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Synthesis of single-phase HgBa2Ca2Cu3O8+δ superconductor
PHYSICA Physica C 232 (1994) 5-9
ELSEVIER
Synthesis of single-phase HgBa2Ca2Cu308 superconductor H.M. Shao a, L.J. Shen b, J.C. Shen a, X.Y. Hua b, P.F. Yuan b, X.X. Yao a Department of Physics, National Laboratory of Solid State Microstructure, Nanjing University, Nanjing 210008, China b Department of Physics, Nanjing Institute of Chemical Technology, Nanjing 210009, China
Received 3 May 1994; revised manuscript received 8 August 1994
Abstract
The third member (Hg-1223) of the recently discovered homologous series HgBa2Ca,_ i CunO2n+2+ti, having a Tc of 118 K (as-synthesized sample) and 133 K (oxygen-annealed sample), was successfully prepared by the solid state reaction and shorttime annealing technique using mixtures of the metal oxides HgO, BaO, CaO and CuO with starting composition HgBa2Ca2CuaOs+6. We found that the combination of very short annealing times with optimum temperatures 715-725"C gives rise to a very effective sintering of the constituents and avoids an excessive loss of Hg. Samples so obtained display a sharp superconducting transition determined both magnetically and resistively. The X-ray diffraction patterns of both the 118 K phase and the 133 K phase indicate a nearly single phase, corresponding to the tetragonal structure of space group P4/mmm and with lattice parameters a = 3.85 A and c = 15.85 A. The resistivity p of such a sample was very low, ~ 12 × 10- 3 f~ cm at 300 K, and the p versus T curve was linearly extrapolated to zero resistance at 0 K, being similar to the cases of high-quality high-To cuprates. Energy dispersive X-ray analysis (EDX) data of several grains of the Hg-1223 sample are in agreement with the proposed chemical formula.
1. Introduction
The recent discovery of high-temperature superconductivity in the HgBa2Can_lCunO2n+2+~ Hgbased homologous series o f layered Cu-mixed oxides, namely Hg- 12 (n - 1 ) n ( n = 1, 2, 3 ), with superconducting transition temperatures Tc = 96, 120 and 134 K, generates considerable interest, not only because this system set a new record T~, but also because it provides us with a new family o f compounds for studying the superconducting properties and mechanism [ 1-7]. However, the difficulties in preparing single-phase and high-density samples are some o f the obstacles in studying mercury-based compounds [ 8 ]. Mercury and its c o m p o u n d s are among the most toxic inorganic materials. Safety procedures are extremely important in the preparation o f Hg-contain-
ing compounds. HgO, which is one o f the starting oxides used to prepare the high-T¢ superconducting mercury-cuprates, is highly volatile at temperatures above its melting point o f 500°C [8 ]. In preparing these high-To superconducting oxides, prevention o f mercury loss is the most important factor, not only for safety purposes, but also for maintaining the stoichiometry o f products to ensure the formation o f the desired phase. In order to prevent severe mercury loss during the reaction, usually there are two methods to follow: (a) Using a high-pressure synthesis technique [ 8,9 ]. The use o f high pressure, possibly, lowers the mercury oxide decomposition. It also leads to a decrease o f the stability o f CaHgO2, whose synthesis at the first stage o f the reaction inhibits the formation o f rig-based compounds [ 10]. (b) Using a completely sealed reaction system
0921-4534/94/$07.00 @ 1994 Elsevier Science B.V. All rights reserved SSD10921-4534 (94) 00532-X
6
H.M. Shao et al./Physica C 232 (1994) 5-9
[ 4,5 ]. In order to prevent the decomposition of HgO and the removal of Hg from the starting mixture in the synthesis of Hg-based compounds, samples have been wrapped in noble metal foils, such as Au, Ag and Pt foils. We have found that Au foil works better than other foils, in terms of preventing Hg loss, because it is very inert and slightly melts at the high reaction temperatures, forming a tight seal which prevents severe mercury loss. However, high-pressure techniques are not suitable for preparing a relatively large amount of samples. As previously reported [4,5 ], the encapsulation process was likely to be suitable for the synthesis of Hg-based superconductors
[8]. However, to date, no one has succeeded in obtaining single-phase samples of the Hg-1223 phase [8 ]. Even if high-purity samples can be prepared, their densities are very low. In this paper we report on the preparation conditions of high density and high purity of bulk samples of the Hg- 1223 superconductor in a completely sealed reaction system using the combination of very short annealing times with optimum annealing temperatures, which gives rise to a very effective sintering of the constituents and avoids an excessive loss of Hg.
