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Structural features and anomalies in the temperature dependence of resistance in superconducting Bi-2212 single crystals
Journal of Crystal Growth 198/199 (1999) 619—625
Structural features and anomalies in the temperature dependence of resistance in superconducting Bi-2212 single crystals L. Leonyuk *, G.-J. Babonas, V. Maltsev , A. Vetkin , V. Rybakov , A. Reza Chair of Crystallography, Geological Faculty, Moscow State University, 119899 Moscow, Russia Semiconductor Physics Institute, LT-2600 Vilnius, Lithuania
Abstract The structure of the series of Bi-2212-type single crystals was studied. Two types of Bi-2212 crystals were distinguished which differ in the structure of Bi—O layer. The drops of resistance at anomaly high temperatures were observed. The presence of anomalies was correlated with the structural features of these samples: the occurrence of intergrowth and monoclinic distortion in the structure of one phase. The structure of Bi-2212-type compounds was discussed in the polysomatic approximation. 1999 Elsevier Science B.V. All rights reserved. PACS: 61.50.Ks; 74.70.Vy Keywords: Cuprates; Crystal structure
1. Introduction In spite of intensive studies provided on the Bi2212-type compounds during 10 years since their discovery, some of the experimental observations related to their physical properties are not well understood. It is known that in the Bi(Pb)—Sr— Ca(Y,RE)—Cu—O (where RE are rare-earth elements) system the mono-phase samples grown from the melt present only a small part of the crystals in
* Corresponding author. Tel.: #7 095 939 2881; fax: #7 095 932 8889; e-mail: [email protected].
the crucible [1]. This is a possible reason of ambiguity in the estimation of the superconducting (SC) transition temperature ¹ for the phases under con sideration [2]. A well-known feature is the structural modulations observed in Bi Sr CaCu O and other Bi-containing phases. It is absent in Bi-2212 doped by Pb up to 40% with respect to Bi and in Tl—Ba2212 compounds [3]. A particular feature of SC Bi-compounds is anomaly high-¹ values which have been frequently noticed. However, so far there is no model which would be widely accepted to interpret this interesting phenomenon. The goal of the present work was to study the relationship between the structural features and the
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specific properties of single crystals of the Bi-2212type phases.
2. Experiment and results Single crystals of the 2212-type structure were synthesized by slow cooling of the oxide melts with various Bi/Pb and Ca/RE concentration ratios. The crystals were grown by a self-flux method in alumina crucibles at ambient atmosphere. At temperatures lower than 1000°C the interaction between crucible and melt was negligible. After multiple probes the melt composition 2Bi O — 3SrO—3CaO—4CuO was chosen. For doping, Pb (up to 50% with respect to stoichiometric Bi amount) and RE elements from the Y-group (up to 10% with respect to Ca) were used. The growth conditions were described in details elsewhere [1]. According to the X-ray phase analysis and EPMA data the Bi-2212-type phases form four series A—D of the solid solutions (Fig. 1, Table 1). About 20 stable isostructural 2212-type phases, which were slightly different by the composition and lattice constant values, have been indicated on the basis of X-ray phase and spectral analyses, EPMA data [1] and Raman spectroscopy [4]. The most typical phases (Table 2) were studied by single-crystal X-ray structural analysis, detailed Xray phase analysis, goniometry, electron microscopy and spectroscopic ellipsometry. The results obtained are summarized in the following statements: (i) The satellite reflections are not characteristic of all Bi-2212-type phases. They were absent in a SC (¹ "85 K) optically anisotropic crystal (N 555 in Table 2) and in a nonsuperconducting (NSC) optically isotropic crystal (N 998 in Table 2). The satellites in the X-ray diffraction pattern were indicated for the crystals with lattice constant c"30.6 A> assigned to the A and B groups (NN 1059 and 1313 in Table 2) (Fig. 1). These samples exhibit intermediate optical anisotropy. (ii) Under a polarizing microscope, well-defined growth steps parallel to the b-axis along which the lattice parameter is modulated (Fig. 2a, sample N 1313 in Table 2) were observed. When the lattice modulation is absent, the square steps are observed
Fig. 1. Four series (A—D) of the isostructural Bi-2212-type phases.
