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New cuprates with (Bi, Cu)-O monolayers: (Bi,Cu)Sr2LnCeCu2O9−δ
Materials Letters 15 (1992) North-Holland
141-145
New cuprates with (Bi,Cu ) -0 monolayers: (Bi,Cu) Sr,LnCeCu209_8 A. Schilling,
J.D. Guo, M. Cantoni, F. Hulliger, B. Xue and H.R. Ott
Laboratorium fiir Festkiirperphysik, ETH Hiinggerberg, Zurich, Switzerland Received
10 August
1992
New Bi-based cuprates with the “l-2-2-2” structure have been synthesized. The tetragonal structure of (Bi,Cu)Sr,LnCeCuzOg_, (space group 14/mmm) can be derived from that of the recently discovered superconductors (Bi,Cu)SrZYCuzO,_,, in which Y separates the CuO, layers responsible for the occurrence of superconductivity. When Y is replaced by fluorite-type blocks, LnCeOz (with Ln=Y, Sm, Eu, Gd, Dy and Ho), and the residual upper half of each unit cell is shifted by [f t 01, the resulting (Bi,Cu)Sr,LnCeCu20,_, are isostructural to the superconductor (Pb,Cu) (Sr,Eu)z(Eu,Ce)ZCuzOg_,. The compounds (Bi,Cu)Sr,YCu@_. and (Bi,Cu)Sr,LnCeCu20r,_6 with (Bi,Cu)-0 monolayers can be considered as sister series analogous to the Bi&,CaCu,Os and the Bi,Sr,(Ln,Ce)zCuzO,, compounds, which both contain double Bi-0 layers. A quantitative analysis of almost-single-phase material for Ln = Ho with respect to cation contents by EDS techniques suggests an approximate stoichiometry Bi,.,Sr,.,Ho,.2Ce0.9Cu2.209_-6. When synthesized at ambient pressure, the investigated samples are not superconducting above T= I .9 K, for any choice of Ln.
1. Introduction Since the discovery of (Nd,Ce),CuO,_,, Bi,Sr,(Ln,Ce)zCu20,0 and T12Ba2(Ln,Ce)2Cu20,0 by Tokura and co-workers [ 1,2], many other cuprates containing fluorite-type ( Ln,Ce)zO, blocks have been reported in the literature: (Ln,Ce, Sr)ZCu04--6 [3], (Tl,Pb) (Sr,La),(Ln,Ce),Cuz09 141, TlBa2(Eu,Ce)Xu209+6 ]5,61, PbzSrl(BE,Ce),CujOlo [7,81, (Pb,Cu)(Sr,Eu)2(Eu,Ce)2Cu20, [91, Pb(Ba,Sr)2(Ln,Ce)2Cu30, [lOI, (Pb,Cu ) Srz(Ho,Ce)&u20i1 [ll I, (Ln,Ce)2(Ba,Ln)zCu3010 1121, (Ln,Ce),Sr,Cu,(Cu,M)O,+,,+, (n= l-3, M=Cu, Fe, Al or Pb) [ 13,141, MSrz(Ln,Ce)2Cu20y (M=Ta,NborTi) [15],andGaSrz(Y,Ce)ZCu209_-s [ 161. Any oxocuprate with two CuOz layers in its structure separated by Ca, (Ca,Sr), (Ln,Ca) or Ln (including yttrium) can be considered as a candidate for the application of a general “recipe” for designing compounds containing (Ln,Ce),O, blocks, as illustrated in fig. 1. The former structural elements can be replaced by an isoelectric ( Ln,Ce),Oz block, shifting the residual upper half of each unit cell by [ 4 4 0] (see fig. 1). With this type of sub0167-577x/92/$
05.00 0 1992 Elsevier Science Publishers
stitution, one obtains, e.g., BizSrz(Ln,Ce)zCuzO,o [ 2 ] from the compound Bi2SrzCaCuzOs [ 17 1. Here, the divalent Ca can, in principle, be replaced by a LnzOz block without Ce addition [ 18,191, although a partial substitution of Ln by Ce [ 2 ] or Th [ 18 ] seems to be favourable for the stabilization of the “22-2-2” structure. In this communication, we report the successful synthesis of compounds with the idealized formula (Bi,Cu)SrzLnCeCu209_6 (Ln=Y, Sm, Eu, Cd, Dy and Ho). Motivated by the work of Ehmann and coworkers [20], who reported the synthesis of superconductors with the composition ( Bi,Cu)Sr,YCuzO7_-z, we postulated the existence of related compounds according to the above described “recipe”. We expected that Y may ideally be replaced by a block ( Lnl_-xCeX)202 with xz 0.5 to account for the trivalency of the yttrium ion. A further argument supporting the idea of the existence of (Bi,Cu)SrzLnCeCuzOg_, cuprates originated in the structural analogy to the known thallium and lead-based compounds, (Tl,Pb)(Sr,La),(Ln,Ce)$&O, [4] and (Pb,Cu)(Sr,Eu)2(Eu,Ce)zCuz0,, [9], respectively.
