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Synthesis of the Bi,Pb-2223 high-Tc superconductor through a novel oxide nitrate route
Physica C 403 (2004) 1–8 www.elsevier.com/locate/physc
Synthesis of the Bi,Pb-2223 high-Tc superconductor through a novel oxide nitrate route K. Gibson, P. Ziegler, H.-J. Meyer
*
Abteilung f€ ur Festk€orperchemie und Theoretische, Anorganische Chemie, Eberhard-Karls-Universit€at T€ubingen, Auf der Morgenstelle 18, D-72076 T€ubingen, Germany Received 31 July 2003; received in revised form 3 November 2003; accepted 21 November 2003
Abstract A novel route is presented for the synthesis of the high-Tc superconductor (Bi,Pb)2 Sr2 Ca2 Cu3 O10þd . Solid mixtures of metal nitrates or of metal oxides and nitrates are reacted with approximate 2:2:2:3 metal ion composition in a reactor (Staurohr) under NOx atmosphere to form the oxide nitrate precursor (M1 –Mn )2 O2 NO3 with M1 –Mn ¼ Bi, Pb, Sr, Ca, plus side phases. When heated in air, the oxide nitrate precursor reacts with side phases to yield a standard precursor powder, containing 2212, Ca2 PbO4 , and Ca0:85 CuO2 , that is being converted into 2223. A detailed analysis of the reaction pathway is presented for the preparation of 2223. All important reaction stages and phase compositions are analyzed by powder XRD. 2003 Elsevier B.V. All rights reserved. PACS: 74.72.Hs; 74.62.Bf Keywords: 2223; Precursor; Reactivity; NOx atmosphere; Oxide nitrate
1. Introduction The high-temperature superconductor (Bi,Pb)2 Sr2 Ca2 Cu3 O10þx with Tc ¼ 110 K [1] is one of the most relevant superconducting materials for applications [2]. The synthesis of a high-quality standard precursor and its conversion into superconducting 2223 has been strained for almost two
*
Corresponding author. Tel.: +49-7071-29-76226; fax: +497071-29-5702. E-mail address: [email protected] (H.-J. Meyer). URL: http://www.uni-tuebingen.de/AK-Meyer/.
decades with significant progress. In spite of numerous publications on precursor syntheses [2– 12], a reliable synthetic route for a high-quality standard precursor for 2223 materials is still not established because all classical syntheses possess specific deficiencies and disadvantages. Some of them are: (a) Costly pre-treatments of raw materials through spray-drying, freeze-drying, or spray-pyrolysis [2–12]. (b) Absence of a standard precursor material with a defined phase composition. (c) Incomplete transformation reactions of precursors, due to the presence of stable multinary
0921-4534/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2003.11.007
2
K. Gibson et al. / Physica C 403 (2004) 1–8
oxides that slow down or inhibit continuous reaction performances [13,14]. (d) Poor reproducibilities under equivalent reaction conditions. Small or insignificant differences of the raw materials cause different precursor properties that require individually optimized transformation conditions into 2223. Recently we discovered a family of oxide nitrate compounds with the general formula (M1 –Mn )2 O2 NO3 as a flexible host for cations M1 –Mn ¼ Bi, Pb, Sr, Ca, . . . that can serve as precursors for oxide materials. Crystal structures have been solved for the examples BiSrO2 NO3 and BiBaO2 NO3 [15]; isotypic indexing was presented for BiPbO2 NO3 [16], BiCaO2 NO3 [17], and some other multinary (M1 –Mn )2 O2 NO3 phases have been studied. (M1 –Mn )2 O2 NO3 compounds or solid solutions are formed even from less reactive (mixtures of) oxides under appropriate temperature and NOx conditions. They are stable up to temperatures as high as 800 C under moderate NOx pressures, and they can be easily decomposed into (multinary) oxides once they are exposed to air at elevated temperatures. The NOx pressure can control the reactivity in the Bi–Pb–Sr–Ca–Cu–O system through the formation of (M1 –Mn )2 O2 NO3 compounds [18]. Applications were attempted for the synthesis of Bi2 Sr2 CuO6þd (2201), Bi2 Sr2 CaCu2 O8þd (2212), and Bi2 Sr2 Ca2 Cu3 O10þd (2223) superconductors, having Tc values around 10, 85 and 110 K [1]. Recently we have described the synthesis of 2201 through a homogeneous BiSrO2 NO3 /CuO mixture [19]. The synthesis of Bi(Pb)-2212 via (Bi,(Pb,)Sr,Ca)2 O2 NO3 /CuO is subject of a different contribution [20]. In this paper we present the corresponding route for the preparation of Bi,Pb-2223, and a detailed analysis of the reaction pathway.
2. Experimental Mixtures with total masses between 1 and 1.5 g of solid Bi(NO3 )3 Æ 5H2 O (Merck, DAB 6), Pb(NO3 )2 (Merck, 99.5%), Sr(NO3 )2 (Merck, 99%), Ca(NO3 )2 Æ 4H2 O (Merck, p.a.), and Cu(NO3 )2 Æ
3H2 O (Merck, 99.5%) were ground in an agate mortar according to the stoichiometry Bi1:80 Pb0:33 Sr1:87 Ca2:0 Cu3:0 Ox . Each sample was transferred in a corundum beaker and placed into a silica tube (V 30 cm3 ) equipped with a pressure control valve (Staurohr). The samples were heated up to different temperatures, always treated with the same heating ramp, in order to monitor and to investigate every reaction step on course of the 2223 synthesis. Reacted materials were analyzed by powder X-ray diffraction (XRD) using germanium monochromated Cu-Ka1 radiation (STADI-P, STOE). Lattice parameters were refined for selected intermediate phases. A calibration curve is plotted for BiPbx Ca1x O2 NO3 in Fig. 2 from separately prepared samples with nominal compositions corresponding to x ¼ 0, 0.2, 0.4, 0.6, 0.8 and 1.0 in order to analyze solid state cation exchange reactions. A calibration curve for BiSrx Ca1x O2 NO3 is provided in [20]. Two intermediate precursor stages and the final product were characterized by powder XRD structure refinements using the program WinPLOTR [21]. Magnetic susceptibility measurements were performed in gelatin capsules with a SQUID magnetometer between 5 and 120 K. In a different set of reactions, mixtures of solid nitrates and oxides (e.g. Bi2 O3 , PbO, Sr(NO3 )2 , Ca(NO3 )2 Æ 4H2 O, and CuO) were used as starting materials with equal success, aimed to minimize NOx emissions.
