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Alteration of the doping state induced by thallium loss in Tl-2201
Physica C 436 (2006) 68–71 www.elsevier.com/locate/physc
Alteration of the doping state induced by thallium loss in Tl-2201 Hongqi Chen *, Lars-Gunnar Johansson Department of Inorganic Chemistry, Go¨teborg University, 412 96 Goteborg, Norway Received 9 November 2005; received in revised form 12 January 2006; accepted 13 January 2006 Available online 15 March 2006
Abstract Influence of thallium content in Tl-2201 on superconductivity is investigated by annealing at different temperatures at 0.2 oxygen pressure. Deeply overdoped, non-superconducting Tl-2201s undergo a transition from non-superconducting to superconducting and then to non-superconducting state with increasing annealing temperature. By analysis of mass loss of Tl-2201 at high temperature, effects of thallium and oxygen content on superconductivity were separated. The loss of thallium results in that the Tl-2201 undergoes an alteration from over-doping to under-doping state. During annealing at temperatures lower than 500 °C, the thallium lose is ignored. The mainly feature of Tl-2201 exhibits absorption of oxygen and an increase in the mass of Tl-2201. At temperatures higher than 500 °C, the loss of thallium in Tl-2201 becomes dominant, meantime accompanying with a release of oxygen. A phase diagraph with respect to the relationship between the hole carrier concentration, the interstitial oxygen and the substitution Cu2+ for Tl3+ was discussed. The under-doped Tl-2201s only appear at low substitution Cu2+ for Tl3+ region. Ó 2006 Published by Elsevier B.V. PACS: 74.72.Fq; 74.62.Dh; 74.25.Dw Keywords: Tl-2201; Carrier concentration; Thallium content; Doping state
1. Introduction It is well known that high temperature superconductivity is closely related to the hole doping in the CuO2 planes. An internal charge transfer takes place in the double layer thallium cuprates. Electronic charge is transferred from the CuO2 planes to the charge buffer layers, creating positive holes in CuO2 planes and causing Tl to exhibit an average valence <3+ [1–3]. The formation of positive holes in the CuO2 planes is a pre-requisite for superconductivity. This argument refers to stoichiometric Tl-2201. Any deviation from ideal stoichiometry that affects the distribution of the electric charge of the charge buffer layers is expected to affect the hole doping of the CuO2 planes and, thereby to influence the superconducting properties. The most common deviations of these types in Tl-2201 are (a) oxygen non-stoichiometry (interstitial oxygen *
Corresponding author. E-mail address: [email protected] (H. Chen).
0921-4534/$ - see front matter Ó 2006 Published by Elsevier B.V. doi:10.1016/j.physc.2006.01.043
atoms), (b) thallium non-stoichiometry (Tl vacancies or substitution of Cu2+ for Tl3+). As-synthesized Tl-2201 is usually overdoped. When annealed in vacuum or in Ar to remove some of the interstitial oxygen, Tc increases. It has been reported that such annealing can increase Tc from 0 to 93 K [4–6]. As noted above, the hole concentration in Tl-2201 is affected by thallium content. Any distortion of the Tl-O sheets influences the energy of the Tl6s band and thereby the relative energy with respect to the Cu2p bands, and therefore the superconductivity of Tl-2201. It has been reported that thallium stoichiometry influences Tc and structural symmetry [7]. It also has been reported that the occupancy of thallium site in a synthesized Tl-2201 is near to 99% based on resonant synchrotron X-ray diffraction data [8]. For a Tl-2201 with low thallium content, the substitution of Cu2+ for Tl3+ appears, resulting in structural transition from orthorhombic to tetragonal phase. It is expected that the vacancy of thallium site due to a postannealing at a high temperature also results in distortion
H. Chen, L.-G. Johansson / Physica C 436 (2006) 68–71