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@J 0 Fig. 1. Schematic representation of the “recipe” for designing Cu02 layers, replacing Ca, (Ca,Sr ) , (Ln,Ca) or Ln [ 19 1.
structures
2. Experimental details and results
The samples of nominal composition Bio,s_xSr2_yLnl+,Ce,_-r+yCu2.5--x09--6 were prepared by standard solid-state reaction techniques for several values of x, y, and z, and for Ln = Y, Sm, Eu, Gd, Dy and Ho. The metal oxides and Sr ( NO3 )2 were mixed and heated at 800°C for 1 h in air. The resulting materials were then ground and pressed into pellets, and sintered in air at 970°C during 5 h. Finally, the samples were re-ground and sintered at 980°C for 1 h. We observed that the reaction temperatures could be raised up to 1060°C without melting the samples.
Bio.dhHol
Fig. 2. X-ray diffraction
142
pattern
containing
a fluorite-like
(Ln,Ce),Or
block (B) between two
However, the higher synthesis temperatures did neither substantially improve the sample quality, nor was the amount of parasitic phases reduced. We realized that experiments with even stoichiomen-y (i.e. x=y=z=O) did not result in phase-pure samples, although a large amount of the “l-2-2-2” phase was always detected. Further experiments for Ln = Ho showed that nominal compositions around x=0.1, y=O and ~~0.1 yielded samples of the desired quality. On Guinier photographs taken with Cu Ku, radiation, several weak unidentified lines due to parasitic phases were still present. However, on the powder diffraction pattern shown in fig. 2, those lines
.iCeo.&~2.409-6
of the sample with nominal
composition
Bio.4SrzHo,.,Ceo.sCu2.409_a,
taken with Cu Ku, radiation.
