of transition, resistivity slope, residual resistivity and normal resistivity of Hp samples.
Fig. 1. XRD patterns of Hp Y:123 samples before and after annealing.
Orth. Dis.
Fig. 1 shows XRD patterns for Hp samples before and after Ar heat treatment. The same is done for Lp samples. The structure of the samples, after annealing by Ar up to 60 h, maintain a clearly single phase and no additional peaks could be formed as before Ar treatment. The obtained peaks (0 1 0), (0 1 3), (1 0 3), (0 0 5), (1 1 3), (0 2 0), (2 0 0), (1 2 3) and (2 1 3) belong to the well known peaks reported for Y:123 superconductors. Although no extra peaks are observed in the XRD patterns for all samples, there is a little variation of the lines intensity of the peaks with increasing annealing time. The marginal shift in the position of lines is mainly attributed to the change in the c-lattice parameter produced by Ar annealing. The samples annealed at t¼ 0 and 12 h are completely orthorhombic, being nearly evident from crystallographic splitting of (0 2 0), (2 0 0) and (1 2 3), (2 1 3). But with increase in annealing time up to 60 h, the crystallographic splitting of (0 2 0), (2 0 0) and (1 2 3), (2 1 3) disappears and seem to be one broader peak which indicates that the structure is changed to tetragonal. The c-parameters, orthorhombic distortion (OD) [(b a)/b] and oxygen deficiency for all samples are listed in Table 1(a and b). It is clear that c-parameter and oxygen deficiency are increased by increasing annealing time for all samples, while OD continuously decreases. The c-parameter and oxygen deficiency for LpAr samples are comparatively more than observed in HpAr samples, while the vice is versa for OD. The change of c-parameter after annealing is probably related to the
˚ C (A)
3. Results and discussion
HP
found such as carbon dioxide (0.03%) and carbon monoxide (0.001%). The samples are tested for phase purity by X-ray diffraction (XRD) using Semen’s D-500 with CuKa radiation of ˚ The oxygen content is measured by idometery titra1.541838 A. tion method. The electrical resistivity of the samples are obtained using the standard four-probe technique in closed cycle refrigerator [cryomech compressor package with cryostat Model 8101812212, USA] within the range of (10–300) K. Nanovoltameter Keithley 2182, current source Keithley 6220 and temperature controller 9700 (0.001 K resolution) are used in this experiment. After that, Ar heat treatments, at a temperature of 450 1C for different times of 12, 24, 36, 48 and 60 h are separately made on the two considered samples with intermediate characterization and measurements.
(dq/dT) (mO.cm/K)
A. Sedky / Physica B 410 (2013) 233–236
Table 1 (a) C- parameter, orthorhombic distortion, oxygen deficient and effective Cu valance, width of transition, resistivity slope , residual resistivity and normal resistivity of Hp samples
234
A. Sedky / Physica B 410 (2013) 233–236
235
Resistivity (mohm.cm)
1.2 1 0.8 0.6 HpAr0 LpAr0
0.4 0.2 0
Resistivity (mohm.cm)
4
1.4
0
50
200 100 150 Temperature (K)
3.5 3 2.5 2 1.5 0.5
18
45
16
40
14
35
12 10 8 6 4
HpAr36 LpAr36
2 0
0
50
200 100 150 Temperature (K)
250
HPAr12 LpAr12
1 0
300
250
Resistivity (mohm)
Resistivity (mohm.cm)
1.6
0
50
200 100 150 Temperature (K)
250
300
HpAr48 LpAr48
30 25 20 15 10 5
300
0
0
50
200 100 150 Temperature (K)
250
300
Resistivity (mohm.cm)
30 25 20 15 10 HpAr60 LpAr60
5 0
0
50
100 150 200 Temperature (K)
250
300
Fig. 2. (a) Resistivity versus temperature for Y:123 samples before annealing, (b) resistivity versus temperature for Y:123 samples annealed at 12 h, (c) resistivity versus temperature for Y:123 samples annealed at 24 h, (d) resistivity versus temperature for Y:123 samples annealed at 36 h, and (e) resistivity versus temperature for Y:123 samples annealed at 60 h.