and moisture [ 11 ]. Then, the mixtures were pressed into rectangular bars of 2 X 4 × 20 mm 3. Each bar wrapped in silYer foil was placed on a quartz tube placed in a preheated tube furnace at 715-725°C for 3-4 min. The short-time annealing process was repeated four times with thorough intermediate grinding of the powder. After the final step the sample was wrapped again with silver foil and then encapsulated in evacuated ( ~ 10 -2 Torr) quartz tubes. The encapsulated samples were then sintered at 750°C for 1-10 h and rapidly cooled to room temperature in about 3 min. Subsequently, some of the sintered samples were annealed at 300°C for 8 h in flowing oxygen or argon gas. The annealed samples were characterized by Xray diffraction with Cu K~ radiation, transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), as well as resistivity and susceptibility measurements. Electrical contacts for the resistivity measurement were formed using conductive silver paste on the surface of the annealed sample. The resistivity was measured by a standard four-probe method.
3. Results and discussion 2. Experimental The mixed powders were obtained by two different methods as follows: (a) First, a precursor with nominal composition Ba2Ca2Cu3Ox was prepared by mixing high-purity Ba(NO3)2, Ca(NO3)2"4H20, and Cu(NO3)z'3H20. The mixture of nitrates was initially heated at 600°C in air for 12 h, then ground and heated at 925°C in 02 flow for 72 h with three intermediate regrindings [ 10 ]. Then appropriate amounts of HgO were added to the precursor with starting composition HgBa2CazCu3Oa+a and completely mixed. (b) Using metal-oxide powders of HGO(99%), BAO(99%), CAO(99%) and CUO(99%) as starting materials, CaO and BaO were obtained by decomposing highly pure ( > 99.5%) CaCO3 and BaCO3 in a vacuum furnace at 950°C-1000°C for 6 and 24 h; the metal-oxide powders were weighted to a Hg- 1223 composition, and mixed thoroughly. Powder mixing as described above was done in a dry box to prevent the powders from absorbing CO2
If the mixtures of the metal oxides HgO, BaO, CaO and CuO with starting composition HgBazCa2Cu3Oa + a are sealed in evacuated quartz tubes and sintered without short-time heat treatment, because of the low melting point of HgO (only 500°C) and its liability to evaporate, the quartz tubes will explode during the sintering process even if there is a slight increase in the speed of temperature rising [2 ]. On the other hand, in the temperature range between 500-550°C, the Hg vapor cannot get into the mixture of metal oxides and form the lower member of Hgbased homologous series [ 11 ]. But in this temperature range the CaHgOz compound can easily be formed, which will inhibit the formation of rig-based compounds, thus the single-phase Hg-1223 compound of high quality cannot be obtained [ 1,2 ]. If the above mixtures of metal oxides get short-time heat treatments at 715-725°C for 3-4 rain, which process was repeated four times with thorough intermediate grindings of the powder, we have found that 3-4 rain heating at the optimum temperature of 715-
H.M. Shao et al. / Physica C 232 (1994) 5-9
725°C is sufficient for the formation of Hg- 1201 and Hg-1212 phases in a non-closed system, because once the mercury is stabilized in the structure of the lower member of the Hg-based homologous series, the vaporization of Hg becomes very low during the later reaction [ 11,12 ]. So when we sealed the intermediate product for a short time into the evacuated quartz tube to sinter, the speed of temperature rising need not to be limited. In fact, no explosion occurred with the quartz tubes that we used in our laboratory. Also, the high speed of temperature rising can shorten the time the intermediate product has to stay in the