Table 1 The concentration ratios of the basic elements in Bi-2212 crystals for series A—D Concentration ratio
A
B
C
D
RE/(RE#Ca) Pb/(Bi#Pb)
0—0.55 0
0—0.70 0.05—0.07
0.08—0.60 0.11—0.12
0.55—0.65 0.26—0.34
on the crystal surface (Fig. 2b, sample N 1317 in Table 2). (iii) The goniometric investigations have shown that the bulk-type formations (Fig. 3a, sample N 1321 in Table 2) represent the single crystalline samples of a monoclinic symmetry. The growth steps with angles 112° and 135° are usually developed on the surface of the platelet-shaped samples (Fig. 3b, sample N 1073 in Table 2). (iv) The single-crystalline X-ray structural analysis was performed on the platelet-shaped sample. The analysis of experimental reflection intensities for the crystal Bi Sr CaY Cu O from the A-group (N 1059 in Table 2, Pnnn; a"5.403 A> , b"27.034 A> , c"30.56 A> , R "8.6%) indicated FIJ that the symmetry of averaged unit cell corresponded to the monoclinic (P112/b or P11b) rather than to a usually fixed orthorhombic symmetry. The modulation was better developed in the [Bi O ] layer than in the fragment [(Ca,Y)Cu Sr O ]. The
A
D
B B B
B A B
—”—
555
998
1317 1313 1334
1107 1059 1080
1080f in 740
B
Group
Sample no.
Sr Ca Cu O V
(Bi Pb )Sr Ca Cu O V
(Bi Pb )(Sr Ca )(Ca Y )Cu O V
(Bi Pb )Sr Ca Cu O V Bi Sr CaY Cu O (Bi Pb )(Sr Ca )(Ca Y )Cu O V
(Bi Pb )Sr Ca Er Cu Al O V (Bi Pb )(Sr Ca )(Ca Y )Cu O V (Bi Pb )(Sr Ca )(Ca Y )Cu O V (Bi Pb )Sr Ca Y Cu O V
Bi
Chemical composition
Table 2 Characteristics of the crystals studied
3.826(3)
5.403
5.401(6) 5.403 10.8
5.396(11)
5.421(3)
a 30.87(3)
c
3.823(4)
27.034
5.400(2) 27.034 30.6
F-cell P-cells
c1"30.8 c2"30.6
Pnnn P-cells
F-cell Pnnn C2/c b"112°
P-cell
P-cell
Symmetry
15.29(1)
c2"30.4
30.73 30.56 c1"30.6
30.69(6) 30.56 29.0
5.421(11) 30.50(9)
27.07(2)
b
Lattice parameters (A> )
141, 85
82 80 266, 249 204, 136 43
82 80 75
NSC
85
¹ (K)
After cooling—heating runs
Modulations along b-axis
Bulk sample
Optically anisotropic, modulations along b-axis [5] Optically isotropic, without modulations [5]
Remarks
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Fig. 2. The basal plane of SC (¹ "80 K) Bi-2212-type crystals: Bi (Sr Ca )(Ca Y )Cu O , Pnnn (a"5.403 A> , V b"27.034 A> , c"30.56 A> ) with structural modulations (a); and (Bi Pb )(Sr Ca )(Ca Y )Cu O , (F-symmetry, V a"5.401(6) A> , b"5.400(2) A> , c"30.69(6) A> ) without structural modulations (b) under polarizing microscope. The sample images were obtained at the magnification 600;.
latter fragment is characterized by an independent structural type [6], while former one represents the oxygen-deficient unit Bi O . \V (v) The Bi-2212-type sample, which originally represented the intergrowth of two phases (c"30.6 and 30.4 A> ), had shown the metal—superconductor transitions at 266 and 249 K at sample cooling and heating, respectively, for the first run. The corresponding temperatures for the second run were 204 and 136 K. The resistance of the sample in the normal state before the transition to SC state was equal to 1.8 ). The experiments were performed during two days. After three weeks the temperature
Fig. 3. Bi-2212-type phases with monoclinic distortion: (a) bulk sample of single crystal (Bi Pb )Sr (Ca Y )Cu O V (¹ "75 K) (4;3;3 mm); (b) basal plane of the crystal (Bi Pb )(Sr Ca )CaCu O (¹ "82 K) with growth V steps at the angle 112° (600").