change in the coupling between CuO2 planes due to the loss of oxygen from CuO chains during annealing operation [13–15]. These results indicate that oxygen loss from CuO chains for Lp samples is higher than Hp samples, and consequently the relative change in c-parameter and OD is different. From the values of oxygen content, the effective Cu valence P ¼ 2y7 is calculated 3 and listed in Table 1 [11]. It is clear that P slightly decreases with annealing time, but the relative decrease is higher in Lp samples when compared to Hp samples. This of course is related to decreasing of the hole carriers in the CuO2 planes [10,11]. In Fig. 2(a–e), we plot the resistivity as a function of temperature for all samples before and after Ar annealing. In general the r(T) curves of the samples annealed at t¼0 and 12 h involve a linear region, which extends from room temperature down to a temperature close to the onset temperature Ton. The interesting point here is that Tc ¼90 K for the two samples before Ar annealing irrespective of the purity of oxygen. This is consistent with the reported data based on Y:123 superconducting systems. While Ar annealing up to 12 h, the Tc is decreased for both samples, but it is higher in Hp
sample (84 K) than that of Lp sample (76 K). With increasing annealing time up to 24 and 36 h, the r(T) curves involve a semiconductor behavior for both samples, but still show superconductivity at Tc (72 and 60 K) for Hp sample, and (52 and 22 K) for Lp sample. With more increase in annealing time up to 48 and 60 h, Lp samples show semiconductor behavior with decreasing temperature to 20 K, while Hp sample at 48 h shows a transition to superconducting state without zero resistivity, and 60 h Hp sample is non superconductor. However, a variation of critical temperature Tc against annealing time for all samples is shown in Fig. 3(a). A quick glance at this figure shows the depression in Tc with annealing time. It is remarkable to note that the relative decrease in Tc is more in Lp samples when compared with those of Hp samples. For a comparison between the different ways of oxygen purity, we plot the difference in Tc for (Hp Lp); see Fig. 3(b). It is clear that the difference in Tc increases with increasing annealing time. These results indicate that the oxygen purity is controlling the suppression of superconductivity by Ar annealing for Y:123 samples.
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A. Sedky / Physica B 410 (2013) 233–236
Y:123 pure superconducting systems. But the present study indicate that the random selection of oxygen purity is the main parameter responsible for the variance in Tc depression by annealing between the two samples. However, this study leads towards two different behaviors of oxygen disorder according to the purity of oxygen. The relative loss of oxygen from Lp samples is more than Hp samples, and consequently the structural and transport properties are different. Anyhow, the present study opens a new trend for the impact of oxygen purity on superconductivity, and really emphasizes the need of paying more attention by using different facilities, such as neutron diffraction and SQUID magnetometer, which are not available in our research Lab.
100 HP
90
Lp
80
Tc (K)
70 60 50 40 30 20 10 0
0
10
20
30 40 Anealing time (h)
50
60
70
4. Conclusion Impact of argon heat treatment at 450 1C for different times on two oxygenated Y:123 samples with different oxygen purity is investigated. We have shown that c-parameter, orthorhombic distortion, oxygen deficiency, effective Cu valence, r300, DTc, r0, dr/dT and the Tc behaviors are significantly different by Ar annealing in Hp samples with respect to Lp samples. This study leads towards different behaviors of oxygen disorder and hole carriers according to the purity of oxygen, which is not obtained before annealing for Y:123 superconducting system.
45
[TcHp - TcLp (K)]
40 35 30 25 20 15 10 5 0
Acknowledgments 0
10
20
30 40 Anealing time (h)
50
60
70
Fig. 3. (a) Tc versus annealing time for Y:123 samples and (b) (TcHp TcLp ) versus annealing time for Y:123 samples.