temperature range between 500-550°C, so that the CaHgO2 compound cannot easily be formed [ 11 ]. This is of vital importance to ensure the formation of the desired phase. We still cannot understand the miracle that although the short-time annealing process with the mixed powder obtained by method (a) as described above, which was the mixture of the precursor Ba2Ca2Cu3Ox and the oxide HgO, was also repeated four times under the same reaction conditions, but when the intermediate product so obtained was sealed into the evacuated quartz tube and sintered, we still did not obtain the superconductivity of the resulting products, which we will work on later. Hg-1223 samples prepared in a non-stoichiometric starting composition may form non-superconducting crystallites and this results in lower quality samples. So the quality and density of the samples can be improved significantly if, after the initial short-time annealing, the exact amount of HgO that was lost (usually about 5-10%) is added and the bars are reground, pressed again and reheated. By this method the density of the bars can be as high as 85% theoretically, so the quality and density of the samples can be increased by adding the exact amount of HgO that was lost during the initial heating, followed by regrinding, pressing again and reheating under the same conditions. The as-synthesized sample displays a sharp transition. The width of the resistive transition is ~ 1 K with a zero-resistance value of 118 K. Especially, for the sample p at 300 K was ~ 12 mf~ cm, and exhibited a linear T dependence in the temperature range 180-300 K. This linear p versus T curve can be extrapolated to zero resistance at 0 K, which has been
7
known as a characteristic property of high-quality superconductors [ 7,11 ]. After post-annealing in flowing oxygen at 300oc for 6 h, all the samples exhibited diamagnetism at ~ 133 K. The ac susceptibility (X' and Z") for the sample after oxygenation at 300°C for 6 h is shown in Fig. 1. The volume susceptibility ;to of the sample was determined from the measured mass susceptibility Zm by X v = X , , ~ o / ( 1 - D x ~ o ) , where p is the X-ray density (6.176 g / c m 3) and D is the demagnetization factor. [-
!
!
!
I
. . . . . . .
|
90
140 165 190 T(K) Fig. I. Temperature dependence of ac magnetic susceptibilities for ( © ) as-synthesized ( Tc= I 18 K) and ( • ) post-oxygenated at 300°C for 6 h (T¢= 133 K) samplesof HgBa2Ca2Cu3Os+a.
20.0
I15
.
,
16.0[
so
,
8.0 4.0
13o
180
T(K) Fig. 2. Resistivity curves as a function temperature for ( O ) assynthesized and ( • ) post-oxygenatedat 300°C for 6 h samples of HgBa2Ca2Cu3Og+6.The inset shows the region of the superconducting transition.
8
ll.M. Shao et al./Physica C 232 (1994) 5-9
I# 20
I
I
30
40
I
5O
I
I
6O
70
2e Fig. 3. X-raydiffractionpattern for an as-synthesizedsampleof HgBa2Ca2Cu3Oa+~. Impuritypeaks from BaCuO2are markedby ( • ) .
Fig. 2 shows the temperature dependences of electrical resistivity (p versus Tcurve) of the sample after oxygenation at 300°C for 6 h, p undergoes a superconducting transition with a/'co ~ 140 K, Tom~ 134 K and becomes zero at ~ 133 K. The XRD pattern of the as-synthesized sample was very similar to that of the post-oxygen-annealed at 300°C for 6 h sample, the latter is shown in Fig. 3. Almost all of the XRD peaks in both samples could be indexed to the Hg-1223 phase having a tetragonal unit cell ( P 4 / m m m ) with lattice parameters of a = 3.85 A and c= 15.85 A. As observed from the SEM image given in Fig. 4, the bulk compacts were plate-like crystals. The resuits of quantitative elemental analyses by EDX for these plate-like crystals were Hg : Ba : Ca : C u = 1 : 2.24 : 2.26 : 2.92, which were considered to correspond to the Hg- 1223 phase [ 5 ].