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Volume 15, number 3
Table 1 Lattice parameters of the “1-2-2-2” phase for various Ln. The nominal composition was Bi,.,SrzLnCeCu2.509_d. except for Ln=Sm (Bio.4SrzSm,.zCeo.sCuz.409_a) and HO (2) (Bio4Sr,Ho,.,Ceo.9Cu~.~O~_~). Note that the a and c values in Cu 2.5_ X0 9_ 8depend on the real values Bi,,-~r2-vLn,+iCel-r+v of x, y, and z of the “l-2-2-2” phase in the respective samples (see text f
_
Ln
a (At
C(A)
cla
Y
3.8272(2) 3.8543(2) 3.8502(2) 3.8475(2) 3.8347(2) 3.8206(3) 3.8228(2)
28.857(2) 29.005(2) 28985(Z) 28.974(2) 28.895(3) 28.933(3) 28.979(2)
7.5399(10) 7.5253(9) 7.5283(7) 7.5305(7) 7.5.585( 10) 7.5728( 13) 7.5805(7)
Sm EU
Gd DY Ho (I) Ho (2)
are hardly visible because of their low intensity, compared to the lines belonging to the “1-2-2-2” phase. The lattice parameters, for samples with Ln=Y, Sm, Eu, Gd, Dy and Ho, are given in table 1. The dependence of a and c on the radius of the trivalent Ln3’ ion is monotonic but not exactly linear. Since the stoichiomet~ parameters x, y and z are not yet known for all samples on a microscopic scale (except for Ln = Ho, see below), a systematic interpretation of the lattice parameters as a function of Ln is dif-
November 1992
licult at present. The clear difference in the lattice parameters for the compounds with Ln=Ho and Ln=Y, i.e. with two elements of similar ionic radii in eight-coordination, may indicate that the respective x, y and z values differ markedly in both compounds. Fig. 3 shows a schematic representation of the unit cell of the (Bi,Cu)Sr~LnCeCu~O~_~ com~unds. The cell is body-centered tetragonal (space group 141 mmm). The structure is identical to that of the (Pb,Cu) ( Sr,Eu)z( Eu,Ce)&uzO, compound, with similar lattice parameters [ 91. Nevertheless, our analysis of the Ce:Ln content in (Bi,Cu)Sr,LnCeCu,O,_d results in a Ce:Ln ratio remarkably different from that found in the lead-based superconductor, where [ Ce] / [ Ln + Ce] = 25% [ 9 1. From our energy-dispersive X-ray spectrometry investigations (EDS) using a Philips CM 30 ST transmission electron microscopes we deduce an approximate stoichiometry Bio.3Sr,,gHo,.2Ceo.gCu~.~O~_~,thus suggesting [Ce]/[HofCe]z50%. This ratio is in reasonable agreement with that obtained by simple valence-counting arguments (i.e. the replacement of y3+ by an isoelectric (HoCeOz)3+ block in (Bi~Cu)Sr*YCu~O,_~, see above), or with the optimum ratio for phase purity found in our synthesis experiments. Unlike the double Bi-0 layer compounds with the
Fig. 3. Schematic representation of the unit cells of the (Bi,Cu)Sr,YCuzO,_, 2-2-2”) compounds.
[20] (“l-2-1-2”) and the (Bi,CufSr~HoCeCu~O,_~ (“I-
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Bi sites fully occupied, (Bi,Cu) Sr2YCu207 _-zwas reported to exhibit low occupancy factors gx 0.3-0.4 for Bi and Cu, respectively, within the (Bi,Cu)-0 monolayer [ 201. Even lower occupancy factors gzO.2 were found for the lead sites in the Pb analogue of the “l-2-2-2” phase, (Pb,Cu) (Sr,Nd)2(Ho,Ce),Cu,O, [ 211. From our synthesis experiments and the EDS result, it seems safe to conclude that the (Bi,Cu)-0 layers in the Bi-based “l-2-2-2” compounds reported here must also show a low occupancy at the metal sites, of the order of gz0.3. Transmission electron microscopic investigations have additionally been used to check the presence of ordered defects, stacking faults, or a superstructure due to the suggested low occupancy of the (Bi,Cu) sites. Neither selected-area electron-diffraction patterns (SAED, see fig. 4a) nor high-resolution images (HREM, see fig. 4b) show any kind of superstruc-
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November
1992
ture. In the HREM images, a regular spacing without stacking faults of the different layers can be observed. The inset of fig. 4b shows a respective contrast simulation, using an occupancy factor g=O.3 for the (Bi,Cu) sites. None of the as-sintered samples of the (Bi,Cu)Sr,LnCeCu,Og_a series investigated here were superconducting above 1.9 K as evidenced by susceptibility measurements with a SQUID magnetometer. It can be excluded that the absence of superconductivity in the compounds reported here is particularly related to the low occupancy of Bi and Cu within the (Bi,Cu) layers. The compounds (Bi,Cu)Sr2YCu207_-z [20] and (Pb,Cu)(Sr,Nd)z(Ho,Ce),Cu20, [ 211 with similarly low occupancies of (Bi,Cu) and (Pb,Cu) (see above) do show superconductivity, with T, E 68 K [ 201 and T, = 10 K [ 2 11, respectively.