On the other hand, there are also changes in the values of r300,
DTc, r0 and dr/dT, listed in Table 1(a and b), between Hp and Lp samples. The r(T) curves has a slope (dr/dT) which is connected with carrier–carrier scattering, and the extrapolation of r(T) to T¼0 K provides the residual resistivity r0 which is connected with impurity scattering [16]. It is clear that both r300, r0 and DTc are increasing with increasing annealing time up to 60 h. This behavior indicates that the electron scattering by impurities is also affected by annealing time as well as normal state resistivity r300. The (dr/dT) behavior indicates that all samples have a negative slope except the samples annealed at t¼0 and 12 h. This means that the normal state behavior is metallic for these two samples, and semiconductor for the rest of the samples. It has been reported that superconducting property of Y:123 systems are found to vary with both oxygen content and the local arrangement of oxygen vacancies either in the planes or in chains [6]. Furthermore, these vacancies may be destroying the pairing conductions to some extent in the superconducting systems [12]. So, we believe that beside oxygen disorder produced by Ar annealing, some of other parameters, such as the local arrangement of oxygen vacancies and the nature of pair formation, might be affected by the purity of oxygen [17,18]. Moreover, the effective Cu valance in the Cu–O2 planes is also affected and generally decreased the hole carriers for the two samples. Let us now look for the exact reason responsible for the variance in Tc depression by Ar annealing. Of course, the researches may be believed that Tc is not affected by the purity of oxygen as well as in
The author would like to thank the physics department, King Faisal University for maintenance support during the present investigation, and also thank Mr/ Bander Abu-Ziad for his cooperation during measurements. References [1] R.J. Cava, A.W. Hewat, E.A. Hewat, B. Batlogg, M. Merezio, K.M. Rabe, J.J. Krajewski, W.F. Peck, L.W. Ruppjr, Physica C 165 (1990) 419. [2] J.D. Jorgensen, B.W. Weal, A.P. Paulikas, L.J. Nowicki, G.W. Crabtree, H. Claus, W.K. Kwok, Phys. Rev. B 41 (1990) 1863. [3] B.W. Weal, A.P. Paulikas, J.W. Downey, H. Claus, K. Vandervoort, G. Tomlins, H. Shi, M. Jenes, L. Morss, Physica C 162–64 (1989) 97. [4] M. Kogachi, S. Nakanishi, K. Nakahigashi, H. Sasakura, S. Minamigawa, N. Fukuoka, A. Yanase, Jpn. J. Appl. Phys. 28 (1989) L609. [5] H. Shaked, B.W. Weal, J. Faber Jr, R.L. Hitterman, U. Balachandran, G. Tomlins, H. Shi, M. Jensen, L. Morss, A.P. Paulikas, Phys. Rev. B 41 (1990) 4173. [6] J.D. Jorgensen, D.G. Hinks, P.G. Radaeilli, S. Pei, P. Lightfoot, B. Dabrowski, C.U. Segre, B.A. Hunter, Physica C 185–189 (1991) 184. [7] F. Yakhou, J.Y. Henry, P. Burlet, V.P. Plakhty, M. Vlasov, S. Moshkin, Physica C 333 (2000) 146. [8] J.L. Tallon, C. Bernhard, H. Shaked, R.L. Hitterman, J.D. Jorgensen, Phys. Rev. B 51 (1995) 12911. [9] P. Schleger, R.A. Hadfield, H. Casalta, N.H. Anderson, H.F. Poulsen, M. Von Zimmermann, J.R. Schneider, R. Liang, P. Dosanjh, W.N. Hardy, Phys. Rev. Lett. 74 (1995) 1446. [10] Anurag Gupta, A. Sedky, A.V. Narlikar, D.P. Singh, J. Mater. Sci. 37 (2002) 1557. [11] A. Sedky, Bander Abu-Ziad, Physica C 470 (2010) 659. [12] B.D. Weaver, E.M. Jackson, G.P. Summers, E.A. Jackson, Phys. Rev. B 46 (1992) 1134. [13] V.P.S. Awana, S.B. Samanta, P.K. Dutta, E. Gmelin, A.V. Narlikar, J. Phys. Condens. Matter 3 (1991) 8893. [14] Anurag Gupta, A. Sedky, A.V. Narlikar, D.P. Singh, J. Mater. Sci. 37 (2002) 1557. [15] A. Sedky, Bander Abu-Ziad, Physica C 470 (2010) 659. [16] S. Ravi, V.Seshu Bai, Phys. Rev. B 49 (1994) 13082. [17] A.K. Ghosh, A.N. Basu, Supercond. Sci. Technol. 11 (1998) 852. [18] A.V. Narlikar, Anurag Gupta, S.B. Samanta, C. Chen, Y. Hu, P. Wander, B.M. Wanklyn, J.W. Hodby, Phil. Mag. B 79 (1999) 717.