4. Summary
Fig. 4. Scanning electron micrographs for the surface of an assynthesized sample of HgBa2CazCu308+~ which was heated at 750"C for 8 h. (a) 1000 times and (b) 3000 times magnification.
The Zv of the sample shows ~ 80% of - 1 / 4 n in 10 Oe, where Xm= 10.5 × 10- 3 emu/g at 120 K. The details will be given elsewhere.
While preparing the single-phase Hg-1223 compound, in order to ensure the formation of the desired phase, one must [ 11,12 ] (a) prevent the loss of Hg lest it should destroy the starting stoichiometric composition of HgBazCa2Cu3Os+6; (b) avoid the formation of the CaHgO2 compound; (c) manage to form the lower member of the Hg-based homologous series during the early period of the reaction. It is helpful to the formation of higher members of the Hgbased homologous series. Then, high-quality and single-phase samples of the
H.M. Shao et al. / Physica C 232 (1994) 5-9
Hg-1223 compound can be successfully prepared by the solid state reaction and short-time annealing technique using mixtures of the metal-oxide powders HgO, BaO, CaO and CuO with the nominal composition of HgBa2CaECU30~+6. The lattice parameters of a=3.85 /k and c= 15.85 A were determined by XRD measurements. The as-synthesized sample shows the superconducting transition temperature T¢= l 18 K and the oxygen-annealed sample T¢= 133 K.
Acknowledgements This work was supported by the Chinese National Center for R & D Superconductivity. We are grateful to X.C. Jin for kind help.
References [ 1] S.N. Putilin, E.V. Antipov, O. Chmaissen and M. Marezio, Nature (London) 362 ( 1993 ) 226.
9
[2] A. Schilling, M. Cantoni, J.D. Guo and H.R. Ott, Nature (London) 363 (1993) 56. [3] S.N. Putilin, E.V. Antipov and M. Marezio, Physica C 212 (1993) 266. [4 ] M. Itoh, A. Tokiwa-Yamamoto, S. Adachi and H. Yamauchi, Physica C 212 (1993) 271. [5 ] A. Tokiwa-Yamamoto, K. Isawa, M. Itoh, S. Adachi and H. Yamauchi, Physica C 216 (1993) 250. [6 ] M. Hirabayshi, K. Tokiwa, M. Tokumoto and H. Ihara, Jpn. J. Appl. Phys. 32 (1993)L 1206. [7] Z.J. Huang, R.L Meng, X.D. Qiu, Y.Y. Sun, J. Kulik, Y.Y. Xue and C.W. Chu, Physica C 217 ( 1993 ) 1. [ 8 ] K. Isawa, A. Tokiwa-Yamamoto, M. Itoh, S. Akachi and H. Yamauchi, Physiea C 217 (1993) 11. [9] M. Hirabayashi, K. Tokiwa, H. Ozawa, Y. Noguchi, M. Tokumoto and H. Ihara, Physica C 219 ( 1993 ) 6. [ 10] E.V. Antipov, S.M. Loureiro, C. Chaillout, J.J. Capponi, P. Bordet, J.L. Tholence, S.N. Putilin and M. Marezio, Physica C215 (1993) 1. [ 11 ] R.L. Meng, L. Beauvais, X.N. Zhang, Z.J. Huang, Y.Y. Sun, Y.Y. Xue and C.W. Chu, Physica C 216 (1993) 21. [ 12 ] S.M. Loureiro, E.V. Antipov, J.L. Tholence, J.J. Capponi, O. Chmaissem, Q. Huang and M. Marezio, Physica C 217 (1993) 253.