3. The reaction pathway to 2223 A detailed reaction pathway is shown in the flow scheme in Fig. 1, presenting a complete listing of all crystalline phases at different reaction stages, as detected by powder XRD. It may be noted that this scheme represents the reaction pathway on slow heating, rather than a recipe for an effective synthesis, which is provided in Section 4. The synthesis of 2223 may be subdivided into three important stages, corresponding to the atmosphere changes from air, to NOx , to air again. All reactions performed under NOx are displayed
K. Gibson et al. / Physica C 403 (2004) 1–8
2h
Nitrate Mix
24 h 450 C
24 h 620 C
4h 830 C
250 C
BiPbxCa1-xO2NO3 Pb0,33Sr0,67(NO3)2 Sr(NO3)2 (little) Ca(NO3)2 (little) CuO 2
BiSrxCa1-xO2NO3 Sr(NO3)2 Ca2PbO4 Ca0,85CuO2 CuO 6
Precursor: 2212 Ca2PbO4 Ca0,85CuO2
24 h 450 C
BiONO3 Pb(NO3)2 Sr(NO3)2 Ca(NO3)2 Cu-NO3-OH
1
BiPbxCa1-xO2NO3 (Pb,Sr,Ca)(NO3)2 CuO
24 h 450 C
3
24 h 640 C
16 h 850 C
BiSrxCa1-xO2NO3 Sr(NO3)2 Ca2PbO4 Ca0.85CuO2 CuO 7
BiPbxCa1-xO2NO3 Ca0,33Sr0,67(NO3)2 Ca2PbO4 CuO
24 h 550 C
4
24 h 650 C
18 h
2212 (Pb) 2223
9
3
850 C
Nitrate Precursor: BiSrxCa1-xO2NO3 Ca2PbO4 Ca0.85CuO2 CuO 8
5
4h 820 C
68 h
2223 2212 (Pb)
10
BiCaO2NO3 Sr(NO3)2 Ca2PbO4 CuO
850 C
2223
11
12
Fig. 1. Reaction flow scheme for the synthesis of Bi,Pb-2223 on heating solid nitrates of Bi:Pb:Sr:Ca:Cu in 1.8:0.33:1.87:2.0:3.0 molar ratio. All listed compounds are identified from XRD. Reactions performed under NOx atmosphere are shown with gray background. Numbers are used to label all (intermediate) reaction products.
with gray background in Fig. 1. Each intermediate product mixture is numbered in Fig. 1 to serve as a reference in the following text.
powders after the first reaction stage (see product no. 1 in Fig. 1). 3.2
3.1 The reaction is initiated by heating the solid nitrate mixture in air from room temperature to 250 C within 1 h in an open system. During this procedure some nitrates melt with their crystal water to generate a homogeneous mixture of reactants. During the evaporation of water, some nitrates start to decompose: BiðNO3 Þ3 5H2 O ! BiONO3 H2 O þ 2HNO3 þ 3H2 O 4CuðNO3 Þ2 3H2 O ! CuðNO3 Þ2 3CuðOHÞ2
Samples were heated from 250 to 650 C in a closed reaction vessel (Staurohr), thereby generating an NOx atmosphere, during the reaction cascade 2–8 (see Fig. 1). Pressures in our reaction were released through a pressure valve once they exceeded 1 bar. After heating 24 h at 450 C, the oxide nitrate BiPbx Ca1x O2 NO3 , the double nitrate Pb0:33 Sr0:67 (NO3 )2 , and CuO are formed besides small residues of Sr(NO3 )2 and Ca(NO3 )2 (see product no. 2 in Fig. 1). BiONO3 H2 O þ 1 xCaðNO3 Þ2 þ xPbðNO3 Þ2 450 C;–NOx
!
BiPbx Ca1x O2 NO3 þ H2 O
þ 6HNO3 þ 6H2 O Only the three nitrates Pb(NO3 )2 , Sr(NO3 )2 , and Ca(NO3 )2 remain fully intact as finely divided
1=3PbðNO3 Þ2 þ 2=3SrðNO3 Þ2 450 C;–NOx
!
Pb0:33 Sr0:67 ðNO3 Þ2
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K. Gibson et al. / Physica C 403 (2004) 1–8
Table 1 Oxide nitrate phase, phase no., lattice parameters (in pm), and unit cell volumes (in 106 pm3 ) Phase
No.
Lattice parameters
V
BiPbx Ca1x O2 NO3 a BiPbx Ca1x O2 NO3 a BiCaO2 NO3 a BiSrx Ca1x O2 NO3 b BiSrx Ca1x O2 NO3 b BiSrx Ca1x O2 NO3 b
3 4 5 6 7 8
a ¼ b ¼ 394:8ð4Þ c ¼ 1423ð1Þ a ¼ b ¼ 395:1ð2Þ c ¼ 1421ð2Þ a ¼ b ¼ 397:9ð2Þ c ¼ 1401ð1Þ a ¼ 1413ð2Þ b ¼ 565:1ð4Þ c ¼ 573:8ð4Þ a ¼ 1439:8ð6Þ b ¼ 566:0ð2Þ c ¼ 580:2ð2Þ a ¼ 1444ð2Þ b ¼ 566:6ð3Þ c ¼ 580:3ð8Þ
221.8(8) 220.0(4) 221.8(3) 458.1(9) 472.8(4) 474.8(8)
a b
Tetragonal, I4/mmm, Z ¼ 2. Orthorhombic, Cmmm, Z ¼ 4.
450 C
CuðNO3 Þ2 3CuðOHÞ2 ! 4CuO þ 2HNO3 þ 2H2 O
BiCaO2 NO3 þ xSrðNO3 Þ2 þ yCuO 550 C–650 C;–NOx
!
BiSrx Ca1x O2 NO3 þ yCa0:85 CuO2
The strontium uptake into tetragonal BiCaO2 NO3 introduces an orthorhombic distortion for BiSrx Ca1x O2 NO3 . This is indicated by the splitting of the tetragonal (1 1 0) XRD reflection into orthorhombic (0 0 2) and (0 2 0) reflections in Fig. 3. In addition, as a result of the bigger ionic radius of strontium versus calcium, the lattice parameters of BiSrx Ca1x O2 NO3 increase with x (Table 1), as
a... (Pb,Sr,Ca)(NO3)2 o b... Ca2PbO4 c... CuO d... Ca0.85CuO2 o a
1485 1480 1475 1470 1465 1460 1455 1450 1445 1440 1435 1430 1425 1420 1415 1410
Intensity
Lattice constant c/pm
After another 24 h at 450 C, some nitrates combine to form a (Pb,Sr,Ca)(NO3 )2 solid solution as part of a fairly simple phase composition represented by product no. 3 in Fig. 1. The lead content of BiPbx Ca1x O2 NO3 remains nearly constant during this stage, as estimated from calculated c lattice parameters listed in Table 1 and the calibration curve displayed in Fig. 2. The (Pb,Sr,Ca)(NO3 )2 solid solution starts to decompose into Ca0:33 Sr0:67 (NO3 )2 and Ca2 PbO4 if heating at 450 C is prolonged (see product no. 4). Lead is also released from BiPbx Ca1x O2 NO3 while some calcium is taken up until BiCaO2 NO3 is present after heating 24 h at 550 C (Table 1). Sr(NO3 )2 remains the only stable nitrate at 550 C (see product no. 6). Its decomposition begins
around 620 C when strontium is incorporated into BiCaO2 NO3 to yield BiSrx Ca1x O2 NO3 , while calcium is continuously driven out of BiSrx Ca1x O2 NO3 to form Ca0:85 CuO2 through reaction with CuO at 640 C (product no. 7).
