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Crystal structure of Bi2Sr2CuO6: A structure based on periodic crystallographic shear planes in the “2201” structure
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0038 - 1098/93 $6.00+. 00 Pergamon Press Ltd
Solid State Communications, Vol. 86, No 4, pp. 227-230, 1993 Printed in Great Britain.
CRYSTAL STRUCTURE OF Bi2Sr2CuO6: A STRUCTURE BASED ON PERIODIC CRYSTALLOGRAPHIC SHEAR PLANES IN THE "2201" STRUCTURE
J. Darriet, F. Weill, B. Darriet, X. F. Zhang* and J. Etourneau Laboratoire de Chimie du Solide du CNRS, Universit6 de Bordeaux I, 351 Cours de la Lib6ration, 33405 Talence Cedex, France. *EMAT - Universiteit Antwerpen (RUCA), Groenenborgerlaan, 171, B 2020 Antwerpen, Belgie.
(Received 5january 1993 by D. Van Dyck) The crystal structure of the insulating phase Bi2Sr2CuO 6 is reported. The refinements were performed on X-ray and neutron powder diffraction data. The R values were Ri=10.56% (Rn=6.67%, Rwp=8.40% ) for X-ray and Ri=7.47% (Rn=5.01%, RwD=6.45% ) ,f,dr neutron data. The structure is derived from that of thdpseudo-tetra~onal "2201' phase by a periodic crystallographic shear parallel to the Ctetr axis. Slabs of 2201" type with a thickness of eight octaneara are then formed ~,hich are parallel to (001). The 13 angle of the monoclinic cell results from the periodic crystallographic shear which is a simple translation along the c axis. This structural model has been confirmed by HRTEM where the slabs of "2201" have been observed.
X-ray powder diffraction patterns were registered with a Philips PWl050 goniometer using CuKa radiation. Intensities were measured between 5°<2o<120 ° and scanned with a step size of 0.02 °. Neutron powder diffraction data were collected using the diffractometer at the ClaN Grenoble using a neutron wavelength of 1.344A. The angular range 10° < 2o < 90° was scanned with a step size of 0.1 °. The diffraction data were analyzed using the Rietveld's method [7]. The peaks were assumed to have a pseudoVoigtian line profile for the X-ray data and a Gaussian line profile for the neutron data. Electron diffraction patterns were obtained with a JEOL 2000 FX microscope, the samples were prepared by crushing in alcohol. If the observation of images was generally easy, a special preparation of the samples was necessary in order to observe particular planes especially the (a.c) plane as it will be shown later. The samples were prepared by dispersion of the particles in an epoxy resin and oriented by centrifugation, then thin slabs were cut using an ultramicrotom. High resolution images were obtained with a J E O L 4000 EX microscope.
INTRODUCTION The Bi2+xSr2.yCuO6+ 6 system actually contains two distinct phases with closely related compositions [1, 2 and references therein]. The first one corresponds to a solid solution and will be refered to as "2201" phase. This phase shows superconductivity and was itially mentioned as being the n= 1 member of the general series A2B2Can_lCUnO4+2n. The structure is one-dimensionally modulated and the modulation vector decreases with increasing x [3,4]. The second compound of the system is only single phase at composition very close to 2 2 1 [1,2,5] though this composition has been the subject of controversy [3,4]. This paper deals with the crystal structure of this l~hase which has been determined by neutron and ,'~-ray powder diffraction using a profile analysis method and confirmed by HRTEM observations. This phase will be refered to as Bi2Sr2CuO 6 (m): m meaning monoclinic. EXPERIMENTAL Bi2Sr2CuO 6 (m) has been observed for the first time by R.S. Roth et al. [6] but the preparation were performed in air or under oxygen flow and then it was difficult to control the oxygen content. A mixture with the "2201" phase is often observed on the X-ray powder spectra. In order to avoid this problem the reaction were performed in argon flow or under vacuum [5]. The optima conditions to prepare Bi2Sr2CuO 6 (m) as a single phase is to start from stoichlometric amounts of Bi20 3 and Sr2CuO 3 in the 1:1 molar ratio and to heat at 800 C during 24h. Sr2CuO 3 was prepared by the reaction of 2SrCO3+CuO at 1000 C - 1100 C for 24h. The phase composition of the reacted samples was detected by powder X-ray diffraction and confirmed by chemical analysis (inducting coupling plasma) at the Service Central d'Analyse du CNRS. The experimental atomic percentages: Bi=56.1%, Sr=23.7% and Cu=8.7% agree very well with the Bi2Sr2CuO 6 formula. The corresponding theoretical percentages are Bi=55.5%, Sr =23.3% and Cu=8.4%.