ELSEVIER
Synthesis of single-phase HgBa2Ca2Cu308 superconductor H.M. Shao a, L.J. Shen b, J.C. Shen a, X.Y. Hua b, P.F. Yuan b, X.X. Yao a Department of Physics, National Laboratory of Solid State Microstructure, Nanjing University, Nanjing 210008, China b Department of Physics, Nanjing Institute of Chemical Technology, Nanjing 210009, China
Received 3 May 1994; revised manuscript received 8 August 1994
Abstract
The third member (Hg-1223) of the recently discovered homologous series HgBa2Ca,_ i CunO2n+2+ti, having a Tc of 118 K (as-synthesized sample) and 133 K (oxygen-annealed sample), was successfully prepared by the solid state reaction and shorttime annealing technique using mixtures of the metal oxides HgO, BaO, CaO and CuO with starting composition HgBa2Ca2CuaOs+6. We found that the combination of very short annealing times with optimum temperatures 715-725"C gives rise to a very effective sintering of the constituents and avoids an excessive loss of Hg. Samples so obtained display a sharp superconducting transition determined both magnetically and resistively. The X-ray diffraction patterns of both the 118 K phase and the 133 K phase indicate a nearly single phase, corresponding to the tetragonal structure of space group P4/mmm and with lattice parameters a = 3.85 A and c = 15.85 A. The resistivity p of such a sample was very low, ~ 12 × 10- 3 f~ cm at 300 K, and the p versus T curve was linearly extrapolated to zero resistance at 0 K, being similar to the cases of high-quality high-To cuprates. Energy dispersive X-ray analysis (EDX) data of several grains of the Hg-1223 sample are in agreement with the proposed chemical formula.
1. Introduction
The recent discovery of high-temperature superconductivity in the HgBa2Can_lCunO2n+2+~ Hgbased homologous series o f layered Cu-mixed oxides, namely Hg- 12 (n - 1 ) n ( n = 1, 2, 3 ), with superconducting transition temperatures Tc = 96, 120 and 134 K, generates considerable interest, not only because this system set a new record T~, but also because it provides us with a new family o f compounds for studying the superconducting properties and mechanism [ 1-7]. However, the difficulties in preparing single-phase and high-density samples are some o f the obstacles in studying mercury-based compounds [ 8 ]. Mercury and its c o m p o u n d s are among the most toxic inorganic materials. Safety procedures are extremely important in the preparation o f Hg-contain-
ing compounds. HgO, which is one o f the starting oxides used to prepare the high-T¢ superconducting mercury-cuprates, is highly volatile at temperatures above its melting point o f 500°C [8 ]. In preparing these high-To superconducting oxides, prevention o f mercury loss is the most important factor, not only for safety purposes, but also for maintaining the stoichiometry o f products to ensure the formation o f the desired phase. In order to prevent severe mercury loss during the reaction, usually there are two methods to follow: (a) Using a high-pressure synthesis technique [ 8,9 ]. The use o f high pressure, possibly, lowers the mercury oxide decomposition. It also leads to a decrease o f the stability o f CaHgO2, whose synthesis at the first stage o f the reaction inhibits the formation o f rig-based compounds [ 10]. (b) Using a completely sealed reaction system
0921-4534/94/$07.00 @ 1994 Elsevier Science B.V. All rights reserved SSD10921-4534 (94) 00532-X
6
H.M. Shao et al./Physica C 232 (1994) 5-9
[ 4,5 ]. In order to prevent the decomposition of HgO and the removal of Hg from the starting mixture in the synthesis of Hg-based compounds, samples have been wrapped in noble metal foils, such as Au, Ag and Pt foils. We have found that Au foil works better than other foils, in terms of preventing Hg loss, because it is very inert and slightly melts at the high reaction temperatures, forming a tight seal which prevents severe mercury loss. However, high-pressure techniques are not suitable for preparing a relatively large amount of samples. As previously reported [4,5 ], the encapsulation process was likely to be suitable for the synthesis of Hg-based superconductors
[8]. However, to date, no one has succeeded in obtaining single-phase samples of the Hg-1223 phase [8 ]. Even if high-purity samples can be prepared, their densities are very low. In this paper we report on the preparation conditions of high density and high purity of bulk samples of the Hg- 1223 superconductor in a completely sealed reaction system using the combination of very short annealing times with optimum annealing temperatures, which gives rise to a very effective sintering of the constituents and avoids an excessive loss of Hg.