a
o... BiMO2NO3
b b
o bo b
o
a
o
o
d
c
c d
c
c
c
o a
o
a
b
o a
25
26
c
a 27
28
c 29
30
31
32
33
34
35
36
2θ 0.0
0.2
0.4
0.6
0.8
1.0
x in BiPbxCa1-xO2NO3
Fig. 2. Calibration curve for BiPbx Ca1x O2 NO3 based on lattice parameters of prepared samples with nominal compositions of x ¼ 0, 0.2, 0.4, 0.6, 0.8, and 1.0, refined from XRD data.
Fig. 3. Powder XRD patterns of the reaction progress along tetragonal BiPbx Ca1x O2 NO3 produced at 450 C, tetragonal BiCaO2 NO3 at 550 C, and orthorhombic BiSrx Ca1x O2 NO3 at 620 and 640 C, respectively (products no. 3, 5–7, from bottom to top).
K. Gibson et al. / Physica C 403 (2004) 1–8
indicated by the shift of XRD reflections of this phase in the powder patterns (Fig. 3). Thereby, the unit cell volume increases from 2 222 ¼ 444 106 pm3 for x ¼ 0 at 550 C to 475 106 pm3 for x 0:9 at 650 C (Table 1). The value of x is estimated from [20]. A powder XRD pattern analysis on the nitrate precursor (product no. 8, further annealed in a silica tube at 680 C) is shown in Fig. 4. The nitrate precursor contains BiSrx Ca1x O2 NO3 besides small amounts of Ca2 PbO4 and Ca0:85 CuO2 /CuO. After the nitrate precursor is heated for 4 h at 820 C, the reaction vessel (Staurohr) is opened and the NOx is released to air. 3.3 The precursor is formed after 4 h heating at 830 C in air and initiates the final reaction cascade (9–12) towards the 2223 formation. The precursor contains 2212, besides Ca0:85 CuO2 and small amounts of Ca2 PbO4 , according to the XRD pattern fit shown in Fig. 5. The reaction progress on the precursor conversion into 2223 is displayed by the XRD patterns in Fig. 6. The reaction of the precursor is initiated
5
at 850 C when lead is incorporated into 2212 after 16 h (product no. 10). The distortion from the lead-free tetragonal 2212 structure towards the lead containing orthorhombic structure is indicated by the appearance of distinct (0 2 0) and (0 0 2) reflections slightly above 2H ¼ 33 in Fig. 6. After another 18 h at 850 C 2223 is already formed besides 2212 (product no. 11). The cuprate phases Ca0:85 CuO2 and CuO seem to be X-ray amorphous during these reaction stages. Finally, the amount of 2212 in the sample decreases until it is fully converted into 2223. The powder XRD pattern fit for Bi, Pb-2223 is shown in Fig. 7 (product no. 12, pellet, 16 h annealed). A Rietveld calculation reveals a yield of 94.0(6)% Bi,Pb-2223 and 6.0(2)% 2212. The magnetic susceptibility measurement of Bi,Pb-2223 shows a transition temperature of 108 K and confirms 2212 impurities in the zero field cooled curve of Fig. 8.
4. An effective precursor synthesis for Bi,Pb-2223 An effective conversion of a 1 g sample of mixed metal nitrates was performed as follows: The
Fig. 4. Powder XRD pattern of the nitrate precursor annealed at 650 C, assigned for BiSrx Ca1x O2 NO3 , Ca2 PbO4 , and Ca0:85 CuO2 , with structure refinement fit and difference curve (product no. 8 in Fig. 1).
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K. Gibson et al. / Physica C 403 (2004) 1–8
Fig. 5. Powder XRD pattern of the nitrate-free precursor containing Bi-2212, Ca2 PbO4 , and Ca0:85 CuO2 , with structure refinement fit and difference curve (product no. 9 in Fig. 1).
a... 2223 b... 2212
c... Ca2 PbO4 d... Ca0.85 CuO2 a
a a a
a a a
Intensity
a a b
b
a
b
a a b
b
b ab
a
a
b
b
b
b
b b
25
b 26
27
28
29
5. Summary and conclusion c
30
31
h, and annealed at 650 C for another 4 h before the temperature was raised to 820 C within 1 h. After 2 h at 820 C the reaction vessel was opened, allowing NOx continuously to escape while the sample was heated from 820 C to 830 C (within 20 min). The obtained precursor was finally annealed at 830 C for 4 h in air.
c
c
32
d 33
b 34
2θ
Fig. 6. Reaction progress of the conversion of the precursor into Bi,Pb-2223 recorded by XRD (from bottom to top) on annealing between 830 and 850 C (products no. 9–12). Important peaks are assigned for 2223, 2212, Ca2 PbO4 , and Ca0:85 CuO2 .
mixture of solid nitrates was homogenized in a corundum beaker through the formation of a melt and solidification while being heated from room temperature to 250 C within 1 h in air. Afterwards the beaker with the sample was locked into the Staurohr, heated from 250 to 650 C within 4
A new synthesis is described for Bi,Pb-2223. The detailed reaction pathway is given through an exemplary study with extended reaction times and crystallizations of intermediate products. The role of oxide nitrates with the general formula (M1 – Mn )2 O2 NO3 as a variable host and as a precursor deserves special attention. The phase BiPbx Ca1x O2 NO3 is already formed in the initial stage of the reaction under NOx atmosphere. Through the uptake of calcium and release of lead, BiCaO2 NO3 is formed afterwards. Then, strontium is taken up and calcium is partially released under the formation of BiSrx Ca1x O2 NO3 . These reactions demonstrate the capability of compounds (M1 – Mn )2 O2 NO3 to incorporate metal ions from ni-
K. Gibson et al. / Physica C 403 (2004) 1–8
7
Fig. 7. Powder XRD pattern of an annealed Bi,Pb-2223 sample (product no. 12 in Fig. 1) with structure refinement fit and difference curve.