RESULTS and DISCUSSION Using electron diffraction, it was possible to deduce the cell parameters and to index the X-ray pattern [1,2,5]. The symmetry is monoclipic (C face-centered ) with a=24.451(5)A, b=5.425(2)A, c--21.954(5)A anti 13= 105.41(1). The experimental density d = 7.07g/cm j is in ~agreement with the calculated value of d=7.13 g/cm a for Z = 16 formula units Bi2Sr2CuO 6 per unit cell. There is a close relationship between the cell paramete[s of the pseudo-tetragonal "2201" phase (a-~b~5.5A) and those of Bi2Sr2CuO6(m): amonocl" =Ctetr" , bmonocl" ~atetr " and Cmonocl" ~4.atetr ' The electron diffraction pattern corresponding to the [010] zone axis con,firm the existence of a superstructure along the c direction compared to the pseudo-tetragonal phase (fig.la). The other electron diffraction patterns along the [100] and [001] zone axes are given in [5]. Crystallographic considerations of the 227
228
CRYSTAL STRUCTURE OF Bi2Sr2CuO 6
:
'
m'
i.~ . . . . .
21
:!
2Ag:m.,
2 '92~
•
Fig. 1 -(a) Selected area diffraction pattern along [010] zone axis, (b) Corresponding HREM (the arrow indicates a large slab) (c) Details of regular slabs of eight octahedra.
structutal model of "2201" phase and the above relationships between the cell parameters allowed us to propose a structural model for the structure of Bi2Sr2CuO 6 (m). First of all, the model was tested with the C2/m space group and for the X-ray spectrum for which the contribution of the heavy atoms (cations) is the most important. Then, the atomic positions of the oxygen have been refined from the neutron data. The atomic coordinates, given in Table I, corresponds to the final refinement (X-ray) where the oxygen positions were fixed as deduced from the neutron data. This procedure was dictated by the fact that there was too many variable parameters to be refined compared to the number of experimental points. The B factors have been also fixed to 1.5• 2 for the oxygen and 1.0/~2 for all the cations. With these conditions, the refinement resulted in R-factors of Ri=10.56%,
Vol. 86, No. 4
Rp=6.67%,Rwo=8.40% for X-ray and Ri=7.47%, R~=5.01% anti Rwo=6.45% for neutron data. The observed, calculated gnd difference diffraction profiles of Bi2Sr2CuO 6 (m) are given in figure 2. It can be observedthat all the peaks at small values of 20 are interpreted. The atomic parameters are given in Table I. A projection of the structure on the (010) plane is shown in figure 3. The strontium atoms occupy similar positions in the two structures and have been omitted to clarify the figure. The similitude with the "2201" structure is obvious. The structure of Bi2Sr2CuO 6 (m) derives from that of "2201" by periodic crystallographic shears parallel to the Ctetr" axis of "2201". The crystallographic shear is a simple translation of about Ctetr/5 along this axis which explains the value of the 13 angle of the monoclinic cell. Periodic slabs of "2201" hase with a thickness of eight octahedra are then rmed (fig.3) These slabs have been observed by electron microscopy. Low resolution images (fig.4) clearly show these slabs are regular, whereas higher resolution images (fig.l) prove that the sequence corresponds to that of