2. Experimental Bulk Tl-2201 samples were picked up from two Tl-2201 crucibles which are as Tl2O source for thallization of precursor films. One of them was multi-time used as a Tl2O source, the Tc of the Tl-2201 crucible deceased to below 20 K. Another was new and has a Tc of 80 K. The preparation of Tl-2201 containers was described in reference [9]. Bulk Tl-2201 samples were put in a gold crucible, then they are sealed in a quartz ampoule filed with air. The assembled ampoule was subjected to annealing at different temperatures. The Tcs of samples were determined by susceptibility measurement and the loss of the mass was determined by an electronic balance with a sensitivity of 0.01 mg. 3. Results and discussion Fig. 1 shows Tcs for two group samples subjected to a series of 2 h annealing at 300, 350, 400, 450, 500 °C. . .. The two group samples have original transition temperatures of 80 and <4.2 K. First group samples with a Tc of 80 K were annealed, respectively at 300, 350, and 400 °C resulted in a decrease in critical temperature, Tc lows down <4.2 K after the 2 h anneal at 400 °C. This is the behaviour expected from an overdoped sample. The extra oxygen results in decreasing or even vanishing of Tc. Annealing at 550 and 600 °C for the two group samples results in a partial restoration of Tc. The Tc reaches the maximum 43 K and 60 K for two group samples at 620 and 680 °C,
100
80
Tc (K)
of the local lattice and influences the distribution of chargers, therefore, affects superconductivity. However, it is difficult to change the thallium content of a sample (e.g., by vaporization) without changing the oxygen content. As the oxygen content has a very strong influence on Tc in Tl-2201. Therefore, the relationship between Tc and thallium content in Tl-2201 tends to be obscured by the effect of changing oxygen content. This work also origins from some considerations of the lifetime of a bulk Tl-2201 crucible as a TlO2 source in the so-called crucible method for preparing Tl-2201 superconducting films, in which the thallium-free precursor films react with saturation vapour of Tl2O to form Tl-2201 phase. However, it was found that Tc of a thallized Tl-2201 film depends on the history of Tl-2201 crucibles, and that the lifetime of a Tl-2201 crucible is very short and is only used for a few times. It was believed that this is due to the thallium loss in surface of a bulk Tl-2201 crucible, even though the total loss of thallium related to the crucible is very small. In the crucible method, the vapour pressure of Tl2O and O2 in the crucible are of importance. So the thallium loss results in a change in vapour pressure of Tl2O and O2 in the crucible. In present work, we study features of thallium loss and oxygen absorption at different temperatures and separated the effects of oxygen and thallium content on Tc in Tl-2201.
69
60
40
20
0
0
200
400 600 Annealing temperature (°C)
800
1000
Fig. 1. Tc of Tl-2201 samples as a function of annealing temperature.
respectively. With increasing annealing temperature, the Tc decreases and vanishes at 760 and 800 °C, respectively. It seems that Tl-2201 undergoes a transition from underdoped to overdoped state. Why the Tl-2201 exhibits so abnormal behaviour after annealing at different temperatures? In the annealing processes, two things were changed, oxygen and thallium content in Tl-2201. It can be described by a simply chemical reaction. Tl2 Ba2 CuO6þd ¼ Tl2y Ba2 CuO6þdþD þ y=2Tl2 O y=2O2 D=2O2
ð1Þ
Here, the ideal thallium stoichiometry is for simplicity. The hole carrier concentration can be described by P ¼ 2ðd þ DÞ þ 3y.
ð2Þ
Clearly, a decrease in Tc and even a destruction in superconductivity below 400 °C is due to Tl-2201 becoming an over-doped state. Under this annealing temperature, the thallium loss is ignorable; therefore an appearance of an over-doped state can contribute to an increase of the interstitial oxygen. That is, y = 0, and D > 0. What happen at higher than 500 °C? In order to separate effects of oxygen and thallium content on Tc in Tl-2201, we simulated experiment conditions of the two group samples and studied the mass loss at different temperatures, using the bulk samples, that are from the same as-synthesized bulk Tl-2201. It was found that changes in the mass of a sample are a function of annealing time and annealing temperature. Annealing for a short time (12 min at below 800 °C in air) always results in an increase in the mass, and the increase of mass decreased with annealing temperature. In contrast, annealing for 15 h at higher than 550 °C in air always leads to a decrease in mass and the decrease of mass increased with annealing temperature. Changes in mass for bulk Tl-2201 samples as a function of annealing temperature are shown in Fig. 2. It is argued that the increase in mass after annealing in air is due to the absorption of oxygen from air while the decrease in mass after annealing is due to thallium loss.
