High-resolution electron microscopy of triclinic modulations in bismuth-barium cuprates

Physica C 213 (1993) 276-282 North-Holland

High-resolution electron microscopy of triclinic modulations in bismuth-barium cuprates S. T a o , H . - U . N i s s e n , M . C a n t o n i a n d J . D . G u o Laboratorium ffir FestkOrperphysik, ETH-HOnggerberg CH-8093 Ziirich, Switzerland Received 22 April 1993

In order to investigate the role of incommensurate modulations in high-To superconductor oxide structures, a compound with bulk composition BizBa2NdCu2Oa+6 has been investigated by high-resolution transmission electron microscopy (HRTEM) as well as selected area electron diffraction (SAD) techniques. The SAD patterns reveal a tetragonal unit cell of this material, with parameters a= 5.60 A and c= 31.4/~; these are significantly larger than those of the bismuth-strontium analogue, Bi2Sr2CaCu2Os. The HRTEM images and the satellites (i.e. their positions as well as their intensities) observed in the SAD patterns suggest that the basic structure is modulated by two different types of modulation. These two superlattices both have triclinic symmetry; however, they differ in their cell parameters.

I. Introduction Since the discovery o f superconductivity in the b i s m u t h - b a s e d cuprates [1,2], the structure o f the compounds with the general formula Bi2Sr2CanCun+~O2n+4+6 has been intensively studied [ 3-11 ]. The basic structure in these c o m p o u n d s was f o u n d to contain an i n c o m m e n s u r a t e m o d u l a tion, and a different wave vector o f this m o d u l a t i o n was found for several materials having the same crystal structure but different compositions. The m o d u l a t i o n s observed previously in the b i s m u t h s t r o n t i u m cuprates have o r t h o r h o m b i c s y m m e t r y in the 2212- ( n = l ) a n d the 2223-phase ( n = 2 ) and monoclinic s y m m e t r y in the 2201-phase ( n = 0) [ 1217 ]. These c o m p o u n d s have transition t e m p e r a t u r e s To~ 10, 85, 110 K for n = 0 , 1, 2, respectively. In order to study the influence o f s t r o n t i u m doping on the superconductivity in the b i s m u t h - b a s e d 2212-phase, N a u m o v et al. [ 18 ] synthesised a new b a r i u m - c o n taining 2212-phase with the composition Bi2Ba2NdCu2Os+~. This new c o m p o u n d was found to have a tetragonal unit cell, with d i m e n s i o n s a = 5 . 5 4 9 ~ and c = 3 1 . 0 4 A, and these p a r a m e t e r s are significantly larger than those o f the Sr-containing 2212 cuprates ( a ~ 5 . 4 A, c ~ 3 0 . 8 A.). Also, no superconductivity was found in this c o m p o u n d down

to 4 K. Recently, P h a m et al. [ 19 ] have reported the synthesis o f another b a r i u m - c o n t a i n i n g 2212-phase, with c o m p o s i t i o n BizBaz+xLa~_xCu2Os+a, and they found that the modulations in these compounds have monoclinic symmetry, i.e. a s y m m e t r y which differs from that observed in the B i - S r and the T1-Ba " 2 2 1 2 " - c o m p o u n d s . In view o f these previously obt a i n e d results, we considered it is necessary to study the m o d u l a t i o n s o f the new b a r i u m - c o n t a i n i n g 2212phase with the aim of clarifying the relations between the superconductivity, the composition, the cell p a r a m e t e r s and the m o d u l a t i o n s o f the structure. We report here on the results o f a study of the compound having nominal composition Bi2BazNdCu208+a, using selected area diffraction ( S A D ) and high-resolution transmission electron microscopy ( H R T E M ) techniques. In the course of this work, two different kinds of m o d u l a t i o n , both with triclinic symmetry, could be distinguished.

2. Experimental Samples with nominal composition BizBa2NdCu2Os+~ were p r e p a r e d from Bi203, BaCO3, Nd203 and CuO. The starting materials were ground and mixed in the ratio of

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S. Tao et al. / HREM ofbismuth-bariurn cuprates

Bi: Ba: Nd : Cu = 2 : 2 : 1 : 2. Subsequently, the mixed powder was first calcined in Ar at 800°C for 10 h, and then the calcined powder was again ground in an agate mortar and pressed into pellets. Finally, these pellets were sintered in Ar at 820°C for 24 h. For investigation by SAD and HRTEM, small pieces of the specimen were ground in an agate mortar with water-free alcohol; the suspensions were then dispersed on carbon holey foils. The SAD- and HRTEM-results were mainly achieved using a Jeol JEM 200CX and a Philips CM30 electron microscope, respectively. The former was operating at 200 kV and was equipped with a _+35 ° tilting stage, while the CM30 instrument was operating at 300 kV and equipped with a super-twin lens having theoretical point-to-point resolution of 0.19 nm.