dependence of resistance was stable and metallic like. A partial decrease of resistance from 3 to 2.5 ) was noticed at 80 K. The sample resistance increased up to 4 ) and the onset of the resistance drop was at 60 K. The zero-resistance was indicated at 43 K. The X-ray structural analysis of the stable sample has shown a new phase of orthorhombic F-symmetry with lattice parameters a" 3.826(3) A> , b"3.823(4) A> , c"15.29(1) A> . Similar anomalies in the temperature dependence of resistance were indicated in a series of six samples that represent the intergrowth of Bi-2212type phases of various composition and slightly different lattice constants (Table 3). The c-parameter of one phase was close to 30.6 A> . In this phase the monoclinic distortion was always
L. Leonyuk et al. / Journal of Crystal Growth 198/199 (1999) 619–625
623
Table 3 The chemical composition, c-parameter values and the temperatures of the resistance drop ¹ of the Bi-2212 (B1,Pb) (Sr,Ca) (RE,Ca)Cu O crystals with anomalies in the temperature dependence of resistance V Pb/Bi
Ca/Sr
RE/Ca
c (A> )
c (A> )
¹ (K)
0.04
0.07
0
3077
141, 85
1217
0.04—0.07
0.04—0.05
0.31—0.47
30.78
171—142, 36
1313 1119 1317
0.08—0.09 0.05—0.07 0.11—0.12
0.22—0.23 0.10—0.15 0—0.17
0.31—0.33 0.37—0.50 0.20—0.30
30.5 7 30.6 3 30.6 30.6 30.6
30.4 30.4 30.4
144, 41 207—128, 62 208—90, 62
1068
0.05—0.06
0.20
0.28—0.44
30.87
180—140, 71
1080
0.05—0.06
0.19—0.23
0.30—0.41
30.5 7 30.5 6
30.35
266—136, 43
Sample no. 740
indicated. The concentration ranges of basic elements (Table 3) illustrate the sample nonuniformity of the phase with c"30.6 A> . It should be noted that a resistance drop to zero at 141 K was indicated in one of the samples from the batch N 740. However, the transition temperature ¹ "85 K was determined for this sample by the measurements of magnetic susceptibility, which have been performed after several months. For the other samples from the batch N 740 the value ¹ "85 K was determined. It should be also noted that according to EPMA data, the Bi-concentration exceeded locally the stoichiometric value up to 40% in the samples with resistance anomalies. (vi) The multiple melting and quenching of the initial load before the final slow cooling in the growth procedure leads to the formation of the SC Bi-2212-type phase without structural modulations (sample N 1317 in Table 2). (vii) An annealing in oxygen (one day at 500°C) leads to the re-crystallization of all the samples except those from the D-series. As a rule, the new Bi-2212-type phases do not possess the structural modulations if RE are absent in the composition of the samples. After oxygen annealing of the samples with RE, the structural modulations were still observed.
3. Discussion The experimental data obtained on the twophase samples can be reasonably interpreted using the assumption that, in general, two Bi-2212-type phases can be formed, one with structural modulations (and monoclinic distortion) and another without modulations. The structural modulations are apparently caused by the structure of the Bi—O fragment. If the Bi—O fragment 2(BiO) is characterized by the rock-salt structure, the Bi-2212-type phase is the analog of a well-known TlBa-2212 phase. If the Bi—O fragment retains the structure of monoclinic bismuth oxide (c-Bi O , defect fluorite structural type [7]), the structural modulations occur due to the difference in the inter-atomic distances and symmetry between the fragments [YCuO ] (infinite layer type structure) and [Bi O ]. In \V addition, in the latter case the so-called extra oxygen atoms are observed in the Bi—O layer by the X-ray diffraction. The deformation of intermediate SrO layer of the rock salt structure is weaker than in Bi—O fragment but stronger than in the Cu—O plane. In the case of the crystals with structural modulation, the growth steps having a 112° angle characteristic of bismuth oxide are observed on the (ab)surface. The anomalies observed in the temperature
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dependence of resistance are not to be interpreted as the real transition temperatures to superconducting state as they were not confirmed by the magnetic susceptibility data. However, these anomalies can be unambiguously correlated to the intergrowth of isostructural phases one of which possesses the monoclinic (or even triclinic) distortion. The ¹ -values of mono-phase monoclinic bulk Bi-2212 samples do not exceed 85 K. The structural modulations of Bi-2212 phases can be destroyed by multiple re-crystallization during which the similarity between Bi—O fragment and bismuth oxide structure is gradually reduced. The presence of the structural modulations correlates with the development of the Raman peak at 100 cm\ [4] assigned to the A -symmetry mode of Bi vibrations [8].