Fig. 4. Left picture: Selected-area electron-diffraction pattern (SAED) of a (Bi,Cu)Sr,HoCeCu,O,_, sample, taken with the electron beam parallel to the [ 1001 direction. The (020) and the (0’0’12) reflections are indicated by arrows. Right picture: Representative HREM image of a (Bi,Cu)Sr,HoCeCu,0,_6 fragment. The crystallographic orientation with the [ 1001 direction normal to the image plane is illustrated by a sketch of the “1-2-2-2” unit cell in the inset. A corresponding contrast simulation, using an occupancy factor g=O.3 of the (Bi,Cu) sites to account for the EDS results (see text), is also shown in an inset to the HREM image. The HoCeOz blocks are indicated by “HC”, while the positions of the other atomic layers are shown in the sketch of the “1-2-2-2” unit cell. The white arrows in the HREM image show the [ f i 0] shift of the (Bi,Cu),,5SrCuOs.s blocks in the [ 1001 projection (see text and figs. 1 and 3).
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From the isostructural (Pb,Cu) ( Sr,Nd)2( Ho,Ce)&uzO, [ 2 1 ] and the closely related double Bi-0 layer compounds BizSr,(Ln,Ce)zCuzO,O [2], it is known that the application of 80-90 atm oxygen pressure at annealing temperatures between 500600°C induces superconductivity in the considered compounds as a result of an increased oxygen concentration and thus a larger hole density per [ Cu-0] unit, compared to samples prepared at ambient oxygen pressure. From this analogy, it seems almost that superconducting (Bi,Cu)Sr,Lncertain CeCuz09_6 can be obtained by an appropriate heat treatment under high oxygen pressure for certain choices of Ln, with critical temperatures up to T, = 30 K. Such attempts to induce superconductivity in these new Bi-based “l-2-2-2” cuprates are underway.
Acknowledgement We would like to thank S. Siegrist for taking the Guinier photographs. This work was in part supported by the Schweizerische Nationalfonds zur Fordering der Wissenschaftlichen Forschung.
References [I ] Y. Tokura, H. Takagi and S. Uchida, Nature 337 (1989) 345. [2] Y. Tokura, T. Arima, H. Takagi, S. Uchida, T. Ishigaki, H. Asano, R. Beyers, AI. Nazzal, P. Lacorre and J.B. Torrance, Nature 342 (1989) 890. [ 31 J. Akimitsu, S. Suzuki, M. Watanabe and H. Sawa, Japan. J. Appl. Phys. 27 (1988) L1859.
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[4] T. Mochiku, T. Nagashima, Y. Saito, M. Watahiki, H. Asano and Y. Fukai, Japan. J. Appl. Phys. 29 ( 1990) L588. [ 51 C. Martin, D. Bourgault, M. Hervieu, C. Michel, J. Provost and B. Raveau, Mod. Phys. Letters B 3 ( 1989) 993. [ 6 ] R.S. Liu, M. Hervieu, C. Michel, A. Maignan, C. Martin, B. Raveau and P.P. Edwards, Physica C I97 ( 1992) 13 1. [ 71 T. Mochiku, M. Osawa and H. Asano, Japan. J. .4ppl. Phys. 29 (1990) L1406. [8] A.L. Kharlanov, E.V. Antipov, L.M. Kovba, L.C. Akselrud, I.G. Muttik, A.A. Gippius and