Bi2Sr2CuO 6 (m) structure determined by X-ray and neutron refinements. The unit cell parameters calculated from these high resolution images are in good agreement with those given above. Nevertheless some defects have been observed. The arrows in figure lb indicates a larger slab than the normal periodic ones. This larger slab corresponds indeed to a thickness of nine octahedra instead of eight ones. Similar defects with only seven octahedra have been also observed, but these slabs either larger or narrower than the normal ones are randomly distributed in the material. A structure with regular slabs of seven octahedra has been reported by Hervieu et al. in the Bi14 + xS.r7Ba7Cu7:xO42 + x/2 system [8]. The observation on me images of these defaults is consistent with the presence of streaking or even incommensurate modulation along the c direction pointed out on some of the electron diffraction patterns given in [9]. Furthermore Garcia-Alvarado et al. have shown that incommensurate modulation is strongly dependent on both the strontium and the
Table I - Atomic coordinates* for Bi2Sr2CuO 6 (see text for the oxygen positions). Y
Y Bil
Bi2 Bi3 Bi4 Bi5 Bi6 Bi7 Bi8 Srl Sr2 Sr3 Sr4 Sr5 Sr6 Sr7 Sr8 Cul Cu2 Cu3 Cu4
.2522 .1533 O1 .2518 .1797 02 .0701(6) .2510 .2039 03 .1768(7) l/2 .2505 .2210 04 .3070(6) 0 1/4 1/4 05 .4286(7) l/2 0 .2598 .5589(7) 06 0 1/2 .0881 07 .6683(7) 1/2 0 .1125 08 .7986(7) 0 1/2 .1401 09 .0763(15) 1/2 0 .1714 O1( .1967(14) 0 1/2 .1820 Dll .3073(15) 1/2 .2081 0 O12 .4297(14) 0 0 .0597 O12 .5582(17) 1/2 1/2 -.0276 O14 .6853(17) 0 -.0035 0 O15 .8082(16) 1/2 1/2 .0313 O1( 1/2 .0556(13) .0626 0 Oli .0554(23) 0 1/2 .0717 O1~ .1785(21) 1/2 .1953(23) .0998 0 O1~ .2940(19) 0 .2113(22) 1/2 .1226 02( .4319(25) 1/2 .2264(18) *The B factors have been fixed :Boxy" = 1.5~ 2 and Bcat.= 1.0~ 2
.1420(7) -.0275(6) -.0030(7) .0249(7) .0529(7) .0766(6) .0982(7) .1227(7) .2455(13) .1166(14) .1400(16) .1702(15) .1896(16) .2173(15) .2396(15) .0780(12) .1659(25)
1/2 0
-.0705(8)
.0010 .1167 .2404 .3646
1/2
.0604 .1911 .3067 .4305 .5512 .6850 .8001 .0531 0715 .1931 .3046 .4478 .5415 .6640 .7956
Vol. 86, No. 4
CRYSTAL STRUCTURE OF Bi2Sr2CuO6
229
18253
,, .u,',,"' ,' ,'! ,",' !,,!~
10
20
--',,--:.,,
.........
, .........
30
40
, .........
50
, .........
60
, .........
, .........
70
80
, .........
90
, .........
100
, .........
110
'~[~e) 120
Bi2Sr2CuO~ (X ray)
118287
0 |
I 11 Ill IIIIIIIIIIIIIIIIIWIIIHIMWlIIIIINIIII.d-....riII ~i~
(~e)
i--,~IIiIII
I l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l J|lllllllllll|llllilllrlllll
20
30
40
50
60
70
flO
Bi2Sr2CuO6 (Neutron)
Fig.2 -The observed (...), calculated (--) and difference X-ray and neutron profiles of Bi2Sr2CuO 6 (m). oxygen contents [10]. In the unit cell, there are forty independent positions for all the atoms and hence it is clear that the
accuracy of the results is limited especially for the oxygen atoms. Indeed, the contribution of the oxygen in the refinement is small compared to that of the
230
CRYSTAL STRUCTURE OF Bi2Sr2CuO 6
Vol. 86, N o 4
Fig.3 -Projections of the structure perpendicular to the b axis of Bi2Sr2CuO 6 (m), the z axis is horizontal. (Cu: bold octahedron, Bi: shaded environment).
