Preparation and electron diffraction study of Bi2Sr2CuO6

t

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Solid State Communications, Vol. 77, No. 9, PP. 679-682, 1991. Printed in Great Britain.

0038-1098/9153.00+.00 Pergamon Press plc

PREPARATION AND ELECTRON DIFFRACTION STUDY OF Bi2Sr2CuO 6 F. Weill, B. Darriet, M. Ducau, J. Darriet and J. Etourneau Laboratoire de Chimie du Solide du CNRS, 351 Cours de la Lib&ation Universit~ de Bordeaux I , 33405 Talence Cedex France

( Received 13 November 1990 by P. Burlet )

The compound Bi2Sr2CuO 6 has been prepared by solid state reaction under inert gas flow (Ar). at 800"C. It has been observed that the control of the gas flow is a prnme necessity to prepare it as single phase. The same results have been obtained when the solidstate reaction are made under vacuum. Using electron diffraction and high resolution electron microscopy, it has been possible to deduce the cellparameters and to index tile x-ray patterr] . The symmet/-y is monoclinic (Ccentered) with a=24.451(5)A, b=5.425(2)A, c--21.954(5)Aand 13= 105.41(1) ° . The experimental density confirms the proposed unit-cell parameters and also that the compound does not contain excess of oxygen above the stoichiometric value of six oxygens per formula unit. These results are in good agreement with reference [13]. Bi2Sr2CuO 6 ,which is normally the n = l member of the SUl~erconducting series A2B2Can.lCunO4+2r ~ (A=Sr, Ba;B=Bi,TI), seems to be a different phase from that expected in this sernes.

INTRODUCTION

700°C and 750°C ; In the final step, the samples were heated at 800°C during 24 hours and furnace cooled to room temperature . When the reactions are made under vacuum, one take care to avoid the volatility of Bi O3 around. 800°C. by increasing, the reaction time to 482hours m the mtermedmte steps. Transmission electron diffraction (TEM) observations were performed in a Jeol 2000 FX microscope operating at 200 kV and using a side-entry double tilt angles of_+ 45*.

The phase in the Bi-Sr-Cu-O system which was first mentioned to show superconductivity , was originally reported by Michel et al. [1] with the composition Bi2Sr2Cu207+y . They proposed a Ccentered orthorhombic space group with the cell parameters : a=5.32A , b=26.6A and c=48.8A . Several authors have reinvestigated the system and observed that the composition of this phase corresponds in fact to the n = 1 member of the general series A2B2Can_lCUnO4+2n(A=Sr , Ba ; B--Bi , Tl)(n = 1 , 2 or 3)[2-5]. For this reason this phase has been called "2201" . I t has been shown that the real structure is a long period ineommensurately modulated superstructure. However, recent results on this system have shown that the true composition o f the superconducting p h a s e is actually non stoichiometric with a deficient SrO [6-11]. Recently Roth et al. [12] reported that the actual composition Bi2Sr2CuO 6 represented a phase which has a quite different x-ray pattern. However, they observed that small amounts of the "2201" and Sr14Cu24041 phases are always present in their products. H e r e , we report our results ,on the preparation and the electron diffraction study of Bi2Sr2CuO 6 .

RESULTS and DISCUSSION For all the preparations , the x-ray diffraction patterns appear to be identical (Table I) . The peaks characteristic of the "2201"phase are not observed in the limit of the sensibility of the x-ray. EspecialJy, the peaks (002) (d=12.4A) and (113)(d=3.46A) are absent . Our spectrum agrees very well with that of reference [13] except that some peaks are absent in,our case (see for example the intense peak at d = 4.072A in ([12]). If the product is now heated in air or in oxygen flow, one observed the formation of the "2201" phase. U p to now it has not been possible to prepare single crystals which could be investigated by x-ray. Samples have been prepared to be studied by transmission electron microscope (TEM). The diffraction patterns l(a) and l(b) , corresponding to small g values along two different zone axis , were pointed out by selected area diffraction (SAD) . Pattern l(a) consists mainly in a rectangular lattice (pseudo square) of [he basic spots. Along a line of strong spots such as c in the pattern l(a) , the main reflections are split into seven weak spots (1/8 of the f~ndamental vector). The direction B orthogonal to c (fig.l(a)) is common to both diffraction patterns l(a) and l(b). In the pattern l(b) this direction B is also orthogonal to a line of.strong spots. The pattern l ( c ) , which has an oblique lattice of basic spots , was obtained from the l(a) one by