3. Results and discussion 3. I. S A D results

Several compounds with nominal composition Bi2Ba2NdCu2Os +6 were first investigated by the SAD technique. Figure 1 presents electron diffraction patterns taken along the [ 100]-, the [011 ]- and the [001 ]-direction, respectively. The main diffraction spots which can be recognised in fig. 1 can be indexed using indices hkl, where h, k and l are either all odd or all even; this demonstrates that the average structure in this specimen has a body-centred tetragonal unit cell, with parameters a = 3 . 9 6 A and c = 31.4 ~, or, alternatively, a face-centred tetragonal unit cell, with a = 5.60 A and c = 31.4 A. The cell parameters measured in SAD patterns basically agree with the results reported by Naumov et al. [18]. However, the structure described here has a larger c value. In addition to the main diffraction spots, a set of satellites can clearly be recognised in fig. 1. Obviously, this set of satellites has an overall symmetry different from that observed in bismuth-strontiumcontaining cuprates with the 2212-phase. The [ 100]-SAD patterns presented in fig. 1 (a) reveal the following features: (1) The main diffraction spots are slightly diffuse along the c*-axis. (2) The satellite sequences are not oriented parallel to the b*-axis, but roughly along the [01i]*-direc-

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tion. The value q of the modulation wave vector can be measured with high accuracy using an enlargement of the [ 100]-diffraction pattern. The b*-component of the vector q is exactly ~ , and the c*-component is approximately ½; this value is not exact, as indicated by the streaking of the satellite intensities. Consequently, the modulation wave vector can be expressed as q = 2b* / 11 - c* / 5, where b* and ¢* are the reciprocal vectors of b and c, respectively, and Ib*l=la*l=RL/la], Ic*l = R L / I c [ RL being the experimental camera constant. (3) The satellites are elongated parallel to the c*-axis. For this reason, some satellites caused by the intersection of the diffraction plane with the streaked diffraction spots can be observed in the [011]- and [001 ]-SAD patterns. This will be discussed in detail in the following paragraphs. (4) A set of satellites with wave vector q' = 2 b * / 11 + c * / 5 can additionally be observed; this set is assumed to be caused by the effects of twinning. In the following part of this paper, this will be proved by HRTEM images. Figure 1 (c) presents the SAD patterns taken along the [011 ]-direction. Besides the main diffraction spots with indices (h k l), twenty-two satellites located between the main diffraction spots (for example, between the (000)*- and the (022)*-spot) can clearly be recognised. However, the arrangement of relatively strong and relatively weak satellites demonstrates that there cannot be simply a twentytwo-fold modulation along the b-axis in the structure of the bismuth-barium-containing compound. Moreover, it can be seen in fig. 1 (a) that there are only eleven satellites between the (000)*- and the (025)*-spot. In a similar way to the [011 ]-diffraction patterns, the SAD patterns taken along the [001 ] direction, which are presented in fig. 1 (d), also contain twentytwo satellites between the (000)*- and the (020)*spot. It is worth pointing out that, even though the satellites are located on the b*-axis in fig. 1 (d), this does not imply that the modulation wave vector is perpendicular to the a*-c* plane. Figure 1 (a) clearly shows that there are no diffraction spots between the (000)*- and the (020)*-spot. However, by cutting the streaks of the satellites recognised in fig. 1 (a), eleven satellites can result between the main diffraction spots in fig. 1 (d). Obviously, the intensities

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Fig. 1. SAD patterns taken along: (a) and (b) [ 100]-, (c) [011 ]- and (d) [001 ]-directions. (a) and (b) were taken from different domains of the same specimen Bi2Ba2NdCu208+6. o f these satellites are much weaker than those o f the satellites in fig. l ( c ) , where the reciprocal plane (O1 T)* nearly cuts the satellites themselves. There-

fore, half o f the satellites appearing in figs. 1 (c) and ( d ) can be interpreted as resulting from a cut through the satellites themselves a n d through their streak

S. Tao et al. / HREM of bismuth-barium cuprates prolongation, respectively. Another set of eleven satellites is, however, present in figs. 1 (c) and (d). In this set some of the satellites, which have been marked by arrows in the corresponding figures, have rather strong intensities. For geometric reasons, these satellites cannot be caused by the modulation wave vector q. This implies that in the specimen another kind of modulation coexists with the set described above. The satellite with index (010)* in fig. 1 (d) also has a strong intensity. However, it is not considered to be caused by the two modulations mentioned above. Fig. 1 (b), which was taken from another region of the same sample, shows the diffuse diffraction columns parallel to the c*-axis and passing through the (0k0)*-positions ( k = odd). In the following section, an attempt will be made to confront these results on two different sets of satellites obtained from SAD patterns with the evidence from HRTEM images, in order to arrive at a consistent model of the modulation geometry.