4. Polytypic, homologues and polysomatic series in Tl—Ba and Bi—Sr cuprates The difference in the structure of the Bi—O fragment allows one to assign the Bi-2212-type phases to various generically different structural series. Up to the present time Bi and Tl compounds are assigned to the same group according to the crystalchemistry classification of layered cuprates [9]. The crystal structure was firstly determined for Tl compounds. The Bi compounds were considered as analogs due to the similar values of the lattice parameters. Various sets of the compounds were called as homologues and polytypic, however the relationship between the structures were not studied in detail. The whole family of Tl-cuprates can be divided into the following three groups (Table 4). Using the definitions presented in Refs. [10,11], each group can be considered as homologues series whereas the compounds from the second and third groups form the polytypic series. One part of Bi compounds can be considered as analogs of Tl-2201 from the first group and of all phases from the third group. They can be treated as both polytypic and homologues series. These Bi compounds are characterized by Bi—O layer of the rock-salt-type structure and show no structural modulations. The Bi compounds from the second
Table 4 Selection of groups in the family of Tl-cuprates First group
Second group
1201 1212 ((Tl,Bi)Sr CuO ) (TlBa CaCu O ) 2201(Tl Ba CuO ) 1223 (TlBa Ca Cu O ) 1234 (TlBa Ca Cu O ) 1245 (TlBa Ca Cu O )
Third group 2122 (Tl Ba CaCu O ) 2122 (Tl Ba CaCu O ) 2223 (Tl Ba Ca Cu O ) 2234 (Tl Ba Ca Cu O ) 2245 (Tl Ba Ca Cu O )
group are characterized by monoclinic distortion and structural modulations because the Bi—O layer retains the structure of Bi O . From the view point of polysomatism [12], these Bi-2212-type phases can be considered as polysomes AB in the series with the end-members Bi O (A) and \V (La,Sr,Ca) (La,Sr,Ca)Cu O (B). The defects and structural distortions are to be located [12] at the interfaces between the slabs A and B, i.e., between Sr—O and Bi—O layers. A tendency of the Bi—O layer to retain the Bi O -type structure should favor the inclusion of additional Bi—O layers into the structure. As a result, total Bi amount in the crystal will be higher than in stoichiometric Bi2212. The solid-state reactions are characteristic of polysomes and occur at the interfaces between the slabs [13]. In the case of Bi-cuprates the solid-state reactions can be provoked by the cooling—heating cycles during experimental runs. This is a possible interpretation of the structural transformation which has been observed after the measurements of the temperature dependence of electrical resistance. It is reasonable to assume that anomalies in the temperature dependence of resistance is related to the formation of metastable Bi-2212-type phases of the symmetry lower than orthorhombic with the Bi amount higher than stoichiometric one. 5. Summary The analysis of Bi-cuprates as the members of polysomatic series allows one to get a deeper
L. Leonyuk et al. / Journal of Crystal Growth 198/199 (1999) 619–625
understanding of their specific structural features. The structural modulation and monoclinic distortion were shown to originate from particular features of the Bi—O double layer. The interfaces between the polysomatic slabs represent the area of the solid-phase reactions and defects. As a result, the epitaxial intergrowths are typical for monoclinic modifications of Bi-2212. The interface between the polysomatic slabs is characterized by largest distortions. Electrical resistance drops to zero values were noticed at the temperatures significantly higher than the usual ¹ -values for Bi-2212 compounds. These particular electrical anomalies do not present the real transition to the superconducting state but can be interpreted as the manifestation of the structural instabilities which occur at the interfaces between the polysomatic slabs.
References [1] L. Leonyuk, G.-J. Babonas, V. Maltsev, A. Vetkin, Supercond. Sci. Technol. 8 (1995) 53. [2] R.P. Gupta, M. Gupta, Phys. Rev. B 49 (1994) 13154.