H. Chen, L.-G. Johansson / Physica C 436 (2006) 68–71 0.4 12min.
dw/w (%)
0.3
0.2
0.1 15hr.
0 -0.1 450
500
550
600 650 700 Temperature (°C)
750
800
850
Fig. 2. Relative mass loss of Tl-2201 changes with annealing temperature (top for annealing time 12 min, bottom for annealing time 15 h).
The results imply that both oxygen absorption and the thallium loss processes at a measurable rate and that oxygen absorption is a much faster process. The facts that Tl-2201 absorbs oxygen at low temperature, ex. at 300 °C, have been demonstrated by enormous number of experiments. Our results prove that as-synthesized Tl-2201 films absorbs oxygen from air at low temperature. At lower temperature, the thallium loss is expected to be very slow in comparison to oxygen uptake. Conversely, the thallium loss will be quite fast above 500 °C. This means that annealing performed in the air at 300 or 400 °C will mainly affect the oxygen content, while annealing carried out at >500 °C will result in a major change in the thallium content in the samples. As mentioned above, it is generally considered that as-synthesized Tl-2201 is overdoped. Annealing which results in oxygen uptaking is therefore expected to cause Tc to decrease. In other words, any increase in Tc for Tl-2201 subjected to such annealing can be considered to be due to the thallium loss. In order to explain how does the thallium loss affect superconducting transition temperature of a Tl-2201 sample, we can make a simply estimation. From Fig. 2, the maximum loss of the mass is about 0.1%. The mass fraction of thallium atoms in a Tl-2201 molecule is about 40%. This means that the mass loss of 0.1% responds to create 0.0025 vacuum positions per Tl-2201 unit cell. If regardless of a change in oxygen content, so little change in thallium content is not sufficient to change the hole concentration in the CuO plane so that the Tl-2201 undergoes a transition from under-doping-like to over-doping-like state. In addition, the thallium loss, in principle, will increase hole carrier concentration in the CuO plane, resulting in much deeply overdoping. Therefore, the effect of the thallium loss on Tc is not a direct procedure. From a view of electric balance, the vacancy of thallium in a unit cell will result in that oxygen ions O2 coordinated with Tl3+ also exhausts out Tl-2201. The exhaustion of the thallium ions does not change hole carrier concentration in Tl-2201. More importance is that the vacancy of thallium creates located lattice
distortion, and the distortion exhausts the interstitial oxygen atoms out the lattice. That is to say that the exhaustions of the interstitial atoms results in the restoration of Tc of Tl-2201. In the view of above analysis, the change of oxygen content during annealing undergoes an increase at low temperature and a decrease at high temperature with the thallium loss. Now let us analyse the interstitial oxygen. Suppose that the hole carrier concentration at optimum doping state is 0.16 [10,11]. For a Tl-2201 without thallium defects, the interstitial oxygen content d is about 0.08. In our samples, the thallium content is 1.85, and they have a tetragonal crystal structure. It has been demonstrated that a part of thallium is substituted by Cu2+ ions in a Tl-2201 with tetragonal structure symmetry. Suppose that the rest of thallium position is replaced by Cu. It can generally be written as Tl2xBa2Cu1+xO6+d. In this case, the d is 0.005 at optimum doping state. In general, the d equals to (0.16 x)/2. With an increase in substitution of Cu2+ for Tl3+, ex. the substitution x reaching to 0.15, the interstitial oxygen d deceases from 0.08 to 0.005 at the optimum doping state. The concentration of the interstitial oxygen in Tl2xBa2Cu1+xO6+d at the optimum doping is very low. This result explains why most of synthesised Tl-2201s are in the over-doping state. In order to describe relationship between doping state and interstitial oxygen and substitution of Cu2+ for Tl3+, Fig. 3 shows a sketch of a phase diagram, indicating the interstitial oxygen as a function of the substitution of Cu2+ for Tl3+. Based on Tc/Tc max = 182.6(0.16p)2 [11], p = 2d x, where, p is the hole carrier concentration, x is the substitution of Cu2+ for Tl3+, the d x plane is divided into under-doping, over-doping and normal regions. For the region with d < 0, it means that the other oxygen deficiency, expect for the interstitial oxygen O(4). For example, it has been reported that there is a small deficiency in O(3) position on Tl–O planes, less than 0.04 [12]. So the d < 0 may indicates that the Tl-2201 phase is unstable. In this figure, we can see that the underdoped Tl-2201 only appears at low substitution region. Our sample locals
0.20 0.18 0.16
Interstitial oxygen δ
70
0.14 0.12 0.10 0.08
Overdoping
0.06 0.04
underdoping
0.02 0.00 0.00
0.05
0.10
0.15
0.20
0.25
0.30
Cu+2 substitution for Tl+3 x Fig. 3. A sketch of the interstitial oxygen as a function of the substitution of Cu2+ for Tl3+.