V.V. Moshalkov, Physica C 169 (1990) 469. [9] T. Maeda, K. Sakuyama, S. Koriyama. A. Ichinose, H. Yamauchi and S. Tanaka, Physica C 169 ( 1990 ) 133. [lo] A. Tokiwa, T. Oku, M. Nagoshi, D. Shindo, M. Kikuchi, T. Oikawa, K. Hiraga and Y. Syono, Physica C 172 ( 1990) 155. [ 1 I ] T. Wada, A. Ichinose, H. Yamauchi and S. Tanaka, Advances in superconductivity III (Springer, Tokyo, I99 I ). [ 121 H. Sawa, K. Obara, J. Akimitsu, Y. Matusi and S. Horiuchi, J. Phys. Sot. Japan 58 (1989) 2252. [ 131 T. Wada, A. Ichinose, H. Yamauchi and S. Tanaka, Physica c 171 (1990) 344. [ 141 T. Wada, A. Nara, A. Ichinose, H. Yamauchi and S. Tanaka. PhysicaC 192 (1992) 181. [ 151 L. Rukang, Z. Yingje, X. Cheng, C. Zuyao, Q. Yitai and F. Chengao, J. Solid State Chem. 94 ( 1991 ) 206. [ 161 L. Rukang, R.K. Kremer and J. Maier, announced in: HighT, Update 6 No. 10 (1992). [ 171 H. Maeda. Y. Tanaka, M. Fukutomi and T. Asano. Japan. J. Appl. Phys. 27 ( 1988) L209. [ 181 A. Schilling, F. Hulliger, S. Samarappuli and H.R. Ott, PhysicaC 185-189 (1991) 659; 1. Schilling, F. Hulliger, S. Samarappuli and H.R. Ott, Mater. Letters 11 ( 199 1) 2 17. [ 191 A. Schilling, Doctoral Thesis, ETH Zurich, Switzerland (1992).
[ 201 A. Ehmann, S. Kemmler-Sack,
S. Msch, M. Schlichenmaier, W. Wischert, P. Zoller, T. Nissel and R.P. Huebener, Physica C 198 (1992) 1. [ 211 T. Maeda, N. Sakai, F. Izumi, T. Wada. H. Yamauchi, H. Asano and S. Tanaka, Physica C 193 ( 1992) 73.
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New cuprates with (Bi,Cu ) -0 monolayers: (Bi,Cu) Sr,LnCeCu209_8 A. Schilling,
J.D. Guo, M. Cantoni, F. Hulliger, B. Xue and H.R. Ott
Laboratorium fiir Festkiirperphysik, ETH Hiinggerberg, Zurich, Switzerland Received
10 August
1992
New Bi-based cuprates with the “l-2-2-2” structure have been synthesized. The tetragonal structure of (Bi,Cu)Sr,LnCeCuzOg_, (space group 14/mmm) can be derived from that of the recently discovered superconductors (Bi,Cu)SrZYCuzO,_,, in which Y separates the CuO, layers responsible for the occurrence of superconductivity. When Y is replaced by fluorite-type blocks, LnCeOz (with Ln=Y, Sm, Eu, Gd, Dy and Ho), and the residual upper half of each unit cell is shifted by [f t 01, the resulting (Bi,Cu)Sr,LnCeCu20,_, are isostructural to the superconductor (Pb,Cu) (Sr,Eu)z(Eu,Ce)ZCuzOg_,. The compounds (Bi,Cu)Sr,YCu@_. and (Bi,Cu)Sr,LnCeCu20r,_6 with (Bi,Cu)-0 monolayers can be considered as sister series analogous to the Bi&,CaCu,Os and the Bi,Sr,(Ln,Ce)zCuzO,, compounds, which both contain double Bi-0 layers. A quantitative analysis of almost-single-phase material for Ln = Ho with respect to cation contents by EDS techniques suggests an approximate stoichiometry Bi,.,Sr,.,Ho,.2Ce0.9Cu2.209_-6. When synthesized at ambient pressure, the investigated samples are not superconducting above T= I .9 K, for any choice of Ln.