625
[3] Y. Inoue, M. Hasegawa, M. Yamanaka, Y. Koyama, Proc. 2nd ISSS’89, ISTEC, 1989, Tsukuba, Japan, pp. 157—160. [4] L.I. Leonyuk, G.-J. Babonas, Vestnik MGU, Ser. Fizika N1 (1997) 39. [5] G.-J. Babonas, R. Dagys, V. Kregzde, L. Leonyuk, G. Pukinskas, A. Vetkin, Supercond. Sci. Technol. 5 (1992) 671. [6] R.J. Cava, A. Santoro, J.J. Krajewski, R.M. Fleming, J.V. Wasczak, W.F. Peck, P. Marsh, Physica C 172 (1990) 138. [7] H.A. Harwig, Z. Anorg. Allg. Chem. 444 (1978) 151. [8] R. Liu, M.V. Klein, P.D. Han, D.A. Payne. Phys. Rev. B 45 13 (1992) 7392. [9] L. Leonyuk, G.-J. Babonas, V. Maltsev, Int. J. Appl. Electromagn. Mech. 8 (1997) 229. [10] E. Makovicky, Modularity — different types and approaches, in: EMU Notes in Mineralogy, vol. 1, 1997, pp. 315—343. [11] A. Guiner, G.B. Bokii, K. Boll-Domberger, J.M. Cowley, S. Durovic, H. Jagodzinski, P. Krishna, P.M. de Wolff, B.B. Zvyagin, D.E. Cox, P. Goodman, Th. Hahn, K. Kuchitsu, S.C. Abrahams, Acta Crystallogr. A 40 (1984) 399. [12] J.B. Thompson, Am. Mineral. 53 (1970) 292. [13] D.R. Veblen, Am. Mineral. 76 (1991) 801.
Structural features and anomalies in the temperature dependence of resistance in superconducting Bi-2212 single crystals L. Leonyuk *, G.-J. Babonas, V. Maltsev , A. Vetkin , V. Rybakov , A. Reza Chair of Crystallography, Geological Faculty, Moscow State University, 119899 Moscow, Russia Semiconductor Physics Institute, LT-2600 Vilnius, Lithuania
Abstract The structure of the series of Bi-2212-type single crystals was studied. Two types of Bi-2212 crystals were distinguished which differ in the structure of Bi—O layer. The drops of resistance at anomaly high temperatures were observed. The presence of anomalies was correlated with the structural features of these samples: the occurrence of intergrowth and monoclinic distortion in the structure of one phase. The structure of Bi-2212-type compounds was discussed in the polysomatic approximation. 1999 Elsevier Science B.V. All rights reserved. PACS: 61.50.Ks; 74.70.Vy Keywords: Cuprates; Crystal structure
1. Introduction In spite of intensive studies provided on the Bi2212-type compounds during 10 years since their discovery, some of the experimental observations related to their physical properties are not well understood. It is known that in the Bi(Pb)—Sr— Ca(Y,RE)—Cu—O (where RE are rare-earth elements) system the mono-phase samples grown from the melt present only a small part of the crystals in
* Corresponding author. Tel.: #7 095 939 2881; fax: #7 095 932 8889; e-mail: [email protected].
the crucible [1]. This is a possible reason of ambiguity in the estimation of the superconducting (SC) transition temperature ¹ for the phases under con sideration [2]. A well-known feature is the structural modulations observed in Bi Sr CaCu O and other Bi-containing phases. It is absent in Bi-2212 doped by Pb up to 40% with respect to Bi and in Tl—Ba2212 compounds [3]. A particular feature of SC Bi-compounds is anomaly high-¹ values which have been frequently noticed. However, so far there is no model which would be widely accepted to interpret this interesting phenomenon. The goal of the present work was to study the relationship between the structural features and the
0022-0248/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 1 6 2 - 2
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specific properties of single crystals of the Bi-2212type phases.
2. Experiment and results Single crystals of the 2212-type structure were synthesized by slow cooling of the oxide melts with various Bi/Pb and Ca/RE concentration ratios. The crystals were grown by a self-flux method in alumina crucibles at ambient atmosphere. At temperatures lower than 1000°C the interaction between crucible and melt was negligible. After multiple probes the melt composition 2Bi O — 3SrO—3CaO—4CuO was chosen. For doping, Pb (up to 50% with respect to stoichiometric Bi amount) and RE elements from the Y-group (up to 10% with respect to Ca) were used. The growth conditions were described in details elsewhere [1]. According to the X-ray phase analysis and EPMA data the Bi-2212-type phases form four series A—D of the solid solutions (Fig. 1, Table 1). About 20 stable isostructural 2212-type phases, which were slightly different by the composition and lattice constant values, have been indicated on the basis of X-ray phase and spectral analyses, EPMA data [1] and Raman spectroscopy [4]. The most typical phases (Table 2) were studied by single-crystal X-ray structural analysis, detailed Xray phase analysis, goniometry, electron microscopy and spectroscopic ellipsometry. The results obtained are summarized in the following statements: (i) The satellite reflections are not characteristic of all Bi-2212-type phases. They were absent in a SC (¹ "85 K) optically anisotropic crystal (N 555 in Table 2) and in a nonsuperconducting (NSC) optically isotropic crystal (N 998 in Table 2). The satellites in the X-ray diffraction pattern were indicated for the crystals with lattice constant c"30.6 A> assigned to the A and B groups (NN 1059 and 1313 in Table 2) (Fig. 1). These samples exhibit intermediate optical anisotropy. (ii) Under a polarizing microscope, well-defined growth steps parallel to the b-axis along which the lattice parameter is modulated (Fig. 2a, sample N 1313 in Table 2) were observed. When the lattice modulation is absent, the square steps are observed
Fig. 1. Four series (A—D) of the isostructural Bi-2212-type phases.