H. Chen, L.-G. Johansson / Physica C 436 (2006) 68–71
at x less than 0.15 region, the thallium loss creates thallium vacancy and results in an extra decrease in oxygen content. The Tl-2201 undergoes an alteration from over-doping to under-doping state, like to indicating in Fig. 3 by a dot line array. It is worth to notice that Tl-2201 is a superconductor or a deeply overdoped non-superconductor under the region with a substitution higher than 0.05, and that there are two non-superconducting regions without substitution or seldom substitution. One has a high interstitial oxygen content and other has a low interstitial oxygen. The results of Aranda et al. [8] demonstrated the existence of the two orthorhombic Tl-2201 phases. The interstitial oxygen content is 0.03 and 0.16, respectively. It is also worth to notice that the maximum value of Tc in both two samples subjected to an annealing at high temperature is lower than their original maximum values. This perhaps is a direct result that the thallium vacancy results in a decrease in maximum Tc. the similar results were also reported in YBCO, in which the Zn-substitution results in the maximum Tc decreasing from 80 to 20 K [13]. In addition, the vacancy of thallium ions in a crystal cell also may alternate the structure of its energy band and affects the band overlap between Cu3d and Tl6s, affecting the charge transition between Cu–O layer and Tl–O layers. Now, let us come back to the lifetime of the crucible. Because the thallium loss in Tl-2201 results in the distortion of the location lattice and the exhaustion of the extra interstitial oxygen, they can alter the saturation vapour pressure of Tl-2201, therefore affect the lifetime of the Tl-2201 crucible. 4. Conclusion Tcs of over-doped Tl-2201s can partially be restored by annealing at a higher temperature. The restoration of Tcs is contributed to the thallium loss in Tl-2201. The loss of thallium creates vacancies of thallium ions and lattice dis-
71
tortion in a location. The lattice distortion results in that the interstitial oxygen is drained out. The Tl-2201 undergoes a transition from the over-doping to the under-doping state.