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Physica B journal homepage: www.elsevier.com/locate/physb
Oxygen purity effect oxygen deficiency by argon heat treatment on Y:123 superconductors A. Sedky a,b,n a b
Physics Department, Faculty of science, King Faisal University, Al-Hassa 31982, P.O.B 400, Saudi Arabia Physics Department, Faculty of science, Assiut University, Assiut, Egypt
a r t i c l e i n f o
abstract
Article history: Received 6 September 2012 Received in revised form 1 November 2012 Accepted 5 November 2012 Available online 19 November 2012
Argon heat treatments at 450 1C for different times of 0, 12, 24, 36, 48 and 60 h on the two Y:123 oxygenated samples with different oxygen purity (Hp 99.99% and Lp 93%) are reported. The results of X-ray diffraction, oxygen deficiency and resistivity measurements are presented in details. Furthermore, the superconducting critical temperature Tc, normal resistivity r300, residual resistivity r0, resistivity slope dr/dT and width of transition DTc are extracted from resistivity data. It is found that both c-parameter and oxygen deficiency increase by increasing annealing time, while OD and effective Cu valance decrease. But, the relative increase/decrease is more in Lp samples than that of Hp samples. Although Tc is decreased by annealing time for the two samples, the relative decrease in Tc is more in Lp samples when compared with those of Hp samples. Our results are discussed in terms of oxygen vacancies and hole carriers which are produced by annealing for the considered samples. & 2012 Elsevier B.V. All rights reserved.
Keywords: Argon annealing Oxygen purity Oxygen deficiency Normal resistivity Width of transition and critical temperatures
1. Introduction The peculiar dependence of normal and superconducting properties on oxygen stoichiometry of Y:123 high Tc superconductor has been interesting land mark in the field of superconductivity [1–5]. It is well known that argon heat treatment at a temperature above 400 1C on oxygenated Y:123 superconducting system can lose oxygen from the poor Cu–O chains and consequently the oxygen content is decreased from seven to six along with a gradual decrease in Tc [6,7], while the rich Cu–O2 planes are supposed to stay unaffected. The Tc versus oxygen deficiency plot exhibits two successive plateaus at 90 and 60 K (orthorhombic I and II). Orthorhombic I is well established and associated with an optimal doping of the CuO2 planes [8], while orthorhombic II results from an alternate ordering of oxygen rich and oxygen poor of Cu–O chains along the a-axis [9]. However, Y:123 samples should have similar mechanism of oxygen transfer through Ar heat treatment, independent on oxygen purity during sintering processes. If the mechanism of oxygen loss stays independent of the degree of oxygen purity, the normal and superconducting properties should be at least qualitatively similar in both samples, as documented for Y:123 systems before annealing
n Correspondence address: Physics Department, Faculty of science, King Faisal University, Al-Hassa 31982, P.O. Box 380, Saudi Arabia. Tel.: þ966507207331; fax: þ 96635899557. E-mail addresses: [email protected], [email protected]
0921-4526/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2012.11.014
[10–12]. However, the present study shows that the oxygen purity results in an entirely different set of properties for the two samples after Ar heat treatments, which are highlights of the present work. To clarify the above, two similar oxygenated Y:123 samples are prepared by the well known solid state reaction method. But one of them is sintered in high purity oxygen (99.99%), while the other is sintered in low purity oxygen (93%). After that Ar heat treatment at a temperature of 450 1C for five different times (0–60 h) is performed.