1. Introduction Since the discovery of (Nd,Ce),CuO,_,, Bi,Sr,(Ln,Ce)zCu20,0 and T12Ba2(Ln,Ce)2Cu20,0 by Tokura and co-workers [ 1,2], many other cuprates containing fluorite-type ( Ln,Ce)zO, blocks have been reported in the literature: (Ln,Ce, Sr)ZCu04--6 [3], (Tl,Pb) (Sr,La),(Ln,Ce),Cuz09 141, TlBa2(Eu,Ce)Xu209+6 ]5,61, PbzSrl(BE,Ce),CujOlo [7,81, (Pb,Cu)(Sr,Eu)2(Eu,Ce)2Cu20, [91, Pb(Ba,Sr)2(Ln,Ce)2Cu30, [lOI, (Pb,Cu ) Srz(Ho,Ce)&u20i1 [ll I, (Ln,Ce)2(Ba,Ln)zCu3010 1121, (Ln,Ce),Sr,Cu,(Cu,M)O,+,,+, (n= l-3, M=Cu, Fe, Al or Pb) [ 13,141, MSrz(Ln,Ce)2Cu20y (M=Ta,NborTi) [15],andGaSrz(Y,Ce)ZCu209_-s [ 161. Any oxocuprate with two CuOz layers in its structure separated by Ca, (Ca,Sr), (Ln,Ca) or Ln (including yttrium) can be considered as a candidate for the application of a general “recipe” for designing compounds containing (Ln,Ce),O, blocks, as illustrated in fig. 1. The former structural elements can be replaced by an isoelectric ( Ln,Ce),Oz block, shifting the residual upper half of each unit cell by [ 4 4 0] (see fig. 1). With this type of sub0167-577x/92/$
05.00 0 1992 Elsevier Science Publishers
stitution, one obtains, e.g., BizSrz(Ln,Ce)zCuzO,o [ 2 ] from the compound Bi2SrzCaCuzOs [ 17 1. Here, the divalent Ca can, in principle, be replaced by a LnzOz block without Ce addition [ 18,191, although a partial substitution of Ln by Ce [ 2 ] or Th [ 18 ] seems to be favourable for the stabilization of the “22-2-2” structure. In this communication, we report the successful synthesis of compounds with the idealized formula (Bi,Cu)SrzLnCeCu209_6 (Ln=Y, Sm, Eu, Cd, Dy and Ho). Motivated by the work of Ehmann and coworkers [20], who reported the synthesis of superconductors with the composition ( Bi,Cu)Sr,YCuzO7_-z, we postulated the existence of related compounds according to the above described “recipe”. We expected that Y may ideally be replaced by a block ( Lnl_-xCeX)202 with xz 0.5 to account for the trivalency of the yttrium ion. A further argument supporting the idea of the existence of (Bi,Cu)SrzLnCeCuzOg_, cuprates originated in the structural analogy to the known thallium and lead-based compounds, (Tl,Pb)(Sr,La),(Ln,Ce)$&O, [4] and (Pb,Cu)(Sr,Eu)2(Eu,Ce)zCuz0,, [9], respectively.
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1992
@J 0 Fig. 1. Schematic representation of the “recipe” for designing Cu02 layers, replacing Ca, (Ca,Sr ) , (Ln,Ca) or Ln [ 19 1.
structures
2. Experimental details and results
The samples of nominal composition Bio,s_xSr2_yLnl+,Ce,_-r+yCu2.5--x09--6 were prepared by standard solid-state reaction techniques for several values of x, y, and z, and for Ln = Y, Sm, Eu, Gd, Dy and Ho. The metal oxides and Sr ( NO3 )2 were mixed and heated at 800°C for 1 h in air. The resulting materials were then ground and pressed into pellets, and sintered in air at 970°C during 5 h. Finally, the samples were re-ground and sintered at 980°C for 1 h. We observed that the reaction temperatures could be raised up to 1060°C without melting the samples.