Acknowledgements This work was supported by the Swedish Superconductivity Consortium, NUTEK, and KVA. References [1] R.P. Vasquez, Z.F. Ren, J.H. Wang, Phys. Rev. B 54 (1996) 6115. [2] R.P. Vasquez, M.P. Siegal, D.L. Overmyer, Z.F. Ren, J.Y. Lao, J.H. Wang, Phys. Rev. B 60 (1999) 4309. [3] K. Tanaka, A. Iyo, N. Terada, K. Tokiwa, S. Miyashita, Y. Tanaka, T. Tsukamoto, S.K. Agarwal, T. Watanabe, H. Ihara, Phys. Rev. B 63 (2001) 064508. [4] C. Allgeier, J.S. Schilling, Physica C 168 (1990) 499. [5] Y. Kubo, Y. Shimakawa, T. Manako, H. Igarashi, Phys. Rev. B 43 (1991) 7875. [6] S.S.P. Parkin, V.Y. Lee, A.I. Nazza, R. Savoy, T.C. Huang, G. Gorman, R. Beyyers, Phys. Rev. B 38 (1988) 6531. [7] Y. Shimakawa, Physica C 204 (1993) 279. [8] M.A.G. Aranda, D.C. Sinclair, J.P. Attfield, A.P. Mackenzie, Phys. Rev.B 51 (1995) 12747. [9] H.Q. Chen, L.-G. Johansson, Z.G. Ivanov, D. Erts, Phys. Stat. Sol. (a) 172 (1999) 165; H.Q. Chen, L.-G. Johansson, Z.G. Ivanov, Physica C 355 (2001) 156; H.Q. Chen, L.-G. Johansson, Z.G. Ivanov, Appl. Phys. Lett. 77 (2000) 1197. [10] M.R. Presland, J.L. Tallon, R.G. Buckley, R.S. Liu, N.E. Flower, Physica C (1991) 95. [11] J.L. Tallon, C. Bernhard, H. Shaked, R.L. Hitterman, J.D. Jorgensen, Phys. Rev. B51 (1995) 12911. [12] J.L. Wagner, O. Chmaissem, J.D. Jorgensen, D.G. Hinks, P.G. Radaelli, B.A. Hunter, W.R. Jensen, Physica C 277 (1997) 170. [13] S.H. Naqib, J.R. Cooper, J.L. Tallon, C. Psnsgopoulos, Physica C 387 (2003) 365.
Alteration of the doping state induced by thallium loss in Tl-2201 Hongqi Chen *, Lars-Gunnar Johansson Department of Inorganic Chemistry, Go¨teborg University, 412 96 Goteborg, Norway Received 9 November 2005; received in revised form 12 January 2006; accepted 13 January 2006 Available online 15 March 2006
Abstract Influence of thallium content in Tl-2201 on superconductivity is investigated by annealing at different temperatures at 0.2 oxygen pressure. Deeply overdoped, non-superconducting Tl-2201s undergo a transition from non-superconducting to superconducting and then to non-superconducting state with increasing annealing temperature. By analysis of mass loss of Tl-2201 at high temperature, effects of thallium and oxygen content on superconductivity were separated. The loss of thallium results in that the Tl-2201 undergoes an alteration from over-doping to under-doping state. During annealing at temperatures lower than 500 °C, the thallium lose is ignored. The mainly feature of Tl-2201 exhibits absorption of oxygen and an increase in the mass of Tl-2201. At temperatures higher than 500 °C, the loss of thallium in Tl-2201 becomes dominant, meantime accompanying with a release of oxygen. A phase diagraph with respect to the relationship between the hole carrier concentration, the interstitial oxygen and the substitution Cu2+ for Tl3+ was discussed. The under-doped Tl-2201s only appear at low substitution Cu2+ for Tl3+ region. Ó 2006 Published by Elsevier B.V. PACS: 74.72.Fq; 74.62.Dh; 74.25.Dw Keywords: Tl-2201; Carrier concentration; Thallium content; Doping state
1. Introduction It is well known that high temperature superconductivity is closely related to the hole doping in the CuO2 planes. An internal charge transfer takes place in the double layer thallium cuprates. Electronic charge is transferred from the CuO2 planes to the charge buffer layers, creating positive holes in CuO2 planes and causing Tl to exhibit an average valence <3+ [1–3]. The formation of positive holes in the CuO2 planes is a pre-requisite for superconductivity. This argument refers to stoichiometric Tl-2201. Any deviation from ideal stoichiometry that affects the distribution of the electric charge of the charge buffer layers is expected to affect the hole doping of the CuO2 planes and, thereby to influence the superconducting properties. The most common deviations of these types in Tl-2201 are (a) oxygen non-stoichiometry (interstitial oxygen *
Corresponding author. E-mail address: [email protected] (H. Chen).