Fig.4 -Low Resolution image of Bi2Sr2CuO6(m) taken with the [010] incidence, showing the regular thickness of the slabs.
heavy atoms as bismuth and strontium. The positions of the oxygen in the shear plane have been deduced by geometrical considerations and then we cannot exclude an other distribution. Nevertheless, the repartition of the different cations (Bi,Sr,Cu) in the unit cell is accurate enough and confirmes the proposed structural model. The mechanism of crystallographic shear is well known and it was introduced for the first time by Wadsley [11]. It was the starting point for understanding various aspects of non stoichlometry in oxides (see the TiO2. 6 system for example). The same phenomenon has been observed in the structure of Bi2Sr2CuO 6 (m) in which the crystallographic shears are periodic and all oriented in the same direction. The
crystallographic shears interrupt the connections of the flat CuO2 planes, which are assumed to be responsible for the superconductivity of the cuprite oxides. To our knowledge, it is the first time that the refinement of a structure demonstrate the presence of periodic shear planes in a structure related to the Bi-Sr-Cu-O high Tc superconductors. Acknowledgments We thank Dr J. L. Soubeyroux for the neutron diffraction measurement. Part of this work (X.F. Zhang) has been performed with financial help of IUAP 48. This work has been supported by a frenchbelgium scientific collaboration (Tournesol program).
REFERENCES
1 B.C. Chakoumakos, P. S. Ebey, B. C. Sales and E. Sonder, J. Mater. Res. 4, 767 (1989). 2 R.S. Roth, C. J. Rawn and L. A. Bendersky, J. Mater. Res. 5, 46 (1990). 3 Y. Ikeda, H. Ito, S. Shimomura, Y. Oue, K. Inaba, Z. Hiroi and M. Takano, Physica C 159, 93 (1989). 4 Z. Hiroi, Y. Ikeda, M. Takano and Y. Bando, J. Mater. Res. 6, 435 (1991). 5 F. Weill, B. Darriet, M. Ducau, J. Darriet and J. Etourneau, Solid State Comm. 77, 679 (1991).
6 R.S. Roth, Am. Phys. Soc. Meeting, New Orleans, LA, March (1988). 7 H.M. Rietveld, J. Appl. Cryst. 2, 65 (1969). 8 M. Hervieu, C. Michel, A. Q. Pham and B.Raveau, J. Solid State Chem. in press. 9 Y. Matsui, S. Takekawa and H. Nozaki and A. Umezono, Jpn. J. Appl. Phys. 28, 602 (1989). 10 F. Gareia-Alvarado, E. Moran and A. AlarioFranco, J. Solid State Chem. 98, 245 (1992). 11 R. S. Roth and A. D. Wadsley, Acta Cryst. 19, 42 (1965).
0038 - 1098/93 $6.00+. 00 Pergamon Press Ltd
Solid State Communications, Vol. 86, No 4, pp. 227-230, 1993 Printed in Great Britain.
CRYSTAL STRUCTURE OF Bi2Sr2CuO6: A STRUCTURE BASED ON PERIODIC CRYSTALLOGRAPHIC SHEAR PLANES IN THE "2201" STRUCTURE
J. Darriet, F. Weill, B. Darriet, X. F. Zhang* and J. Etourneau Laboratoire de Chimie du Solide du CNRS, Universit6 de Bordeaux I, 351 Cours de la Lib6ration, 33405 Talence Cedex, France. *EMAT - Universiteit Antwerpen (RUCA), Groenenborgerlaan, 171, B 2020 Antwerpen, Belgie.
(Received 5january 1993 by D. Van Dyck) The crystal structure of the insulating phase Bi2Sr2CuO 6 is reported. The refinements were performed on X-ray and neutron powder diffraction data. The R values were Ri=10.56% (Rn=6.67%, Rwp=8.40% ) for X-ray and Ri=7.47% (Rn=5.01%, RwD=6.45% ) ,f,dr neutron data. The structure is derived from that of thdpseudo-tetra~onal "2201' phase by a periodic crystallographic shear parallel to the Ctetr axis. Slabs of 2201" type with a thickness of eight octaneara are then formed ~,hich are parallel to (001). The 13 angle of the monoclinic cell results from the periodic crystallographic shear which is a simple translation along the c axis. This structural model has been confirmed by HRTEM where the slabs of "2201" have been observed.