Bio.dhHol
Fig. 2. X-ray diffraction
142
pattern
containing
a fluorite-like
(Ln,Ce),Or
block (B) between two
However, the higher synthesis temperatures did neither substantially improve the sample quality, nor was the amount of parasitic phases reduced. We realized that experiments with even stoichiomen-y (i.e. x=y=z=O) did not result in phase-pure samples, although a large amount of the “l-2-2-2” phase was always detected. Further experiments for Ln = Ho showed that nominal compositions around x=0.1, y=O and ~~0.1 yielded samples of the desired quality. On Guinier photographs taken with Cu Ku, radiation, several weak unidentified lines due to parasitic phases were still present. However, on the powder diffraction pattern shown in fig. 2, those lines
.iCeo.&~2.409-6
of the sample with nominal
composition
Bio.4SrzHo,.,Ceo.sCu2.409_a,
taken with Cu Ku, radiation.
MATERIALS LETTERS
Volume 15, number 3
Table 1 Lattice parameters of the “1-2-2-2” phase for various Ln. The nominal composition was Bi,.,SrzLnCeCu2.509_d. except for Ln=Sm (Bio.4SrzSm,.zCeo.sCuz.409_a) and HO (2) (Bio4Sr,Ho,.,Ceo.9Cu~.~O~_~). Note that the a and c values in Cu 2.5_ X0 9_ 8depend on the real values Bi,,-~r2-vLn,+iCel-r+v of x, y, and z of the “l-2-2-2” phase in the respective samples (see text f
_
Ln
a (At
C(A)
cla
Y
3.8272(2) 3.8543(2) 3.8502(2) 3.8475(2) 3.8347(2) 3.8206(3) 3.8228(2)
28.857(2) 29.005(2) 28985(Z) 28.974(2) 28.895(3) 28.933(3) 28.979(2)
7.5399(10) 7.5253(9) 7.5283(7) 7.5305(7) 7.5.585( 10) 7.5728( 13) 7.5805(7)
Sm EU
Gd DY Ho (I) Ho (2)
are hardly visible because of their low intensity, compared to the lines belonging to the “1-2-2-2” phase. The lattice parameters, for samples with Ln=Y, Sm, Eu, Gd, Dy and Ho, are given in table 1. The dependence of a and c on the radius of the trivalent Ln3’ ion is monotonic but not exactly linear. Since the stoichiomet~ parameters x, y and z are not yet known for all samples on a microscopic scale (except for Ln = Ho, see below), a systematic interpretation of the lattice parameters as a function of Ln is dif-
November 1992
licult at present. The clear difference in the lattice parameters for the compounds with Ln=Ho and Ln=Y, i.e. with two elements of similar ionic radii in eight-coordination, may indicate that the respective x, y and z values differ markedly in both compounds. Fig. 3 shows a schematic representation of the unit cell of the (Bi,Cu)Sr~LnCeCu~O~_~ com~unds. The cell is body-centered tetragonal (space group 141 mmm). The structure is identical to that of the (Pb,Cu) ( Sr,Eu)z( Eu,Ce)&uzO, compound, with similar lattice parameters [ 91. Nevertheless, our analysis of the Ce:Ln content in (Bi,Cu)Sr,LnCeCu,O,_d results in a Ce:Ln ratio remarkably different from that found in the lead-based superconductor, where [ Ce] / [ Ln + Ce] = 25% [ 9 1. From our energy-dispersive X-ray spectrometry investigations (EDS) using a Philips CM 30 ST transmission electron microscopes we deduce an approximate stoichiometry Bio.3Sr,,gHo,.2Ceo.gCu~.~O~_~,thus suggesting [Ce]/[HofCe]z50%. This ratio is in reasonable agreement with that obtained by simple valence-counting arguments (i.e. the replacement of y3+ by an isoelectric (HoCeOz)3+ block in (Bi~Cu)Sr*YCu~O,_~, see above), or with the optimum ratio for phase purity found in our synthesis experiments. Unlike the double Bi-0 layer compounds with the
Fig. 3. Schematic representation of the unit cells of the (Bi,Cu)Sr,YCuzO,_, 2-2-2”) compounds.