2. Experimental details Two similar samples of the series YBa2Cu3O7 are synthesized by the well-known solid state reaction method. The ingredients Y2O3, BaCO3 and CuO of 4 N purity are thoroughly mixed in required proportions and calcined at 900 1C in air for a period of 16 h, and then the furnace is slowly cooled to room temperature. This exercise is repeated three times with intermediate grinding at each stage. The resulting powders are ground, palletized in to two pellets. One of them is sintered in high purity oxygen (99.99% Hp) at 940 1C for a period of 24 h and then the furnace is cooled to room temperature with an intervening annealing for 24 h at 600 1C, while the other is sintered in low purity oxygen (93% Lp) at the same conditions. The 93% oxygen produced from air by the molecular sieve process contains not less than 90% and not more than 96% by volume of oxygen. The remainder consists mostly of argon and nitrogen. Also, some other gases could be
0.004 0.000 0.013 0.041 0.056 0.107 1.47 3.36 6.76 8.19 9.55 10.16 0.329 3.29 10.35 19.29 25.09 38.94 6 19 33 63 00 00 89 76 52 22 00 00 2.280 2.193 2.120 2.073 1.893 1.833 0.08 0.21 0.32 0.39 0.66 0.75 11.683 11.690 11.699 11.705 11.726 11.731 Ar0 Ar12 Ar24 Ar36 Ar48 Ar60
0.018 0.015 0.008 0.005 0.000 0.000
(dq/dT) (mO.cm/K)
q300 (mO.cm) q0 (mO.cm) DTc (K) Tc (K) P Oxy. def. Orth. dis. ˚ C (A) LP
0.003 0.003 0.005 0.006 0.017 0.037 1.07 2.86 4.98 6.13 7.33 8.92 0.09 1.94 6.38 7.67 12.04 18.84 8 11 18 25 40 85 90 84 72 60 40 00 2.273 2.226 2.200 2.147 1.993 1.907 0.09 0.16 0.20 0.28 0.51 0.64 11.683 11.689 11.695 11.702 11.713 11.717 Ar0 Ar12 Ar24 Ar36 Ar48 Ar60
0.019 0.017 0.012 0.009 0.001 0.000
q300 (mO.cm) q0 (mO.cm) DTc (K) Tc (K) P Oxy. def.
(b) C parameter, orthorhombic distortion, oxygen deficient and effective Cu valance, width of transition, resistivity slope, residual resistivity and normal resistivity of Hp samples.
Fig. 1. XRD patterns of Hp Y:123 samples before and after annealing.
Orth. Dis.