0921-4534/$ - see front matter Ó 2006 Published by Elsevier B.V. doi:10.1016/j.physc.2006.01.043
atoms), (b) thallium non-stoichiometry (Tl vacancies or substitution of Cu2+ for Tl3+). As-synthesized Tl-2201 is usually overdoped. When annealed in vacuum or in Ar to remove some of the interstitial oxygen, Tc increases. It has been reported that such annealing can increase Tc from 0 to 93 K [4–6]. As noted above, the hole concentration in Tl-2201 is affected by thallium content. Any distortion of the Tl-O sheets influences the energy of the Tl6s band and thereby the relative energy with respect to the Cu2p bands, and therefore the superconductivity of Tl-2201. It has been reported that thallium stoichiometry influences Tc and structural symmetry [7]. It also has been reported that the occupancy of thallium site in a synthesized Tl-2201 is near to 99% based on resonant synchrotron X-ray diffraction data [8]. For a Tl-2201 with low thallium content, the substitution of Cu2+ for Tl3+ appears, resulting in structural transition from orthorhombic to tetragonal phase. It is expected that the vacancy of thallium site due to a postannealing at a high temperature also results in distortion
H. Chen, L.-G. Johansson / Physica C 436 (2006) 68–71
2. Experimental Bulk Tl-2201 samples were picked up from two Tl-2201 crucibles which are as Tl2O source for thallization of precursor films. One of them was multi-time used as a Tl2O source, the Tc of the Tl-2201 crucible deceased to below 20 K. Another was new and has a Tc of 80 K. The preparation of Tl-2201 containers was described in reference [9]. Bulk Tl-2201 samples were put in a gold crucible, then they are sealed in a quartz ampoule filed with air. The assembled ampoule was subjected to annealing at different temperatures. The Tcs of samples were determined by susceptibility measurement and the loss of the mass was determined by an electronic balance with a sensitivity of 0.01 mg. 3. Results and discussion Fig. 1 shows Tcs for two group samples subjected to a series of 2 h annealing at 300, 350, 400, 450, 500 °C. . .. The two group samples have original transition temperatures of 80 and <4.2 K. First group samples with a Tc of 80 K were annealed, respectively at 300, 350, and 400 °C resulted in a decrease in critical temperature, Tc lows down <4.2 K after the 2 h anneal at 400 °C. This is the behaviour expected from an overdoped sample. The extra oxygen results in decreasing or even vanishing of Tc. Annealing at 550 and 600 °C for the two group samples results in a partial restoration of Tc. The Tc reaches the maximum 43 K and 60 K for two group samples at 620 and 680 °C,
100
80
Tc (K)
of the local lattice and influences the distribution of chargers, therefore, affects superconductivity. However, it is difficult to change the thallium content of a sample (e.g., by vaporization) without changing the oxygen content. As the oxygen content has a very strong influence on Tc in Tl-2201. Therefore, the relationship between Tc and thallium content in Tl-2201 tends to be obscured by the effect of changing oxygen content. This work also origins from some considerations of the lifetime of a bulk Tl-2201 crucible as a TlO2 source in the so-called crucible method for preparing Tl-2201 superconducting films, in which the thallium-free precursor films react with saturation vapour of Tl2O to form Tl-2201 phase. However, it was found that Tc of a thallized Tl-2201 film depends on the history of Tl-2201 crucibles, and that the lifetime of a Tl-2201 crucible is very short and is only used for a few times. It was believed that this is due to the thallium loss in surface of a bulk Tl-2201 crucible, even though the total loss of thallium related to the crucible is very small. In the crucible method, the vapour pressure of Tl2O and O2 in the crucible are of importance. So the thallium loss results in a change in vapour pressure of Tl2O and O2 in the crucible. In present work, we study features of thallium loss and oxygen absorption at different temperatures and separated the effects of oxygen and thallium content on Tc in Tl-2201.
69
60
40
20
0
0
200
400 600 Annealing temperature (°C)
800
1000
Fig. 1. Tc of Tl-2201 samples as a function of annealing temperature.
respectively. With increasing annealing temperature, the Tc decreases and vanishes at 760 and 800 °C, respectively. It seems that Tl-2201 undergoes a transition from underdoped to overdoped state. Why the Tl-2201 exhibits so abnormal behaviour after annealing at different temperatures? In the annealing processes, two things were changed, oxygen and thallium content in Tl-2201. It can be described by a simply chemical reaction. Tl2 Ba2 CuO6þd ¼ Tl2y Ba2 CuO6þdþD þ y=2Tl2 O y=2O2 D=2O2
ð1Þ
Here, the ideal thallium stoichiometry is for simplicity. The hole carrier concentration can be described by P ¼ 2ðd þ DÞ þ 3y.