the in situ generated NOx atmosphere. Third, the precursor (2212, Ca2 PbO4 , Ca0:85 CuO2 /CuO) is formed under air, and being converted into 2223. A similar reaction mechanism can be performed in course of the synthesis of all superconducting compounds in the Bi–Pb–Sr–Ca–Cu–O system. These reactions may be rationalized through the following nitrate precursor stages, whereas for 2223 the nitrate-free precursor is important, too:
Magnetic susceptibility/cm3·g-1
0.000
-0.001
-0.002
FC -0.003
-0.004
ZFC -0.005 10
20
30
40
50
60
70
80
90
100 110 120
Temperature /K
Fig. 8. Magnetic susceptibility versus temperature (FC ¼ field cooled, ZFC ¼ zero field cooled) for Bi,Pb-2223 after annealing at 850 C in air (product no. 12 in Fig. 1, H ¼ 20 Oe).
(1) 2BiSrO2 NO3 + CuO fi Bi2 Sr2 CuO6þd (2201) [19], (2) 2.5(Bi,Pb,Sr,Ca) 2 O2 NO3 + 2CuO fi (Bi,Pb)2 Sr2 CaCu2 O8þd (2212) [20], (3) 3ðBi; Pb; Sr; CaÞ2 O2 NO3 þ 3CuO Nitrate Precursor
trates and thereby release others to form desired oxides, such as Ca2 PbO4 and Ca0:85 CuO2 , before the layered compound BiSrx Ca1x O2 NO3 itself is converted into 2212, through controlled discharge of the NOx atmosphere to air. The complete synthesis may depart from solid nitrates (or oxides and nitrates), treated through three reaction stages. During the first heating stage some nitrate hydrates melt and decompose. Second, intermediate solid solutions of (Bi,Pb,Sr,Ca)2 O2 NO3 control the reactivity of the system through
! 2212; Ca2 PbO4 ; Ca0:85 CuO2 =CuO Precursor
! ðBi; PbÞ2 Sr2 Ca2 Cu3 O10þd
(2223).
The NOx atmosphere plays a crucial role for the stabilization of oxide nitrates in the reaction. It has been shown that an overall pressure up to slightly above 1 bar in the Staurohr is sufficient for the reaction to stabilize oxide nitrates and to avoid the formation of numerous oxides of which some are undesired. The reactivity of the Bi–Pb–Sr–Ca–Cu–O
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K. Gibson et al. / Physica C 403 (2004) 1–8
system can be fully controlled, due do the decent number of compounds or phases that can occur under an appropriate NOx atmosphere. Lower NOx pressures produce undesired oxides (Bi1:4 Sr2x Ca0:6þx O6d [14]), especially strontiumrich phases (Sr14x Cax Cu24 O41 and Bix Pb3x Sr3 Ca2 CuO12y ), as in classical reactions performed in air. It has been demonstrated many times that reactions performed only in air cause many different compounds, and that their parallel reactions with each other sentence any reactivity control to become an extremely difficult task. NOx pressures much higher than 1 bar stabilize nitrates, such as Sr(NO3 )2 , up to higher heating temperatures and thereby slow down the reactivity. We may note that long annealing periods and crystallizations of intermediate products used in this study were aimed to fully describe reaction pathways and intermediate products. Certainly, the reaction can be performed much faster and more efficient.
References [1] H. Maeda, Y. Tanaka, M. Fukutomi, T. Asano, Jpn. J. Appl. Phys. 27 (1988) 209. [2] N.V. Coppa, W.L. Hults, J.L. Smith, J. Brynestad, J. Mater. Res. 9 (1994) 2510. [3] Y.-W. Hsueh, S.C. Chang, R.S. Liu, L. Woodall, M. Gerards, Mater. Res. Bull. 36 (2001) 1653. [4] P. Badica, G. Aldica, S. Madache, Supercond. Sci. Technol. 12 (1999) 162.
[5] O.A. Shlyakhtin, A.L. Vinokurov, A.N. Tretyakov, D. Yu, J. Supercond. 11 (1998) 507. [6] M.G. Smith, J.O. Willis, D.E. Peterson, J.F. Bingert, D.S. Phillips, J.Y. Coulter, K.V. Salazar, W.L. Hults, Physica C 231 (1994) 409. [7] E. Giannini, I. Savysyuk, V. Garnier, R. Passerini, P. Toulemonde, R. Fl€ ukiger, Supercond. Sci. Technol. 15 (2002) 1577. [8] I. Van Driessche, R. Mouton, S. Hoste, Mater. Res. Bull. 31 (1996) 979. [9] B. Sailer, F. Schwaigerer, K. Gibson, H.-J. Meyer, M. Lehmann, L. Woodall, M. Gerards, Trans. Appl. Supercond. 11 (2001) 2975. [10] Q. Xu, L. Bi, D. Peng, G. Meng, G. Zhou, Z. Mao, C. Fan, Y. Zhang, Supercond. Sci. Technol. 3 (1990) 564. [11] J.K.F. Yau, Y.L. Wong, Physica C 339 (2000) 79. [12] A. Jeremie, K. Alami-Yadri, J.-C. Grivel, R. Fl€ ukiger, Supercond. Sci. Techol. 6 (1993) 730. [13] K. Gibson, S. Dill, V. Cauniene, B. Sailer, H.-J. Meyer, M. Lehmann, L. Woodall, M. Gerards, Physica C 372–376 (2002) 995. [14] C.C. Luhrs, M. Morales, F. Sapi~ na, D. Beltran-Porter, A. Fuertes, Solid State Ionic 101–103 (1997) 1107. [15] P. Ziegler, I. Grigoraviciute, K. Gibson, J. Glaser, A. Kareiva, H.-J. Meyer (in preparation). [16] H. Kodama, Special Publication-Royal Society of Chemistry 196 (1997) 39. [17] H. Kodama, Japanese Patent 2,000,086,243, Science and Technology Agency (1998). [18] P. Ziegler, K. Gibson, H.-J. Meyer, German Patent, Patentamt M€ unchen (2003). [19] K. Gibson, P. Ziegler, H.-J. Meyer, Physica C 397 (2003) 112. [20] K. Gibson, P. Ziegler, H.-J. Meyer (in preparation). [21] T. Roisnel, J. Rodrıguez-Carvajal, WinPLOTR: Windows tool for powder diffraction patterns analysis, Materials Science Forum, in: R. Delhez, E.J. Mittenmeijer (Eds.), Proceedings of the Seventh European Powder Diffraction Conference (EPDIC 7), 2000, 118.