Fig. 1 shows XRD patterns for Hp samples before and after Ar heat treatment. The same is done for Lp samples. The structure of the samples, after annealing by Ar up to 60 h, maintain a clearly single phase and no additional peaks could be formed as before Ar treatment. The obtained peaks (0 1 0), (0 1 3), (1 0 3), (0 0 5), (1 1 3), (0 2 0), (2 0 0), (1 2 3) and (2 1 3) belong to the well known peaks reported for Y:123 superconductors. Although no extra peaks are observed in the XRD patterns for all samples, there is a little variation of the lines intensity of the peaks with increasing annealing time. The marginal shift in the position of lines is mainly attributed to the change in the c-lattice parameter produced by Ar annealing. The samples annealed at t¼ 0 and 12 h are completely orthorhombic, being nearly evident from crystallographic splitting of (0 2 0), (2 0 0) and (1 2 3), (2 1 3). But with increase in annealing time up to 60 h, the crystallographic splitting of (0 2 0), (2 0 0) and (1 2 3), (2 1 3) disappears and seem to be one broader peak which indicates that the structure is changed to tetragonal. The c-parameters, orthorhombic distortion (OD) [(b a)/b] and oxygen deficiency for all samples are listed in Table 1(a and b). It is clear that c-parameter and oxygen deficiency are increased by increasing annealing time for all samples, while OD continuously decreases. The c-parameter and oxygen deficiency for LpAr samples are comparatively more than observed in HpAr samples, while the vice is versa for OD. The change of c-parameter after annealing is probably related to the
˚ C (A)
3. Results and discussion
HP
found such as carbon dioxide (0.03%) and carbon monoxide (0.001%). The samples are tested for phase purity by X-ray diffraction (XRD) using Semen’s D-500 with CuKa radiation of ˚ The oxygen content is measured by idometery titra1.541838 A. tion method. The electrical resistivity of the samples are obtained using the standard four-probe technique in closed cycle refrigerator [cryomech compressor package with cryostat Model 8101812212, USA] within the range of (10–300) K. Nanovoltameter Keithley 2182, current source Keithley 6220 and temperature controller 9700 (0.001 K resolution) are used in this experiment. After that, Ar heat treatments, at a temperature of 450 1C for different times of 12, 24, 36, 48 and 60 h are separately made on the two considered samples with intermediate characterization and measurements.
(dq/dT) (mO.cm/K)
A. Sedky / Physica B 410 (2013) 233–236
Table 1 (a) C- parameter, orthorhombic distortion, oxygen deficient and effective Cu valance, width of transition, resistivity slope , residual resistivity and normal resistivity of Hp samples
234
A. Sedky / Physica B 410 (2013) 233–236
235
Resistivity (mohm.cm)
1.2 1 0.8 0.6 HpAr0 LpAr0
0.4 0.2 0
Resistivity (mohm.cm)
4
1.4
0
50
200 100 150 Temperature (K)
3.5 3 2.5 2 1.5 0.5
18
45
16
40
14
35
12 10 8 6 4
HpAr36 LpAr36
2 0
0
50
200 100 150 Temperature (K)
250
HPAr12 LpAr12
1 0
300
250
Resistivity (mohm)
Resistivity (mohm.cm)
1.6
0
50
200 100 150 Temperature (K)
250
300
HpAr48 LpAr48
30 25 20 15 10 5
300
0
0
50
200 100 150 Temperature (K)
250
300
Resistivity (mohm.cm)
30 25 20 15 10 HpAr60 LpAr60
5 0
0
50
100 150 200 Temperature (K)
250
300
Fig. 2. (a) Resistivity versus temperature for Y:123 samples before annealing, (b) resistivity versus temperature for Y:123 samples annealed at 12 h, (c) resistivity versus temperature for Y:123 samples annealed at 24 h, (d) resistivity versus temperature for Y:123 samples annealed at 36 h, and (e) resistivity versus temperature for Y:123 samples annealed at 60 h.
change in the coupling between CuO2 planes due to the loss of oxygen from CuO chains during annealing operation [13–15]. These results indicate that oxygen loss from CuO chains for Lp samples is higher than Hp samples, and consequently the relative change in c-parameter and OD is different. From the values of oxygen content, the effective Cu valence P ¼ 2y7 is calculated 3 and listed in Table 1 [11]. It is clear that P slightly decreases with annealing time, but the relative decrease is higher in Lp samples when compared to Hp samples. This of course is related to decreasing of the hole carriers in the CuO2 planes [10,11]. In Fig. 2(a–e), we plot the resistivity as a function of temperature for all samples before and after Ar annealing. In general the r(T) curves of the samples annealed at t¼0 and 12 h involve a linear region, which extends from room temperature down to a temperature close to the onset temperature Ton. The interesting point here is that Tc ¼90 K for the two samples before Ar annealing irrespective of the purity of oxygen. This is consistent with the reported data based on Y:123 superconducting systems. While Ar annealing up to 12 h, the Tc is decreased for both samples, but it is higher in Hp
sample (84 K) than that of Lp sample (76 K). With increasing annealing time up to 24 and 36 h, the r(T) curves involve a semiconductor behavior for both samples, but still show superconductivity at Tc (72 and 60 K) for Hp sample, and (52 and 22 K) for Lp sample. With more increase in annealing time up to 48 and 60 h, Lp samples show semiconductor behavior with decreasing temperature to 20 K, while Hp sample at 48 h shows a transition to superconducting state without zero resistivity, and 60 h Hp sample is non superconductor. However, a variation of critical temperature Tc against annealing time for all samples is shown in Fig. 3(a). A quick glance at this figure shows the depression in Tc with annealing time. It is remarkable to note that the relative decrease in Tc is more in Lp samples when compared with those of Hp samples. For a comparison between the different ways of oxygen purity, we plot the difference in Tc for (Hp Lp); see Fig. 3(b). It is clear that the difference in Tc increases with increasing annealing time. These results indicate that the oxygen purity is controlling the suppression of superconductivity by Ar annealing for Y:123 samples.