EXPERIMENTAL In the previous studies , the preparations were performed in air or under oxygen flow and then without accurate control of the oxygen stoichiometry. So , in order to avoid the problem of the oxygen content, the reactions were performed under vacuum or in argon flow . Two different ways have been followed : the first one consits of mixing stoichiometric amounts of 2SrCO 3 , Bi20 3 and CuO ; for the second o n e , the starting materials were Bi20 3 and Sr2CuO 3 in the 1:1 molar ratio. The mixtures were heated up to 800°C with intermediate grinding and repelletizing at 679

680

PREPARATION AND ELECTRON DIFFRACTION STUDY OF Bi2Sr2CuO 6

dobs.(/~) 11.80 6.11 5.891 5.323 5.291 4.410 4.063 3.856 3.701 3.539 3.332 3.185 3.183 3.055 3.030 2.950 2.712 2.667 2.646 2.528 2.445 2.417

dobs.(A)

dcalc.(A) h k 1 1% 11.79 6.11 5.893 5.323 5.291 4.409 4.061 3.856 3.698 3.539 3.332 3.185 3.183 3.056 3.030 2.948 2.947 2.712 2.712 2.666 2.646 2.643 2.529 2.528 2.446 2.443 2.443 2.418 2.417 2.416

2 4 4 4 0 2 6 1 3 3 3 5 3 8 8 5 8 0 5 7 0 2 2 6 2 3 10 9 4 4

0 0 16 0-1 4 0 0 4 0 1 3 0 4 3 0 4 2 0-1 60 1-4 6 1-4 6 1 3 10 1-5 18 1 2 8 1 4 8 0 - 2 100 0-1 66 1 3 58 0 0 2 0 18 1 4 1-5 6 0 8 42 2 0 2 2 4 0-8 0 8 62 1 -8 0-2 1-3 4 2 1 0 9

2.355 2.307

2.255 2.138 2.032

1.988 1.956 1.895 1.878

dcalc.(/~) h k 2.357 2.309 2.308 2.306 2.305 2.256 2.256 2.254 2.139 2.137 2.135 2.033 2.032 2.031 2.030 1.989 1.987 1.954 1.954 1.954 1.894 1.893 1.893 1.878 1.877 1.876 1.876

10 9 7 6 2 10 6 6 3 6 8 4 11 12 6 8 8 11 8 6 0 12 11 9 7 11 8

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1 1%

0 0 1-5 1 4 0-9 2-5 0-6 2-1 2-2 1 8 0 7 0-9 2 5 1-1 0-2 2-6 2-4 0-10 1 -6 2 1 2 4 2 8 0-7 1 -7 1 5 1 7 1 2 0 7

2 4

3 5 16

10 12 10 10

Table I : x-ray powder pattern for Bi2Sr2CuO 6 .