Synthesis of the Bi,Pb-2223 high-Tc superconductor through a novel oxide nitrate route K. Gibson, P. Ziegler, H.-J. Meyer
*
Abteilung f€ ur Festk€orperchemie und Theoretische, Anorganische Chemie, Eberhard-Karls-Universit€at T€ubingen, Auf der Morgenstelle 18, D-72076 T€ubingen, Germany Received 31 July 2003; received in revised form 3 November 2003; accepted 21 November 2003
Abstract A novel route is presented for the synthesis of the high-Tc superconductor (Bi,Pb)2 Sr2 Ca2 Cu3 O10þd . Solid mixtures of metal nitrates or of metal oxides and nitrates are reacted with approximate 2:2:2:3 metal ion composition in a reactor (Staurohr) under NOx atmosphere to form the oxide nitrate precursor (M1 –Mn )2 O2 NO3 with M1 –Mn ¼ Bi, Pb, Sr, Ca, plus side phases. When heated in air, the oxide nitrate precursor reacts with side phases to yield a standard precursor powder, containing 2212, Ca2 PbO4 , and Ca0:85 CuO2 , that is being converted into 2223. A detailed analysis of the reaction pathway is presented for the preparation of 2223. All important reaction stages and phase compositions are analyzed by powder XRD. 2003 Elsevier B.V. All rights reserved. PACS: 74.72.Hs; 74.62.Bf Keywords: 2223; Precursor; Reactivity; NOx atmosphere; Oxide nitrate
1. Introduction The high-temperature superconductor (Bi,Pb)2 Sr2 Ca2 Cu3 O10þx with Tc ¼ 110 K [1] is one of the most relevant superconducting materials for applications [2]. The synthesis of a high-quality standard precursor and its conversion into superconducting 2223 has been strained for almost two
*
Corresponding author. Tel.: +49-7071-29-76226; fax: +497071-29-5702. E-mail address: [email protected] (H.-J. Meyer). URL: http://www.uni-tuebingen.de/AK-Meyer/.
decades with significant progress. In spite of numerous publications on precursor syntheses [2– 12], a reliable synthetic route for a high-quality standard precursor for 2223 materials is still not established because all classical syntheses possess specific deficiencies and disadvantages. Some of them are: (a) Costly pre-treatments of raw materials through spray-drying, freeze-drying, or spray-pyrolysis [2–12]. (b) Absence of a standard precursor material with a defined phase composition. (c) Incomplete transformation reactions of precursors, due to the presence of stable multinary
0921-4534/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2003.11.007
2
K. Gibson et al. / Physica C 403 (2004) 1–8
oxides that slow down or inhibit continuous reaction performances [13,14]. (d) Poor reproducibilities under equivalent reaction conditions. Small or insignificant differences of the raw materials cause different precursor properties that require individually optimized transformation conditions into 2223. Recently we discovered a family of oxide nitrate compounds with the general formula (M1 –Mn )2 O2 NO3 as a flexible host for cations M1 –Mn ¼ Bi, Pb, Sr, Ca, . . . that can serve as precursors for oxide materials. Crystal structures have been solved for the examples BiSrO2 NO3 and BiBaO2 NO3 [15]; isotypic indexing was presented for BiPbO2 NO3 [16], BiCaO2 NO3 [17], and some other multinary (M1 –Mn )2 O2 NO3 phases have been studied. (M1 –Mn )2 O2 NO3 compounds or solid solutions are formed even from less reactive (mixtures of) oxides under appropriate temperature and NOx conditions. They are stable up to temperatures as high as 800 C under moderate NOx pressures, and they can be easily decomposed into (multinary) oxides once they are exposed to air at elevated temperatures. The NOx pressure can control the reactivity in the Bi–Pb–Sr–Ca–Cu–O system through the formation of (M1 –Mn )2 O2 NO3 compounds [18]. Applications were attempted for the synthesis of Bi2 Sr2 CuO6þd (2201), Bi2 Sr2 CaCu2 O8þd (2212), and Bi2 Sr2 Ca2 Cu3 O10þd (2223) superconductors, having Tc values around 10, 85 and 110 K [1]. Recently we have described the synthesis of 2201 through a homogeneous BiSrO2 NO3 /CuO mixture [19]. The synthesis of Bi(Pb)-2212 via (Bi,(Pb,)Sr,Ca)2 O2 NO3 /CuO is subject of a different contribution [20]. In this paper we present the corresponding route for the preparation of Bi,Pb-2223, and a detailed analysis of the reaction pathway.
2. Experimental Mixtures with total masses between 1 and 1.5 g of solid Bi(NO3 )3 Æ 5H2 O (Merck, DAB 6), Pb(NO3 )2 (Merck, 99.5%), Sr(NO3 )2 (Merck, 99%), Ca(NO3 )2 Æ 4H2 O (Merck, p.a.), and Cu(NO3 )2 Æ
3H2 O (Merck, 99.5%) were ground in an agate mortar according to the stoichiometry Bi1:80 Pb0:33 Sr1:87 Ca2:0 Cu3:0 Ox . Each sample was transferred in a corundum beaker and placed into a silica tube (V 30 cm3 ) equipped with a pressure control valve (Staurohr). The samples were heated up to different temperatures, always treated with the same heating ramp, in order to monitor and to investigate every reaction step on course of the 2223 synthesis. Reacted materials were analyzed by powder X-ray diffraction (XRD) using germanium monochromated Cu-Ka1 radiation (STADI-P, STOE). Lattice parameters were refined for selected intermediate phases. A calibration curve is plotted for BiPbx Ca1x O2 NO3 in Fig. 2 from separately prepared samples with nominal compositions corresponding to x ¼ 0, 0.2, 0.4, 0.6, 0.8 and 1.0 in order to analyze solid state cation exchange reactions. A calibration curve for BiSrx Ca1x O2 NO3 is provided in [20]. Two intermediate precursor stages and the final product were characterized by powder XRD structure refinements using the program WinPLOTR [21]. Magnetic susceptibility measurements were performed in gelatin capsules with a SQUID magnetometer between 5 and 120 K. In a different set of reactions, mixtures of solid nitrates and oxides (e.g. Bi2 O3 , PbO, Sr(NO3 )2 , Ca(NO3 )2 Æ 4H2 O, and CuO) were used as starting materials with equal success, aimed to minimize NOx emissions.