Table 1 The concentration ratios of the basic elements in Bi-2212 crystals for series A—D Concentration ratio
A
B
C
D
RE/(RE#Ca) Pb/(Bi#Pb)
0—0.55 0
0—0.70 0.05—0.07
0.08—0.60 0.11—0.12
0.55—0.65 0.26—0.34
on the crystal surface (Fig. 2b, sample N 1317 in Table 2). (iii) The goniometric investigations have shown that the bulk-type formations (Fig. 3a, sample N 1321 in Table 2) represent the single crystalline samples of a monoclinic symmetry. The growth steps with angles 112° and 135° are usually developed on the surface of the platelet-shaped samples (Fig. 3b, sample N 1073 in Table 2). (iv) The single-crystalline X-ray structural analysis was performed on the platelet-shaped sample. The analysis of experimental reflection intensities for the crystal Bi Sr CaY Cu O from the A-group (N 1059 in Table 2, Pnnn; a"5.403 A> , b"27.034 A> , c"30.56 A> , R "8.6%) indicated FIJ that the symmetry of averaged unit cell corresponded to the monoclinic (P112/b or P11b) rather than to a usually fixed orthorhombic symmetry. The modulation was better developed in the [Bi O ] layer than in the fragment [(Ca,Y)Cu Sr O ]. The
A
D
B B B
B A B
—”—
555
998
1317 1313 1334
1107 1059 1080
1080f in 740
B
Group
Sample no.
Sr Ca Cu O V
(Bi Pb )Sr Ca Cu O V
(Bi Pb )(Sr Ca )(Ca Y )Cu O V
(Bi Pb )Sr Ca Cu O V Bi Sr CaY Cu O (Bi Pb )(Sr Ca )(Ca Y )Cu O V
(Bi Pb )Sr Ca Er Cu Al O V (Bi Pb )(Sr Ca )(Ca Y )Cu O V (Bi Pb )(Sr Ca )(Ca Y )Cu O V (Bi Pb )Sr Ca Y Cu O V
Bi
Chemical composition
Table 2 Characteristics of the crystals studied
3.826(3)
5.403
5.401(6) 5.403 10.8
5.396(11)
5.421(3)
a 30.87(3)
c
3.823(4)
27.034
5.400(2) 27.034 30.6
F-cell P-cells
c1"30.8 c2"30.6
Pnnn P-cells
F-cell Pnnn C2/c b"112°
P-cell
P-cell
Symmetry
15.29(1)
c2"30.4
30.73 30.56 c1"30.6
30.69(6) 30.56 29.0
5.421(11) 30.50(9)
27.07(2)
b
Lattice parameters (A> )
141, 85
82 80 266, 249 204, 136 43
82 80 75
NSC
85
¹ (K)
After cooling—heating runs
Modulations along b-axis
Bulk sample
Optically anisotropic, modulations along b-axis [5] Optically isotropic, without modulations [5]
Remarks
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Fig. 2. The basal plane of SC (¹ "80 K) Bi-2212-type crystals: Bi (Sr Ca )(Ca Y )Cu O , Pnnn (a"5.403 A> , V b"27.034 A> , c"30.56 A> ) with structural modulations (a); and (Bi Pb )(Sr Ca )(Ca Y )Cu O , (F-symmetry, V a"5.401(6) A> , b"5.400(2) A> , c"30.69(6) A> ) without structural modulations (b) under polarizing microscope. The sample images were obtained at the magnification 600;.