ð2Þ
Clearly, a decrease in Tc and even a destruction in superconductivity below 400 °C is due to Tl-2201 becoming an over-doped state. Under this annealing temperature, the thallium loss is ignorable; therefore an appearance of an over-doped state can contribute to an increase of the interstitial oxygen. That is, y = 0, and D > 0. What happen at higher than 500 °C? In order to separate effects of oxygen and thallium content on Tc in Tl-2201, we simulated experiment conditions of the two group samples and studied the mass loss at different temperatures, using the bulk samples, that are from the same as-synthesized bulk Tl-2201. It was found that changes in the mass of a sample are a function of annealing time and annealing temperature. Annealing for a short time (12 min at below 800 °C in air) always results in an increase in the mass, and the increase of mass decreased with annealing temperature. In contrast, annealing for 15 h at higher than 550 °C in air always leads to a decrease in mass and the decrease of mass increased with annealing temperature. Changes in mass for bulk Tl-2201 samples as a function of annealing temperature are shown in Fig. 2. It is argued that the increase in mass after annealing in air is due to the absorption of oxygen from air while the decrease in mass after annealing is due to thallium loss.
H. Chen, L.-G. Johansson / Physica C 436 (2006) 68–71 0.4 12min.
dw/w (%)
0.3
0.2
0.1 15hr.
0 -0.1 450
500
550
600 650 700 Temperature (°C)
750
800
850
Fig. 2. Relative mass loss of Tl-2201 changes with annealing temperature (top for annealing time 12 min, bottom for annealing time 15 h).
The results imply that both oxygen absorption and the thallium loss processes at a measurable rate and that oxygen absorption is a much faster process. The facts that Tl-2201 absorbs oxygen at low temperature, ex. at 300 °C, have been demonstrated by enormous number of experiments. Our results prove that as-synthesized Tl-2201 films absorbs oxygen from air at low temperature. At lower temperature, the thallium loss is expected to be very slow in comparison to oxygen uptake. Conversely, the thallium loss will be quite fast above 500 °C. This means that annealing performed in the air at 300 or 400 °C will mainly affect the oxygen content, while annealing carried out at >500 °C will result in a major change in the thallium content in the samples. As mentioned above, it is generally considered that as-synthesized Tl-2201 is overdoped. Annealing which results in oxygen uptaking is therefore expected to cause Tc to decrease. In other words, any increase in Tc for Tl-2201 subjected to such annealing can be considered to be due to the thallium loss. In order to explain how does the thallium loss affect superconducting transition temperature of a Tl-2201 sample, we can make a simply estimation. From Fig. 2, the maximum loss of the mass is about 0.1%. The mass fraction of thallium atoms in a Tl-2201 molecule is about 40%. This means that the mass loss of 0.1% responds to create 0.0025 vacuum positions per Tl-2201 unit cell. If regardless of a change in oxygen content, so little change in thallium content is not sufficient to change the hole concentration in the CuO plane so that the Tl-2201 undergoes a transition from under-doping-like to over-doping-like state. In addition, the thallium loss, in principle, will increase hole carrier concentration in the CuO plane, resulting in much deeply overdoping. Therefore, the effect of the thallium loss on Tc is not a direct procedure. From a view of electric balance, the vacancy of thallium in a unit cell will result in that oxygen ions O2 coordinated with Tl3+ also exhausts out Tl-2201. The exhaustion of the thallium ions does not change hole carrier concentration in Tl-2201. More importance is that the vacancy of thallium creates located lattice