and moisture [ 11 ]. Then, the mixtures were pressed into rectangular bars of 2 X 4 × 20 mm 3. Each bar wrapped in silYer foil was placed on a quartz tube placed in a preheated tube furnace at 715-725°C for 3-4 min. The short-time annealing process was repeated four times with thorough intermediate grinding of the powder. After the final step the sample was wrapped again with silver foil and then encapsulated in evacuated ( ~ 10 -2 Torr) quartz tubes. The encapsulated samples were then sintered at 750°C for 1-10 h and rapidly cooled to room temperature in about 3 min. Subsequently, some of the sintered samples were annealed at 300°C for 8 h in flowing oxygen or argon gas. The annealed samples were characterized by Xray diffraction with Cu K~ radiation, transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), as well as resistivity and susceptibility measurements. Electrical contacts for the resistivity measurement were formed using conductive silver paste on the surface of the annealed sample. The resistivity was measured by a standard four-probe method.
3. Results and discussion 2. Experimental The mixed powders were obtained by two different methods as follows: (a) First, a precursor with nominal composition Ba2Ca2Cu3Ox was prepared by mixing high-purity Ba(NO3)2, Ca(NO3)2"4H20, and Cu(NO3)z'3H20. The mixture of nitrates was initially heated at 600°C in air for 12 h, then ground and heated at 925°C in 02 flow for 72 h with three intermediate regrindings [ 10 ]. Then appropriate amounts of HgO were added to the precursor with starting composition HgBa2CazCu3Oa+a and completely mixed. (b) Using metal-oxide powders of HGO(99%), BAO(99%), CAO(99%) and CUO(99%) as starting materials, CaO and BaO were obtained by decomposing highly pure ( > 99.5%) CaCO3 and BaCO3 in a vacuum furnace at 950°C-1000°C for 6 and 24 h; the metal-oxide powders were weighted to a Hg- 1223 composition, and mixed thoroughly. Powder mixing as described above was done in a dry box to prevent the powders from absorbing CO2
If the mixtures of the metal oxides HgO, BaO, CaO and CuO with starting composition HgBazCa2Cu3Oa + a are sealed in evacuated quartz tubes and sintered without short-time heat treatment, because of the low melting point of HgO (only 500°C) and its liability to evaporate, the quartz tubes will explode during the sintering process even if there is a slight increase in the speed of temperature rising [2 ]. On the other hand, in the temperature range between 500-550°C, the Hg vapor cannot get into the mixture of metal oxides and form the lower member of Hgbased homologous series [ 11 ]. But in this temperature range the CaHgOz compound can easily be formed, which will inhibit the formation of rig-based compounds, thus the single-phase Hg-1223 compound of high quality cannot be obtained [ 1,2 ]. If the above mixtures of metal oxides get short-time heat treatments at 715-725°C for 3-4 rain, which process was repeated four times with thorough intermediate grindings of the powder, we have found that 3-4 rain heating at the optimum temperature of 715-
H.M. Shao et al. / Physica C 232 (1994) 5-9
725°C is sufficient for the formation of Hg- 1201 and Hg-1212 phases in a non-closed system, because once the mercury is stabilized in the structure of the lower member of the Hg-based homologous series, the vaporization of Hg becomes very low during the later reaction [ 11,12 ]. So when we sealed the intermediate product for a short time into the evacuated quartz tube to sinter, the speed of temperature rising need not to be limited. In fact, no explosion occurred with the quartz tubes that we used in our laboratory. Also, the high speed of temperature rising can shorten the time the intermediate product has to stay in the