latter fragment is characterized by an independent structural type [6], while former one represents the oxygen-deficient unit Bi O . \V (v) The Bi-2212-type sample, which originally represented the intergrowth of two phases (c"30.6 and 30.4 A> ), had shown the metal—superconductor transitions at 266 and 249 K at sample cooling and heating, respectively, for the first run. The corresponding temperatures for the second run were 204 and 136 K. The resistance of the sample in the normal state before the transition to SC state was equal to 1.8 ). The experiments were performed during two days. After three weeks the temperature
Fig. 3. Bi-2212-type phases with monoclinic distortion: (a) bulk sample of single crystal (Bi Pb )Sr (Ca Y )Cu O V (¹ "75 K) (4;3;3 mm); (b) basal plane of the crystal (Bi Pb )(Sr Ca )CaCu O (¹ "82 K) with growth V steps at the angle 112° (600").
dependence of resistance was stable and metallic like. A partial decrease of resistance from 3 to 2.5 ) was noticed at 80 K. The sample resistance increased up to 4 ) and the onset of the resistance drop was at 60 K. The zero-resistance was indicated at 43 K. The X-ray structural analysis of the stable sample has shown a new phase of orthorhombic F-symmetry with lattice parameters a" 3.826(3) A> , b"3.823(4) A> , c"15.29(1) A> . Similar anomalies in the temperature dependence of resistance were indicated in a series of six samples that represent the intergrowth of Bi-2212type phases of various composition and slightly different lattice constants (Table 3). The c-parameter of one phase was close to 30.6 A> . In this phase the monoclinic distortion was always
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Table 3 The chemical composition, c-parameter values and the temperatures of the resistance drop ¹ of the Bi-2212 (B1,Pb) (Sr,Ca) (RE,Ca)Cu O crystals with anomalies in the temperature dependence of resistance V Pb/Bi
Ca/Sr
RE/Ca
c (A> )
c (A> )
¹ (K)
0.04
0.07
0
3077
141, 85
1217
0.04—0.07
0.04—0.05
0.31—0.47
30.78
171—142, 36
1313 1119 1317
0.08—0.09 0.05—0.07 0.11—0.12
0.22—0.23 0.10—0.15 0—0.17
0.31—0.33 0.37—0.50 0.20—0.30
30.5 7 30.6 3 30.6 30.6 30.6
30.4 30.4 30.4
144, 41 207—128, 62 208—90, 62
1068
0.05—0.06
0.20
0.28—0.44
30.87
180—140, 71
1080
0.05—0.06
0.19—0.23
0.30—0.41
30.5 7 30.5 6
30.35
266—136, 43
Sample no. 740
indicated. The concentration ranges of basic elements (Table 3) illustrate the sample nonuniformity of the phase with c"30.6 A> . It should be noted that a resistance drop to zero at 141 K was indicated in one of the samples from the batch N 740. However, the transition temperature ¹ "85 K was determined for this sample by the measurements of magnetic susceptibility, which have been performed after several months. For the other samples from the batch N 740 the value ¹ "85 K was determined. It should be also noted that according to EPMA data, the Bi-concentration exceeded locally the stoichiometric value up to 40% in the samples with resistance anomalies. (vi) The multiple melting and quenching of the initial load before the final slow cooling in the growth procedure leads to the formation of the SC Bi-2212-type phase without structural modulations (sample N 1317 in Table 2). (vii) An annealing in oxygen (one day at 500°C) leads to the re-crystallization of all the samples except those from the D-series. As a rule, the new Bi-2212-type phases do not possess the structural modulations if RE are absent in the composition of the samples. After oxygen annealing of the samples with RE, the structural modulations were still observed.