X-ray powder diffraction patterns were registered with a Philips PWl050 goniometer using CuKa radiation. Intensities were measured between 5°<2o<120 ° and scanned with a step size of 0.02 °. Neutron powder diffraction data were collected using the diffractometer at the ClaN Grenoble using a neutron wavelength of 1.344A. The angular range 10° < 2o < 90° was scanned with a step size of 0.1 °. The diffraction data were analyzed using the Rietveld's method [7]. The peaks were assumed to have a pseudoVoigtian line profile for the X-ray data and a Gaussian line profile for the neutron data. Electron diffraction patterns were obtained with a JEOL 2000 FX microscope, the samples were prepared by crushing in alcohol. If the observation of images was generally easy, a special preparation of the samples was necessary in order to observe particular planes especially the (a.c) plane as it will be shown later. The samples were prepared by dispersion of the particles in an epoxy resin and oriented by centrifugation, then thin slabs were cut using an ultramicrotom. High resolution images were obtained with a J E O L 4000 EX microscope.
INTRODUCTION The Bi2+xSr2.yCuO6+ 6 system actually contains two distinct phases with closely related compositions [1, 2 and references therein]. The first one corresponds to a solid solution and will be refered to as "2201" phase. This phase shows superconductivity and was itially mentioned as being the n= 1 member of the general series A2B2Can_lCUnO4+2n. The structure is one-dimensionally modulated and the modulation vector decreases with increasing x [3,4]. The second compound of the system is only single phase at composition very close to 2 2 1 [1,2,5] though this composition has been the subject of controversy [3,4]. This paper deals with the crystal structure of this l~hase which has been determined by neutron and ,'~-ray powder diffraction using a profile analysis method and confirmed by HRTEM observations. This phase will be refered to as Bi2Sr2CuO 6 (m): m meaning monoclinic. EXPERIMENTAL Bi2Sr2CuO 6 (m) has been observed for the first time by R.S. Roth et al. [6] but the preparation were performed in air or under oxygen flow and then it was difficult to control the oxygen content. A mixture with the "2201" phase is often observed on the X-ray powder spectra. In order to avoid this problem the reaction were performed in argon flow or under vacuum [5]. The optima conditions to prepare Bi2Sr2CuO 6 (m) as a single phase is to start from stoichlometric amounts of Bi20 3 and Sr2CuO 3 in the 1:1 molar ratio and to heat at 800 C during 24h. Sr2CuO 3 was prepared by the reaction of 2SrCO3+CuO at 1000 C - 1100 C for 24h. The phase composition of the reacted samples was detected by powder X-ray diffraction and confirmed by chemical analysis (inducting coupling plasma) at the Service Central d'Analyse du CNRS. The experimental atomic percentages: Bi=56.1%, Sr=23.7% and Cu=8.7% agree very well with the Bi2Sr2CuO 6 formula. The corresponding theoretical percentages are Bi=55.5%, Sr =23.3% and Cu=8.4%.
RESULTS and DISCUSSION Using electron diffraction, it was possible to deduce the cell parameters and to index the X-ray pattern [1,2,5]. The symmetry is monoclipic (C face-centered ) with a=24.451(5)A, b=5.425(2)A, c--21.954(5)A anti 13= 105.41(1). The experimental density d = 7.07g/cm j is in ~agreement with the calculated value of d=7.13 g/cm a for Z = 16 formula units Bi2Sr2CuO 6 per unit cell. There is a close relationship between the cell paramete[s of the pseudo-tetragonal "2201" phase (a-~b~5.5A) and those of Bi2Sr2CuO6(m): amonocl" =Ctetr" , bmonocl" ~atetr " and Cmonocl" ~4.atetr ' The electron diffraction pattern corresponding to the [010] zone axis con,firm the existence of a superstructure along the c direction compared to the pseudo-tetragonal phase (fig.la). The other electron diffraction patterns along the [100] and [001] zone axes are given in [5]. Crystallographic considerations of the 227
228
CRYSTAL STRUCTURE OF Bi2Sr2CuO 6
:
'
m'
i.~ . . . . .
21
:!