temperature range between 500-550°C, so that the CaHgO2 compound cannot easily be formed [ 11 ]. This is of vital importance to ensure the formation of the desired phase. We still cannot understand the miracle that although the short-time annealing process with the mixed powder obtained by method (a) as described above, which was the mixture of the precursor Ba2Ca2Cu3Ox and the oxide HgO, was also repeated four times under the same reaction conditions, but when the intermediate product so obtained was sealed into the evacuated quartz tube and sintered, we still did not obtain the superconductivity of the resulting products, which we will work on later. Hg-1223 samples prepared in a non-stoichiometric starting composition may form non-superconducting crystallites and this results in lower quality samples. So the quality and density of the samples can be improved significantly if, after the initial short-time annealing, the exact amount of HgO that was lost (usually about 5-10%) is added and the bars are reground, pressed again and reheated. By this method the density of the bars can be as high as 85% theoretically, so the quality and density of the samples can be increased by adding the exact amount of HgO that was lost during the initial heating, followed by regrinding, pressing again and reheating under the same conditions. The as-synthesized sample displays a sharp transition. The width of the resistive transition is ~ 1 K with a zero-resistance value of 118 K. Especially, for the sample p at 300 K was ~ 12 mf~ cm, and exhibited a linear T dependence in the temperature range 180-300 K. This linear p versus T curve can be extrapolated to zero resistance at 0 K, which has been
7
known as a characteristic property of high-quality superconductors [ 7,11 ]. After post-annealing in flowing oxygen at 300oc for 6 h, all the samples exhibited diamagnetism at ~ 133 K. The ac susceptibility (X' and Z") for the sample after oxygenation at 300°C for 6 h is shown in Fig. 1. The volume susceptibility ;to of the sample was determined from the measured mass susceptibility Zm by X v = X , , ~ o / ( 1 - D x ~ o ) , where p is the X-ray density (6.176 g / c m 3) and D is the demagnetization factor. [-
!
!
!
I
. . . . . . .
|
90
140 165 190 T(K) Fig. I. Temperature dependence of ac magnetic susceptibilities for ( © ) as-synthesized ( Tc= I 18 K) and ( • ) post-oxygenated at 300°C for 6 h (T¢= 133 K) samplesof HgBa2Ca2Cu3Os+a.
20.0
I15
.
,
16.0[
so
,
8.0 4.0
13o
180
T(K) Fig. 2. Resistivity curves as a function temperature for ( O ) assynthesized and ( • ) post-oxygenatedat 300°C for 6 h samples of HgBa2Ca2Cu3Og+6.The inset shows the region of the superconducting transition.
8
ll.M. Shao et al./Physica C 232 (1994) 5-9
I# 20
I
I
30
40
I
5O
I
I
6O
70
2e Fig. 3. X-raydiffractionpattern for an as-synthesizedsampleof HgBa2Ca2Cu3Oa+~. Impuritypeaks from BaCuO2are markedby ( • ) .
Fig. 2 shows the temperature dependences of electrical resistivity (p versus Tcurve) of the sample after oxygenation at 300°C for 6 h, p undergoes a superconducting transition with a/'co ~ 140 K, Tom~ 134 K and becomes zero at ~ 133 K. The XRD pattern of the as-synthesized sample was very similar to that of the post-oxygen-annealed at 300°C for 6 h sample, the latter is shown in Fig. 3. Almost all of the XRD peaks in both samples could be indexed to the Hg-1223 phase having a tetragonal unit cell ( P 4 / m m m ) with lattice parameters of a = 3.85 A and c= 15.85 A. As observed from the SEM image given in Fig. 4, the bulk compacts were plate-like crystals. The resuits of quantitative elemental analyses by EDX for these plate-like crystals were Hg : Ba : Ca : C u = 1 : 2.24 : 2.26 : 2.92, which were considered to correspond to the Hg- 1223 phase [ 5 ].