3. The reaction pathway to 2223 A detailed reaction pathway is shown in the flow scheme in Fig. 1, presenting a complete listing of all crystalline phases at different reaction stages, as detected by powder XRD. It may be noted that this scheme represents the reaction pathway on slow heating, rather than a recipe for an effective synthesis, which is provided in Section 4. The synthesis of 2223 may be subdivided into three important stages, corresponding to the atmosphere changes from air, to NOx , to air again. All reactions performed under NOx are displayed
K. Gibson et al. / Physica C 403 (2004) 1–8
2h
Nitrate Mix
24 h 450 C
24 h 620 C
4h 830 C
250 C
BiPbxCa1-xO2NO3 Pb0,33Sr0,67(NO3)2 Sr(NO3)2 (little) Ca(NO3)2 (little) CuO 2
BiSrxCa1-xO2NO3 Sr(NO3)2 Ca2PbO4 Ca0,85CuO2 CuO 6
Precursor: 2212 Ca2PbO4 Ca0,85CuO2
24 h 450 C
BiONO3 Pb(NO3)2 Sr(NO3)2 Ca(NO3)2 Cu-NO3-OH
1
BiPbxCa1-xO2NO3 (Pb,Sr,Ca)(NO3)2 CuO
24 h 450 C
3
24 h 640 C
16 h 850 C
BiSrxCa1-xO2NO3 Sr(NO3)2 Ca2PbO4 Ca0.85CuO2 CuO 7
BiPbxCa1-xO2NO3 Ca0,33Sr0,67(NO3)2 Ca2PbO4 CuO
24 h 550 C
4
24 h 650 C
18 h
2212 (Pb) 2223
9
3
850 C
Nitrate Precursor: BiSrxCa1-xO2NO3 Ca2PbO4 Ca0.85CuO2 CuO 8
5
4h 820 C
68 h
2223 2212 (Pb)
10
BiCaO2NO3 Sr(NO3)2 Ca2PbO4 CuO
850 C
2223
11
12
Fig. 1. Reaction flow scheme for the synthesis of Bi,Pb-2223 on heating solid nitrates of Bi:Pb:Sr:Ca:Cu in 1.8:0.33:1.87:2.0:3.0 molar ratio. All listed compounds are identified from XRD. Reactions performed under NOx atmosphere are shown with gray background. Numbers are used to label all (intermediate) reaction products.
with gray background in Fig. 1. Each intermediate product mixture is numbered in Fig. 1 to serve as a reference in the following text.
powders after the first reaction stage (see product no. 1 in Fig. 1). 3.2
3.1 The reaction is initiated by heating the solid nitrate mixture in air from room temperature to 250 C within 1 h in an open system. During this procedure some nitrates melt with their crystal water to generate a homogeneous mixture of reactants. During the evaporation of water, some nitrates start to decompose: BiðNO3 Þ3 5H2 O ! BiONO3 H2 O þ 2HNO3 þ 3H2 O 4CuðNO3 Þ2 3H2 O ! CuðNO3 Þ2 3CuðOHÞ2
Samples were heated from 250 to 650 C in a closed reaction vessel (Staurohr), thereby generating an NOx atmosphere, during the reaction cascade 2–8 (see Fig. 1). Pressures in our reaction were released through a pressure valve once they exceeded 1 bar. After heating 24 h at 450 C, the oxide nitrate BiPbx Ca1x O2 NO3 , the double nitrate Pb0:33 Sr0:67 (NO3 )2 , and CuO are formed besides small residues of Sr(NO3 )2 and Ca(NO3 )2 (see product no. 2 in Fig. 1). BiONO3 H2 O þ 1 xCaðNO3 Þ2 þ xPbðNO3 Þ2 450 C;–NOx
!
BiPbx Ca1x O2 NO3 þ H2 O
þ 6HNO3 þ 6H2 O Only the three nitrates Pb(NO3 )2 , Sr(NO3 )2 , and Ca(NO3 )2 remain fully intact as finely divided
1=3PbðNO3 Þ2 þ 2=3SrðNO3 Þ2 450 C;–NOx
!
Pb0:33 Sr0:67 ðNO3 Þ2
4
K. Gibson et al. / Physica C 403 (2004) 1–8
Table 1 Oxide nitrate phase, phase no., lattice parameters (in pm), and unit cell volumes (in 106 pm3 ) Phase
No.
Lattice parameters
V
BiPbx Ca1x O2 NO3 a BiPbx Ca1x O2 NO3 a BiCaO2 NO3 a BiSrx Ca1x O2 NO3 b BiSrx Ca1x O2 NO3 b BiSrx Ca1x O2 NO3 b
3 4 5 6 7 8
a ¼ b ¼ 394:8ð4Þ c ¼ 1423ð1Þ a ¼ b ¼ 395:1ð2Þ c ¼ 1421ð2Þ a ¼ b ¼ 397:9ð2Þ c ¼ 1401ð1Þ a ¼ 1413ð2Þ b ¼ 565:1ð4Þ c ¼ 573:8ð4Þ a ¼ 1439:8ð6Þ b ¼ 566:0ð2Þ c ¼ 580:2ð2Þ a ¼ 1444ð2Þ b ¼ 566:6ð3Þ c ¼ 580:3ð8Þ
221.8(8) 220.0(4) 221.8(3) 458.1(9) 472.8(4) 474.8(8)
a b
Tetragonal, I4/mmm, Z ¼ 2. Orthorhombic, Cmmm, Z ¼ 4.
450 C
CuðNO3 Þ2 3CuðOHÞ2 ! 4CuO þ 2HNO3 þ 2H2 O
BiCaO2 NO3 þ xSrðNO3 Þ2 þ yCuO 550 C–650 C;–NOx
!
BiSrx Ca1x O2 NO3 þ yCa0:85 CuO2
The strontium uptake into tetragonal BiCaO2 NO3 introduces an orthorhombic distortion for BiSrx Ca1x O2 NO3 . This is indicated by the splitting of the tetragonal (1 1 0) XRD reflection into orthorhombic (0 0 2) and (0 2 0) reflections in Fig. 3. In addition, as a result of the bigger ionic radius of strontium versus calcium, the lattice parameters of BiSrx Ca1x O2 NO3 increase with x (Table 1), as
a... (Pb,Sr,Ca)(NO3)2 o b... Ca2PbO4 c... CuO d... Ca0.85CuO2 o a
1485 1480 1475 1470 1465 1460 1455 1450 1445 1440 1435 1430 1425 1420 1415 1410
Intensity
Lattice constant c/pm
After another 24 h at 450 C, some nitrates combine to form a (Pb,Sr,Ca)(NO3 )2 solid solution as part of a fairly simple phase composition represented by product no. 3 in Fig. 1. The lead content of BiPbx Ca1x O2 NO3 remains nearly constant during this stage, as estimated from calculated c lattice parameters listed in Table 1 and the calibration curve displayed in Fig. 2. The (Pb,Sr,Ca)(NO3 )2 solid solution starts to decompose into Ca0:33 Sr0:67 (NO3 )2 and Ca2 PbO4 if heating at 450 C is prolonged (see product no. 4). Lead is also released from BiPbx Ca1x O2 NO3 while some calcium is taken up until BiCaO2 NO3 is present after heating 24 h at 550 C (Table 1). Sr(NO3 )2 remains the only stable nitrate at 550 C (see product no. 6). Its decomposition begins
around 620 C when strontium is incorporated into BiCaO2 NO3 to yield BiSrx Ca1x O2 NO3 , while calcium is continuously driven out of BiSrx Ca1x O2 NO3 to form Ca0:85 CuO2 through reaction with CuO at 640 C (product no. 7).