2Ag:m.,
2 '92~
•
Fig. 1 -(a) Selected area diffraction pattern along [010] zone axis, (b) Corresponding HREM (the arrow indicates a large slab) (c) Details of regular slabs of eight octahedra.
structutal model of "2201" phase and the above relationships between the cell parameters allowed us to propose a structural model for the structure of Bi2Sr2CuO 6 (m). First of all, the model was tested with the C2/m space group and for the X-ray spectrum for which the contribution of the heavy atoms (cations) is the most important. Then, the atomic positions of the oxygen have been refined from the neutron data. The atomic coordinates, given in Table I, corresponds to the final refinement (X-ray) where the oxygen positions were fixed as deduced from the neutron data. This procedure was dictated by the fact that there was too many variable parameters to be refined compared to the number of experimental points. The B factors have been also fixed to 1.5• 2 for the oxygen and 1.0/~2 for all the cations. With these conditions, the refinement resulted in R-factors of Ri=10.56%,
Vol. 86, No. 4
Rp=6.67%,Rwo=8.40% for X-ray and Ri=7.47%, R~=5.01% anti Rwo=6.45% for neutron data. The observed, calculated gnd difference diffraction profiles of Bi2Sr2CuO 6 (m) are given in figure 2. It can be observedthat all the peaks at small values of 20 are interpreted. The atomic parameters are given in Table I. A projection of the structure on the (010) plane is shown in figure 3. The strontium atoms occupy similar positions in the two structures and have been omitted to clarify the figure. The similitude with the "2201" structure is obvious. The structure of Bi2Sr2CuO 6 (m) derives from that of "2201" by periodic crystallographic shears parallel to the Ctetr" axis of "2201". The crystallographic shear is a simple translation of about Ctetr/5 along this axis which explains the value of the 13 angle of the monoclinic cell. Periodic slabs of "2201" hase with a thickness of eight octahedra are then rmed (fig.3) These slabs have been observed by electron microscopy. Low resolution images (fig.4) clearly show these slabs are regular, whereas higher resolution images (fig.l) prove that the sequence corresponds to that of Bi2Sr2CuO 6 (m) structure determined by X-ray and neutron refinements. The unit cell parameters calculated from these high resolution images are in good agreement with those given above. Nevertheless some defects have been observed. The arrows in figure lb indicates a larger slab than the normal periodic ones. This larger slab corresponds indeed to a thickness of nine octahedra instead of eight ones. Similar defects with only seven octahedra have been also observed, but these slabs either larger or narrower than the normal ones are randomly distributed in the material. A structure with regular slabs of seven octahedra has been reported by Hervieu et al. in the Bi14 + xS.r7Ba7Cu7:xO42 + x/2 system [8]. The observation on me images of these defaults is consistent with the presence of streaking or even incommensurate modulation along the c direction pointed out on some of the electron diffraction patterns given in [9]. Furthermore Garcia-Alvarado et al. have shown that incommensurate modulation is strongly dependent on both the strontium and the
Table I - Atomic coordinates* for Bi2Sr2CuO 6 (see text for the oxygen positions). Y
Y Bil
Bi2 Bi3 Bi4 Bi5 Bi6 Bi7 Bi8 Srl Sr2 Sr3 Sr4 Sr5 Sr6 Sr7 Sr8 Cul Cu2 Cu3 Cu4
.2522 .1533 O1 .2518 .1797 02 .0701(6) .2510 .2039 03 .1768(7) l/2 .2505 .2210 04 .3070(6) 0 1/4 1/4 05 .4286(7) l/2 0 .2598 .5589(7) 06 0 1/2 .0881 07 .6683(7) 1/2 0 .1125 08 .7986(7) 0 1/2 .1401 09 .0763(15) 1/2 0 .1714 O1( .1967(14) 0 1/2 .1820 Dll .3073(15) 1/2 .2081 0 O12 .4297(14) 0 0 .0597 O12 .5582(17) 1/2 1/2 -.0276 O14 .6853(17) 0 -.0035 0 O15 .8082(16) 1/2 1/2 .0313 O1( 1/2 .0556(13) .0626 0 Oli .0554(23) 0 1/2 .0717 O1~ .1785(21) 1/2 .1953(23) .0998 0 O1~ .2940(19) 0 .2113(22) 1/2 .1226 02( .4319(25) 1/2 .2264(18) *The B factors have been fixed :Boxy" = 1.5~ 2 and Bcat.= 1.0~ 2
.1420(7) -.0275(6) -.0030(7) .0249(7) .0529(7) .0766(6) .0982(7) .1227(7) .2455(13) .1166(14) .1400(16) .1702(15) .1896(16) .2173(15) .2396(15) .0780(12) .1659(25)
1/2 0
-.0705(8)
.0010 .1167 .2404 .3646
1/2
.0604 .1911 .3067 .4305 .5512 .6850 .8001 .0531 0715 .1931 .3046 .4478 .5415 .6640 .7956
Vol. 86, No. 4
CRYSTAL STRUCTURE OF Bi2Sr2CuO6
229
18253
,, .u,',,"' ,' ,'! ,",' !,,!~
10
20
--',,--:.,,
.........