3. Discussion The experimental data obtained on the twophase samples can be reasonably interpreted using the assumption that, in general, two Bi-2212-type phases can be formed, one with structural modulations (and monoclinic distortion) and another without modulations. The structural modulations are apparently caused by the structure of the Bi—O fragment. If the Bi—O fragment 2(BiO) is characterized by the rock-salt structure, the Bi-2212-type phase is the analog of a well-known TlBa-2212 phase. If the Bi—O fragment retains the structure of monoclinic bismuth oxide (c-Bi O , defect fluorite structural type [7]), the structural modulations occur due to the difference in the inter-atomic distances and symmetry between the fragments [YCuO ] (infinite layer type structure) and [Bi O ]. In \V addition, in the latter case the so-called extra oxygen atoms are observed in the Bi—O layer by the X-ray diffraction. The deformation of intermediate SrO layer of the rock salt structure is weaker than in Bi—O fragment but stronger than in the Cu—O plane. In the case of the crystals with structural modulation, the growth steps having a 112° angle characteristic of bismuth oxide are observed on the (ab)surface. The anomalies observed in the temperature
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dependence of resistance are not to be interpreted as the real transition temperatures to superconducting state as they were not confirmed by the magnetic susceptibility data. However, these anomalies can be unambiguously correlated to the intergrowth of isostructural phases one of which possesses the monoclinic (or even triclinic) distortion. The ¹ -values of mono-phase monoclinic bulk Bi-2212 samples do not exceed 85 K. The structural modulations of Bi-2212 phases can be destroyed by multiple re-crystallization during which the similarity between Bi—O fragment and bismuth oxide structure is gradually reduced. The presence of the structural modulations correlates with the development of the Raman peak at 100 cm\ [4] assigned to the A -symmetry mode of Bi vibrations [8].
4. Polytypic, homologues and polysomatic series in Tl—Ba and Bi—Sr cuprates The difference in the structure of the Bi—O fragment allows one to assign the Bi-2212-type phases to various generically different structural series. Up to the present time Bi and Tl compounds are assigned to the same group according to the crystalchemistry classification of layered cuprates [9]. The crystal structure was firstly determined for Tl compounds. The Bi compounds were considered as analogs due to the similar values of the lattice parameters. Various sets of the compounds were called as homologues and polytypic, however the relationship between the structures were not studied in detail. The whole family of Tl-cuprates can be divided into the following three groups (Table 4). Using the definitions presented in Refs. [10,11], each group can be considered as homologues series whereas the compounds from the second and third groups form the polytypic series. One part of Bi compounds can be considered as analogs of Tl-2201 from the first group and of all phases from the third group. They can be treated as both polytypic and homologues series. These Bi compounds are characterized by Bi—O layer of the rock-salt-type structure and show no structural modulations. The Bi compounds from the second
Table 4 Selection of groups in the family of Tl-cuprates First group
Second group
1201 1212 ((Tl,Bi)Sr CuO ) (TlBa CaCu O ) 2201(Tl Ba CuO ) 1223 (TlBa Ca Cu O ) 1234 (TlBa Ca Cu O ) 1245 (TlBa Ca Cu O )
Third group 2122 (Tl Ba CaCu O ) 2122 (Tl Ba CaCu O ) 2223 (Tl Ba Ca Cu O ) 2234 (Tl Ba Ca Cu O ) 2245 (Tl Ba Ca Cu O )
group are characterized by monoclinic distortion and structural modulations because the Bi—O layer retains the structure of Bi O . From the view point of polysomatism [12], these Bi-2212-type phases can be considered as polysomes AB in the series with the end-members Bi O (A) and \V (La,Sr,Ca) (La,Sr,Ca)Cu O (B). The defects and structural distortions are to be located [12] at the interfaces between the slabs A and B, i.e., between Sr—O and Bi—O layers. A tendency of the Bi—O layer to retain the Bi O -type structure should favor the inclusion of additional Bi—O layers into the structure. As a result, total Bi amount in the crystal will be higher than in stoichiometric Bi2212. The solid-state reactions are characteristic of polysomes and occur at the interfaces between the slabs [13]. In the case of Bi-cuprates the solid-state reactions can be provoked by the cooling—heating cycles during experimental runs. This is a possible interpretation of the structural transformation which has been observed after the measurements of the temperature dependence of electrical resistance. It is reasonable to assume that anomalies in the temperature dependence of resistance is related to the formation of metastable Bi-2212-type phases of the symmetry lower than orthorhombic with the Bi amount higher than stoichiometric one. 5. Summary The analysis of Bi-cuprates as the members of polysomatic series allows one to get a deeper
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understanding of their specific structural features. The structural modulation and monoclinic distortion were shown to originate from particular features of the Bi—O double layer. The interfaces between the polysomatic slabs represent the area of the solid-phase reactions and defects. As a result, the epitaxial intergrowths are typical for monoclinic modifications of Bi-2212. The interface between the polysomatic slabs is characterized by largest distortions. Electrical resistance drops to zero values were noticed at the temperatures significantly higher than the usual ¹ -values for Bi-2212 compounds. These particular electrical anomalies do not present the real transition to the superconducting state but can be interpreted as the manifestation of the structural instabilities which occur at the interfaces between the polysomatic slabs.
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