distortion, and the distortion exhausts the interstitial oxygen atoms out the lattice. That is to say that the exhaustions of the interstitial atoms results in the restoration of Tc of Tl-2201. In the view of above analysis, the change of oxygen content during annealing undergoes an increase at low temperature and a decrease at high temperature with the thallium loss. Now let us analyse the interstitial oxygen. Suppose that the hole carrier concentration at optimum doping state is 0.16 [10,11]. For a Tl-2201 without thallium defects, the interstitial oxygen content d is about 0.08. In our samples, the thallium content is 1.85, and they have a tetragonal crystal structure. It has been demonstrated that a part of thallium is substituted by Cu2+ ions in a Tl-2201 with tetragonal structure symmetry. Suppose that the rest of thallium position is replaced by Cu. It can generally be written as Tl2xBa2Cu1+xO6+d. In this case, the d is 0.005 at optimum doping state. In general, the d equals to (0.16 x)/2. With an increase in substitution of Cu2+ for Tl3+, ex. the substitution x reaching to 0.15, the interstitial oxygen d deceases from 0.08 to 0.005 at the optimum doping state. The concentration of the interstitial oxygen in Tl2xBa2Cu1+xO6+d at the optimum doping is very low. This result explains why most of synthesised Tl-2201s are in the over-doping state. In order to describe relationship between doping state and interstitial oxygen and substitution of Cu2+ for Tl3+, Fig. 3 shows a sketch of a phase diagram, indicating the interstitial oxygen as a function of the substitution of Cu2+ for Tl3+. Based on Tc/Tc max = 182.6(0.16p)2 [11], p = 2d x, where, p is the hole carrier concentration, x is the substitution of Cu2+ for Tl3+, the d x plane is divided into under-doping, over-doping and normal regions. For the region with d < 0, it means that the other oxygen deficiency, expect for the interstitial oxygen O(4). For example, it has been reported that there is a small deficiency in O(3) position on Tl–O planes, less than 0.04 [12]. So the d < 0 may indicates that the Tl-2201 phase is unstable. In this figure, we can see that the underdoped Tl-2201 only appears at low substitution region. Our sample locals
0.20 0.18 0.16
Interstitial oxygen δ
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0.14 0.12 0.10 0.08
Overdoping
0.06 0.04
underdoping
0.02 0.00 0.00
0.05
0.10
0.15
0.20
0.25
0.30
Cu+2 substitution for Tl+3 x Fig. 3. A sketch of the interstitial oxygen as a function of the substitution of Cu2+ for Tl3+.
H. Chen, L.-G. Johansson / Physica C 436 (2006) 68–71
at x less than 0.15 region, the thallium loss creates thallium vacancy and results in an extra decrease in oxygen content. The Tl-2201 undergoes an alteration from over-doping to under-doping state, like to indicating in Fig. 3 by a dot line array. It is worth to notice that Tl-2201 is a superconductor or a deeply overdoped non-superconductor under the region with a substitution higher than 0.05, and that there are two non-superconducting regions without substitution or seldom substitution. One has a high interstitial oxygen content and other has a low interstitial oxygen. The results of Aranda et al. [8] demonstrated the existence of the two orthorhombic Tl-2201 phases. The interstitial oxygen content is 0.03 and 0.16, respectively. It is also worth to notice that the maximum value of Tc in both two samples subjected to an annealing at high temperature is lower than their original maximum values. This perhaps is a direct result that the thallium vacancy results in a decrease in maximum Tc. the similar results were also reported in YBCO, in which the Zn-substitution results in the maximum Tc decreasing from 80 to 20 K [13]. In addition, the vacancy of thallium ions in a crystal cell also may alternate the structure of its energy band and affects the band overlap between Cu3d and Tl6s, affecting the charge transition between Cu–O layer and Tl–O layers. Now, let us come back to the lifetime of the crucible. Because the thallium loss in Tl-2201 results in the distortion of the location lattice and the exhaustion of the extra interstitial oxygen, they can alter the saturation vapour pressure of Tl-2201, therefore affect the lifetime of the Tl-2201 crucible. 4. Conclusion Tcs of over-doped Tl-2201s can partially be restored by annealing at a higher temperature. The restoration of Tcs is contributed to the thallium loss in Tl-2201. The loss of thallium creates vacancies of thallium ions and lattice dis-
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tortion in a location. The lattice distortion results in that the interstitial oxygen is drained out. The Tl-2201 undergoes a transition from the over-doping to the under-doping state.
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