, .........
30
40
, .........
50
, .........
60
, .........
, .........
70
80
, .........
90
, .........
100
, .........
110
'~[~e) 120
Bi2Sr2CuO~ (X ray)
118287
0 |
I 11 Ill IIIIIIIIIIIIIIIIIWIIIHIMWlIIIIINIIII.d-....riII ~i~
(~e)
i--,~IIiIII
I l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l J|lllllllllll|llllilllrlllll
20
30
40
50
60
70
flO
Bi2Sr2CuO6 (Neutron)
Fig.2 -The observed (...), calculated (--) and difference X-ray and neutron profiles of Bi2Sr2CuO 6 (m). oxygen contents [10]. In the unit cell, there are forty independent positions for all the atoms and hence it is clear that the
accuracy of the results is limited especially for the oxygen atoms. Indeed, the contribution of the oxygen in the refinement is small compared to that of the
230
CRYSTAL STRUCTURE OF Bi2Sr2CuO 6
Vol. 86, N o 4
Fig.3 -Projections of the structure perpendicular to the b axis of Bi2Sr2CuO 6 (m), the z axis is horizontal. (Cu: bold octahedron, Bi: shaded environment).
Fig.4 -Low Resolution image of Bi2Sr2CuO6(m) taken with the [010] incidence, showing the regular thickness of the slabs.
heavy atoms as bismuth and strontium. The positions of the oxygen in the shear plane have been deduced by geometrical considerations and then we cannot exclude an other distribution. Nevertheless, the repartition of the different cations (Bi,Sr,Cu) in the unit cell is accurate enough and confirmes the proposed structural model. The mechanism of crystallographic shear is well known and it was introduced for the first time by Wadsley [11]. It was the starting point for understanding various aspects of non stoichlometry in oxides (see the TiO2. 6 system for example). The same phenomenon has been observed in the structure of Bi2Sr2CuO 6 (m) in which the crystallographic shears are periodic and all oriented in the same direction. The
crystallographic shears interrupt the connections of the flat CuO2 planes, which are assumed to be responsible for the superconductivity of the cuprite oxides. To our knowledge, it is the first time that the refinement of a structure demonstrate the presence of periodic shear planes in a structure related to the Bi-Sr-Cu-O high Tc superconductors. Acknowledgments We thank Dr J. L. Soubeyroux for the neutron diffraction measurement. Part of this work (X.F. Zhang) has been performed with financial help of IUAP 48. This work has been supported by a frenchbelgium scientific collaboration (Tournesol program).
REFERENCES
1 B.C. Chakoumakos, P. S. Ebey, B. C. Sales and E. Sonder, J. Mater. Res. 4, 767 (1989). 2 R.S. Roth, C. J. Rawn and L. A. Bendersky, J. Mater. Res. 5, 46 (1990). 3 Y. Ikeda, H. Ito, S. Shimomura, Y. Oue, K. Inaba, Z. Hiroi and M. Takano, Physica C 159, 93 (1989). 4 Z. Hiroi, Y. Ikeda, M. Takano and Y. Bando, J. Mater. Res. 6, 435 (1991). 5 F. Weill, B. Darriet, M. Ducau, J. Darriet and J. Etourneau, Solid State Comm. 77, 679 (1991).
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