[20] (“l-2-1-2”) and the (Bi,CufSr~HoCeCu~O,_~ (“I-
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Bi sites fully occupied, (Bi,Cu) Sr2YCu207 _-zwas reported to exhibit low occupancy factors gx 0.3-0.4 for Bi and Cu, respectively, within the (Bi,Cu)-0 monolayer [ 201. Even lower occupancy factors gzO.2 were found for the lead sites in the Pb analogue of the “l-2-2-2” phase, (Pb,Cu) (Sr,Nd)2(Ho,Ce),Cu,O, [ 211. From our synthesis experiments and the EDS result, it seems safe to conclude that the (Bi,Cu)-0 layers in the Bi-based “l-2-2-2” compounds reported here must also show a low occupancy at the metal sites, of the order of gz0.3. Transmission electron microscopic investigations have additionally been used to check the presence of ordered defects, stacking faults, or a superstructure due to the suggested low occupancy of the (Bi,Cu) sites. Neither selected-area electron-diffraction patterns (SAED, see fig. 4a) nor high-resolution images (HREM, see fig. 4b) show any kind of superstruc-
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November
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ture. In the HREM images, a regular spacing without stacking faults of the different layers can be observed. The inset of fig. 4b shows a respective contrast simulation, using an occupancy factor g=O.3 for the (Bi,Cu) sites. None of the as-sintered samples of the (Bi,Cu)Sr,LnCeCu,Og_a series investigated here were superconducting above 1.9 K as evidenced by susceptibility measurements with a SQUID magnetometer. It can be excluded that the absence of superconductivity in the compounds reported here is particularly related to the low occupancy of Bi and Cu within the (Bi,Cu) layers. The compounds (Bi,Cu)Sr2YCu207_-z [20] and (Pb,Cu)(Sr,Nd)z(Ho,Ce),Cu20, [ 211 with similarly low occupancies of (Bi,Cu) and (Pb,Cu) (see above) do show superconductivity, with T, E 68 K [ 201 and T, = 10 K [ 2 11, respectively.
Fig. 4. Left picture: Selected-area electron-diffraction pattern (SAED) of a (Bi,Cu)Sr,HoCeCu,O,_, sample, taken with the electron beam parallel to the [ 1001 direction. The (020) and the (0’0’12) reflections are indicated by arrows. Right picture: Representative HREM image of a (Bi,Cu)Sr,HoCeCu,0,_6 fragment. The crystallographic orientation with the [ 1001 direction normal to the image plane is illustrated by a sketch of the “1-2-2-2” unit cell in the inset. A corresponding contrast simulation, using an occupancy factor g=O.3 of the (Bi,Cu) sites to account for the EDS results (see text), is also shown in an inset to the HREM image. The HoCeOz blocks are indicated by “HC”, while the positions of the other atomic layers are shown in the sketch of the “1-2-2-2” unit cell. The white arrows in the HREM image show the [ f i 0] shift of the (Bi,Cu),,5SrCuOs.s blocks in the [ 1001 projection (see text and figs. 1 and 3).
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From the isostructural (Pb,Cu) ( Sr,Nd)2( Ho,Ce)&uzO, [ 2 1 ] and the closely related double Bi-0 layer compounds BizSr,(Ln,Ce)zCuzO,O [2], it is known that the application of 80-90 atm oxygen pressure at annealing temperatures between 500600°C induces superconductivity in the considered compounds as a result of an increased oxygen concentration and thus a larger hole density per [ Cu-0] unit, compared to samples prepared at ambient oxygen pressure. From this analogy, it seems almost that superconducting (Bi,Cu)Sr,Lncertain CeCuz09_6 can be obtained by an appropriate heat treatment under high oxygen pressure for certain choices of Ln, with critical temperatures up to T, = 30 K. Such attempts to induce superconductivity in these new Bi-based “l-2-2-2” cuprates are underway.
Acknowledgement We would like to thank S. Siegrist for taking the Guinier photographs. This work was in part supported by the Schweizerische Nationalfonds zur Fordering der Wissenschaftlichen Forschung.
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