4. Summary
Fig. 4. Scanning electron micrographs for the surface of an assynthesized sample of HgBa2CazCu308+~ which was heated at 750"C for 8 h. (a) 1000 times and (b) 3000 times magnification.
The Zv of the sample shows ~ 80% of - 1 / 4 n in 10 Oe, where Xm= 10.5 × 10- 3 emu/g at 120 K. The details will be given elsewhere.
While preparing the single-phase Hg-1223 compound, in order to ensure the formation of the desired phase, one must [ 11,12 ] (a) prevent the loss of Hg lest it should destroy the starting stoichiometric composition of HgBazCa2Cu3Os+6; (b) avoid the formation of the CaHgO2 compound; (c) manage to form the lower member of the Hg-based homologous series during the early period of the reaction. It is helpful to the formation of higher members of the Hgbased homologous series. Then, high-quality and single-phase samples of the
H.M. Shao et al. / Physica C 232 (1994) 5-9
Hg-1223 compound can be successfully prepared by the solid state reaction and short-time annealing technique using mixtures of the metal-oxide powders HgO, BaO, CaO and CuO with the nominal composition of HgBa2CaECU30~+6. The lattice parameters of a=3.85 /k and c= 15.85 A were determined by XRD measurements. The as-synthesized sample shows the superconducting transition temperature T¢= l 18 K and the oxygen-annealed sample T¢= 133 K.
Acknowledgements This work was supported by the Chinese National Center for R & D Superconductivity. We are grateful to X.C. Jin for kind help.
References [ 1] S.N. Putilin, E.V. Antipov, O. Chmaissen and M. Marezio, Nature (London) 362 ( 1993 ) 226.
9
[2] A. Schilling, M. Cantoni, J.D. Guo and H.R. Ott, Nature (London) 363 (1993) 56. [3] S.N. Putilin, E.V. Antipov and M. Marezio, Physica C 212 (1993) 266. [4 ] M. Itoh, A. Tokiwa-Yamamoto, S. Adachi and H. Yamauchi, Physica C 212 (1993) 271. [5 ] A. Tokiwa-Yamamoto, K. Isawa, M. Itoh, S. Adachi and H. Yamauchi, Physica C 216 (1993) 250. [6 ] M. Hirabayshi, K. Tokiwa, M. Tokumoto and H. Ihara, Jpn. J. Appl. Phys. 32 (1993)L 1206. [7] Z.J. Huang, R.L Meng, X.D. Qiu, Y.Y. Sun, J. Kulik, Y.Y. Xue and C.W. Chu, Physica C 217 ( 1993 ) 1. [ 8 ] K. Isawa, A. Tokiwa-Yamamoto, M. Itoh, S. Akachi and H. Yamauchi, Physiea C 217 (1993) 11. [9] M. Hirabayashi, K. Tokiwa, H. Ozawa, Y. Noguchi, M. Tokumoto and H. Ihara, Physica C 219 ( 1993 ) 6. [ 10] E.V. Antipov, S.M. Loureiro, C. Chaillout, J.J. Capponi, P. Bordet, J.L. Tholence, S.N. Putilin and M. Marezio, Physica C215 (1993) 1. [ 11 ] R.L. Meng, L. Beauvais, X.N. Zhang, Z.J. Huang, Y.Y. Sun, Y.Y. Xue and C.W. Chu, Physica C 216 (1993) 21. [ 12 ] S.M. Loureiro, E.V. Antipov, J.L. Tholence, J.J. Capponi, O. Chmaissem, Q. Huang and M. Marezio, Physica C 217 (1993) 253.