a
o... BiMO2NO3
b b
o bo b
o
a
o
o
d
c
c d
c
c
c
o a
o
a
b
o a
25
26
c
a 27
28
c 29
30
31
32
33
34
35
36
2θ 0.0
0.2
0.4
0.6
0.8
1.0
x in BiPbxCa1-xO2NO3
Fig. 2. Calibration curve for BiPbx Ca1x O2 NO3 based on lattice parameters of prepared samples with nominal compositions of x ¼ 0, 0.2, 0.4, 0.6, 0.8, and 1.0, refined from XRD data.
Fig. 3. Powder XRD patterns of the reaction progress along tetragonal BiPbx Ca1x O2 NO3 produced at 450 C, tetragonal BiCaO2 NO3 at 550 C, and orthorhombic BiSrx Ca1x O2 NO3 at 620 and 640 C, respectively (products no. 3, 5–7, from bottom to top).
K. Gibson et al. / Physica C 403 (2004) 1–8
indicated by the shift of XRD reflections of this phase in the powder patterns (Fig. 3). Thereby, the unit cell volume increases from 2 222 ¼ 444 106 pm3 for x ¼ 0 at 550 C to 475 106 pm3 for x 0:9 at 650 C (Table 1). The value of x is estimated from [20]. A powder XRD pattern analysis on the nitrate precursor (product no. 8, further annealed in a silica tube at 680 C) is shown in Fig. 4. The nitrate precursor contains BiSrx Ca1x O2 NO3 besides small amounts of Ca2 PbO4 and Ca0:85 CuO2 /CuO. After the nitrate precursor is heated for 4 h at 820 C, the reaction vessel (Staurohr) is opened and the NOx is released to air. 3.3 The precursor is formed after 4 h heating at 830 C in air and initiates the final reaction cascade (9–12) towards the 2223 formation. The precursor contains 2212, besides Ca0:85 CuO2 and small amounts of Ca2 PbO4 , according to the XRD pattern fit shown in Fig. 5. The reaction progress on the precursor conversion into 2223 is displayed by the XRD patterns in Fig. 6. The reaction of the precursor is initiated
5
at 850 C when lead is incorporated into 2212 after 16 h (product no. 10). The distortion from the lead-free tetragonal 2212 structure towards the lead containing orthorhombic structure is indicated by the appearance of distinct (0 2 0) and (0 0 2) reflections slightly above 2H ¼ 33 in Fig. 6. After another 18 h at 850 C 2223 is already formed besides 2212 (product no. 11). The cuprate phases Ca0:85 CuO2 and CuO seem to be X-ray amorphous during these reaction stages. Finally, the amount of 2212 in the sample decreases until it is fully converted into 2223. The powder XRD pattern fit for Bi, Pb-2223 is shown in Fig. 7 (product no. 12, pellet, 16 h annealed). A Rietveld calculation reveals a yield of 94.0(6)% Bi,Pb-2223 and 6.0(2)% 2212. The magnetic susceptibility measurement of Bi,Pb-2223 shows a transition temperature of 108 K and confirms 2212 impurities in the zero field cooled curve of Fig. 8.
4. An effective precursor synthesis for Bi,Pb-2223 An effective conversion of a 1 g sample of mixed metal nitrates was performed as follows: The
Fig. 4. Powder XRD pattern of the nitrate precursor annealed at 650 C, assigned for BiSrx Ca1x O2 NO3 , Ca2 PbO4 , and Ca0:85 CuO2 , with structure refinement fit and difference curve (product no. 8 in Fig. 1).
6
K. Gibson et al. / Physica C 403 (2004) 1–8
Fig. 5. Powder XRD pattern of the nitrate-free precursor containing Bi-2212, Ca2 PbO4 , and Ca0:85 CuO2 , with structure refinement fit and difference curve (product no. 9 in Fig. 1).
a... 2223 b... 2212
c... Ca2 PbO4 d... Ca0.85 CuO2 a
a a a
a a a
Intensity
a a b
b
a
b
a a b
b
b ab
a
a
b
b
b
b
b b
25
b 26
27
28
29
5. Summary and conclusion c
30
31
h, and annealed at 650 C for another 4 h before the temperature was raised to 820 C within 1 h. After 2 h at 820 C the reaction vessel was opened, allowing NOx continuously to escape while the sample was heated from 820 C to 830 C (within 20 min). The obtained precursor was finally annealed at 830 C for 4 h in air.
c
c
32
d 33
b 34
2θ
Fig. 6. Reaction progress of the conversion of the precursor into Bi,Pb-2223 recorded by XRD (from bottom to top) on annealing between 830 and 850 C (products no. 9–12). Important peaks are assigned for 2223, 2212, Ca2 PbO4 , and Ca0:85 CuO2 .
mixture of solid nitrates was homogenized in a corundum beaker through the formation of a melt and solidification while being heated from room temperature to 250 C within 1 h in air. Afterwards the beaker with the sample was locked into the Staurohr, heated from 250 to 650 C within 4
A new synthesis is described for Bi,Pb-2223. The detailed reaction pathway is given through an exemplary study with extended reaction times and crystallizations of intermediate products. The role of oxide nitrates with the general formula (M1 – Mn )2 O2 NO3 as a variable host and as a precursor deserves special attention. The phase BiPbx Ca1x O2 NO3 is already formed in the initial stage of the reaction under NOx atmosphere. Through the uptake of calcium and release of lead, BiCaO2 NO3 is formed afterwards. Then, strontium is taken up and calcium is partially released under the formation of BiSrx Ca1x O2 NO3 . These reactions demonstrate the capability of compounds (M1 – Mn )2 O2 NO3 to incorporate metal ions from ni-
K. Gibson et al. / Physica C 403 (2004) 1–8
7
Fig. 7. Powder XRD pattern of an annealed Bi,Pb-2223 sample (product no. 12 in Fig. 1) with structure refinement fit and difference curve.
the in situ generated NOx atmosphere. Third, the precursor (2212, Ca2 PbO4 , Ca0:85 CuO2 /CuO) is formed under air, and being converted into 2223. A similar reaction mechanism can be performed in course of the synthesis of all superconducting compounds in the Bi–Pb–Sr–Ca–Cu–O system. These reactions may be rationalized through the following nitrate precursor stages, whereas for 2223 the nitrate-free precursor is important, too:
Magnetic susceptibility/cm3·g-1
0.000
-0.001
-0.002
FC -0.003
-0.004
ZFC -0.005 10
20
30
40
50
60
70
80
90
100 110 120
Temperature /K
Fig. 8. Magnetic susceptibility versus temperature (FC ¼ field cooled, ZFC ¼ zero field cooled) for Bi,Pb-2223 after annealing at 850 C in air (product no. 12 in Fig. 1, H ¼ 20 Oe).