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Y:123 pure superconducting systems. But the present study indicate that the random selection of oxygen purity is the main parameter responsible for the variance in Tc depression by annealing between the two samples. However, this study leads towards two different behaviors of oxygen disorder according to the purity of oxygen. The relative loss of oxygen from Lp samples is more than Hp samples, and consequently the structural and transport properties are different. Anyhow, the present study opens a new trend for the impact of oxygen purity on superconductivity, and really emphasizes the need of paying more attention by using different facilities, such as neutron diffraction and SQUID magnetometer, which are not available in our research Lab.
100 HP
90
Lp
80
Tc (K)
70 60 50 40 30 20 10 0
0
10
20
30 40 Anealing time (h)
50
60
70
4. Conclusion Impact of argon heat treatment at 450 1C for different times on two oxygenated Y:123 samples with different oxygen purity is investigated. We have shown that c-parameter, orthorhombic distortion, oxygen deficiency, effective Cu valence, r300, DTc, r0, dr/dT and the Tc behaviors are significantly different by Ar annealing in Hp samples with respect to Lp samples. This study leads towards different behaviors of oxygen disorder and hole carriers according to the purity of oxygen, which is not obtained before annealing for Y:123 superconducting system.
45
[TcHp - TcLp (K)]
40 35 30 25 20 15 10 5 0
Acknowledgments 0
10
20
30 40 Anealing time (h)
50
60
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Fig. 3. (a) Tc versus annealing time for Y:123 samples and (b) (TcHp TcLp ) versus annealing time for Y:123 samples.
On the other hand, there are also changes in the values of r300,
DTc, r0 and dr/dT, listed in Table 1(a and b), between Hp and Lp samples. The r(T) curves has a slope (dr/dT) which is connected with carrier–carrier scattering, and the extrapolation of r(T) to T¼0 K provides the residual resistivity r0 which is connected with impurity scattering [16]. It is clear that both r300, r0 and DTc are increasing with increasing annealing time up to 60 h. This behavior indicates that the electron scattering by impurities is also affected by annealing time as well as normal state resistivity r300. The (dr/dT) behavior indicates that all samples have a negative slope except the samples annealed at t¼0 and 12 h. This means that the normal state behavior is metallic for these two samples, and semiconductor for the rest of the samples. It has been reported that superconducting property of Y:123 systems are found to vary with both oxygen content and the local arrangement of oxygen vacancies either in the planes or in chains [6]. Furthermore, these vacancies may be destroying the pairing conductions to some extent in the superconducting systems [12]. So, we believe that beside oxygen disorder produced by Ar annealing, some of other parameters, such as the local arrangement of oxygen vacancies and the nature of pair formation, might be affected by the purity of oxygen [17,18]. Moreover, the effective Cu valance in the Cu–O2 planes is also affected and generally decreased the hole carriers for the two samples. Let us now look for the exact reason responsible for the variance in Tc depression by Ar annealing. Of course, the researches may be believed that Tc is not affected by the purity of oxygen as well as in
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