Q

Q %,

Fig. 1 - Selected area diffraction (SAD) patterns observed in Bi2Sr2CuO6 : (a) L100] zone axis ; (b) [001] zone axis ; (c) is obtained by tilting of 12.5 ° from (a) around the b axis. tilting of 12.5 ° . These observations suggest that the symmetry of the uttit-cell is monoclinic with a unique axis b normal to c in the pattern l ( a ) . Then one can consider the hypothesis that the line, of strong spots appearing on the pattern l(b) is the a axis. By tilting of several crystals it ha% nqt been possible to observe the reciprocal plarke (a c ) which could allow us to measure directly B . T o , determine i t , we had to tilt the crystal around the b axis. The patterns 2(a) and 2(b) correspond to the tilting angles of 32.4 ° and 48.4 ° respectively,. From the figures l ( a ) , 2(a) and 2(b) the value of B has been determined by a geometrical constpaction (fig.3) . The horizontal line is chosen for the c direction and as origine for the tilting angles.

Two directions are obtained for ct =32.4 ° and 48.4 ° , their fundamental reflections observed in the patterns 2~a) and 2(b) are reproduced . The periodicity of the a axis is also reproduced. The (010) l"~ciprocal plane is then represented from which the B angle can be determined. The cell parameters obtained by this way have been refined by indexing the x-ray powder diffractionp~ttern (table I) . T h e deduced values are: a=24.451(5)A , b = 5 . a 2 5 ( 2 ) A , c=21.954(5)A and B=105.41(1) .This indexation suggests an extinction rule corresponding to a C-centered Bravais ~ttice . The experimental density dexo=7.07(3)g/cm ~ is in good agreement with the calcu/ated one (dcalc=7.13) for Z = 16 formula units per unit cell.

Vol. 77, No. 9

PREPARATION AND ELECTRON DIFFRACTION STUDY OF

Fig. 2 - SAD patterns obtained from l(a) bt( tilting of 32.4 ° (a) and 48.4 ° (b) around the b axis.

Bi2Sr2CuO6

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Fig. 4 - H E M showing a superstructure with a periodicity of 2.1 nm (the arrow shows an area where the stacking is narrower).

a •

Fig. 3 - Geometrical determination of the 13" val~e (reciprocal space section normal to the b axis). As mentioned b e f o r e , the existetnce of ordered strong and weak reflections along the c axis suggests a superstructure along this direction. This phenomenon has been seen clearly on high resolution microlgraph (fig.4) where the periodicity of 2.1nm corresponding to the [001] direction is indicated. Although this image seems u n i f o r m , stacking defects can be observed (of arrow on rigA) . For other crystals of the same preparation the diffraction patterns exhibit sometimes diffusion lines instead of spots between the fundamental reflections and obviously the corresponding high resolution images are less regular than the first ones (fig.5) In conclusion, the preparation of Bi2Sr2CuO 6 as single phase is mainly conditioned by the control of the

Fig. 5 - SAD pattern showing the coexistence of weak spots and diffusion lines. atmosphere of the reaction. The actual composition of the phase does not contain excess oxygen above the stoichiometric value . Our results are in good agreement with those of Roth et al. [12] and the experimental density confirms the proposed cell parameters . There m apparently a close relationship oetween the cell parameters of the pseudotetragonal "2201" phase and those of Bi2Sr2CuO 6 . The relationships are :amono~l =Ctetr , bxnor¥~cl =atetr and Cmonocl =4.atetr . i3'bviousfy , it vail be v e ~ important to" deterrruiae the crystal structure of this phase in order to compare it with the model proposed for the "2201" phase and_also to explain why the substitution of l_a3+ for Sr;z+ in the lattice stabilizes the "2201" phase [13-17].

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PREPARATION AND ELECTRON DIFFRACTION STUDY OF Bi2Sr2CuO 6

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R. Ramesh , L. H. G r e e n e , M. Giroud , M. S. H e d g e & W. R. Mc Kinnon , Phys. Rev. B 3 9 , 4 3 1 6 (1988). J. Darriet , C. J. P. Soethout , B. Chevalier & J. Etourneau , Solid State Commun. 69 , 1093 (1989). W. B a u h o f e r , Hj M a t t a u s c h , R. K. K r e m e r , P. Murugaraj & A. S i m o n , Phys. Rev. B 3 9 , 7 2 4 4 (1990).