(1) 2BiSrO2 NO3 + CuO fi Bi2 Sr2 CuO6þd (2201) [19], (2) 2.5(Bi,Pb,Sr,Ca) 2 O2 NO3 + 2CuO fi (Bi,Pb)2 Sr2 CaCu2 O8þd (2212) [20], (3) 3ðBi; Pb; Sr; CaÞ2 O2 NO3 þ 3CuO Nitrate Precursor
trates and thereby release others to form desired oxides, such as Ca2 PbO4 and Ca0:85 CuO2 , before the layered compound BiSrx Ca1x O2 NO3 itself is converted into 2212, through controlled discharge of the NOx atmosphere to air. The complete synthesis may depart from solid nitrates (or oxides and nitrates), treated through three reaction stages. During the first heating stage some nitrate hydrates melt and decompose. Second, intermediate solid solutions of (Bi,Pb,Sr,Ca)2 O2 NO3 control the reactivity of the system through
! 2212; Ca2 PbO4 ; Ca0:85 CuO2 =CuO Precursor
! ðBi; PbÞ2 Sr2 Ca2 Cu3 O10þd
(2223).
The NOx atmosphere plays a crucial role for the stabilization of oxide nitrates in the reaction. It has been shown that an overall pressure up to slightly above 1 bar in the Staurohr is sufficient for the reaction to stabilize oxide nitrates and to avoid the formation of numerous oxides of which some are undesired. The reactivity of the Bi–Pb–Sr–Ca–Cu–O
8
K. Gibson et al. / Physica C 403 (2004) 1–8
system can be fully controlled, due do the decent number of compounds or phases that can occur under an appropriate NOx atmosphere. Lower NOx pressures produce undesired oxides (Bi1:4 Sr2x Ca0:6þx O6d [14]), especially strontiumrich phases (Sr14x Cax Cu24 O41 and Bix Pb3x Sr3 Ca2 CuO12y ), as in classical reactions performed in air. It has been demonstrated many times that reactions performed only in air cause many different compounds, and that their parallel reactions with each other sentence any reactivity control to become an extremely difficult task. NOx pressures much higher than 1 bar stabilize nitrates, such as Sr(NO3 )2 , up to higher heating temperatures and thereby slow down the reactivity. We may note that long annealing periods and crystallizations of intermediate products used in this study were aimed to fully describe reaction pathways and intermediate products. Certainly, the reaction can be performed much faster and more efficient.
References [1] H. Maeda, Y. Tanaka, M. Fukutomi, T. Asano, Jpn. J. Appl. Phys. 27 (1988) 209. [2] N.V. Coppa, W.L. Hults, J.L. Smith, J. Brynestad, J. Mater. Res. 9 (1994) 2510. [3] Y.-W. Hsueh, S.C. Chang, R.S. Liu, L. Woodall, M. Gerards, Mater. Res. Bull. 36 (2001) 1653. [4] P. Badica, G. Aldica, S. Madache, Supercond. Sci. Technol. 12 (1999) 162.
[5] O.A. Shlyakhtin, A.L. Vinokurov, A.N. Tretyakov, D. Yu, J. Supercond. 11 (1998) 507. [6] M.G. Smith, J.O. Willis, D.E. Peterson, J.F. Bingert, D.S. Phillips, J.Y. Coulter, K.V. Salazar, W.L. Hults, Physica C 231 (1994) 409. [7] E. Giannini, I. Savysyuk, V. Garnier, R. Passerini, P. Toulemonde, R. Fl€ ukiger, Supercond. Sci. Technol. 15 (2002) 1577. [8] I. Van Driessche, R. Mouton, S. Hoste, Mater. Res. Bull. 31 (1996) 979. [9] B. Sailer, F. Schwaigerer, K. Gibson, H.-J. Meyer, M. Lehmann, L. Woodall, M. Gerards, Trans. Appl. Supercond. 11 (2001) 2975. [10] Q. Xu, L. Bi, D. Peng, G. Meng, G. Zhou, Z. Mao, C. Fan, Y. Zhang, Supercond. Sci. Technol. 3 (1990) 564. [11] J.K.F. Yau, Y.L. Wong, Physica C 339 (2000) 79. [12] A. Jeremie, K. Alami-Yadri, J.-C. Grivel, R. Fl€ ukiger, Supercond. Sci. Techol. 6 (1993) 730. [13] K. Gibson, S. Dill, V. Cauniene, B. Sailer, H.-J. Meyer, M. Lehmann, L. Woodall, M. Gerards, Physica C 372–376 (2002) 995. [14] C.C. Luhrs, M. Morales, F. Sapi~ na, D. Beltran-Porter, A. Fuertes, Solid State Ionic 101–103 (1997) 1107. [15] P. Ziegler, I. Grigoraviciute, K. Gibson, J. Glaser, A. Kareiva, H.-J. Meyer (in preparation). [16] H. Kodama, Special Publication-Royal Society of Chemistry 196 (1997) 39. [17] H. Kodama, Japanese Patent 2,000,086,243, Science and Technology Agency (1998). [18] P. Ziegler, K. Gibson, H.-J. Meyer, German Patent, Patentamt M€ unchen (2003). [19] K. Gibson, P. Ziegler, H.-J. Meyer, Physica C 397 (2003) 112. [20] K. Gibson, P. Ziegler, H.-J. Meyer (in preparation). [21] T. Roisnel, J. Rodrıguez-Carvajal, WinPLOTR: Windows tool for powder diffraction patterns analysis, Materials Science Forum, in: R. Delhez, E.J. Mittenmeijer (Eds.), Proceedings of the Seventh European Powder Diffraction Conference (EPDIC 7), 2000, 118.