3.2. H R T E M results In order to distinguish the two modulated lattices, several specimens were additionally investigated by the HRTEM method. Figure 2 shows a H R T E M image taken along the [100]-direction. The corresponding diffraction pattern is presented in fig. 1 (a). A parallelogram-shaped supercell containing sharp and bright dots can be recognised in the thicker part of fig. 2. The two basic vectors of the parallelogram are k~= 1 lb/2 (30.8 ~ ) and k z = b / 2 + c / 2 . Moreover, a twin with the modulation vectors k~ = - b~ 2 + c / 2 or k~ = b / 2 - c / 2 frequently appears in fig. 2. As a rare property of this twin texture, a modulation with vector kd=b+c/2 or kd= --b +C/2 has also been observed in fig. 2. Viewed at a grazing angle along the b-axis of the structure, a sinusoidal wave contrast in the structure can be recognised. Figure 2 demonstrates that the modulations existing in the barium-containing 2212-phase have at least monoclinic symmetry. In order to obtain three-dimensional information on the geometry of the modulations, the [011 ]-direction was chosen for taking H R T E M images, since this projection direction is roughly parallel to the modulation direction defined by the vector k2 in fig.

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2. One of these images is presented in fig. 3. For convenience, this HRTEM image is divided into four parts marked by A, B, C and D, and the direction which is perpendicular to the a-axis is named the b'axis. The corresponding SAD pattern is presented in fig. l ( c ) . The projection of the non-modulated structure along the same direction has a rhombusshaped unit cell, which is shown inserted in fig. 3. The features of the modulation recognised in fig. 3 can be characterised as follows: ( 1 ) In part A, sharp bright dots having the shape of a rhombus can be observed. In the centre of each rhombus there is a rather weak bright dot. In part B, diffused bright dots can be recognised instead of the sharp ones. (2) The distance between parts A and C is 1 lb~/2, where b~ represents the projected b-axis of the unit cell of the ideal 2212-structure, which is shown as a rhombus-shaped inset in fig. 3. (3) The contrast in parts C and D is similar to that in parts A and B, respectively. However, in C and D the units are staggered against each other by a value of l a l / 2 along the a-axis. This implies that the modulation wave vector kl appearing in fig. 2 cannot be parallel to the b-axis and has an a-component. It can therefore be written as k~ = a / 2 + 1 lb/2. (4) Viewed along the directions defined by the longarrows, a stepped arrangement of the bright dots is clearly observable in fig. 3. This feature indicates that the b' value of the rhombus in parts A and C is larger than that of the rhombus in parts B and D. The increase in the b' value, compared to the ideal one, is (22/21-1)b~ =0.26 A. (5) Besides the modulations mentioned above, another kind of contrast change can be recognised in fig. 3, where it is marked by short arrows in parts B and D of the image. The period of this contrast change measured along the b'-axis is 1 lb~/2.

3.3. Two different superlattices occurring in the 2212-phase Two different kinds of modulation contributing to the contrast changes visible in the HRTEM images have been distinguished. One is a triclinic superlattice called superlattice I (SL I), with three basic vectors al=a, hi=a~2+ 1 lb/2 and c~=b/2+c/2. The other one is also a triclinic superlattice called super-

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Fig. 2. HRTEM image taken along the [ 100 ]-direction. The corresponding SAD pattern is shown in fig. 1(a).

Fig. 3. HRTEM image taken along the [ 011 I-direction. The inset is a projection of the non-modulated structure along the same direction, in which the different atoms are identified by circles with different diameters. Additionally, examples for each element are marked by the chemical symbol and horizontal arrow. The corresponding SAD pattern is presented in fig. 1(c). lattice II (SL II) a n d d e f i n e d by the t h r e e basic vectors a z = a / 2 + 1 l b / 4 , b2= - a / 2 + 1 l b / 4 a n d c z = b / 2 + c/2, w h e r e a, b a n d c are the basic v e c t o r s o f the a v e r a g e structure ( [ a [ = [hi ). T h e s e two k i n d s o f m o d u l a t i o n are a s s u m e d here to be c a u s e d by two d i f f e r e n t c r y s t a l - c h e m i c a l features. T h e first one (SL

I) is p r o b a b l y based on h e a v y c a t i o n s u b s t i t u t i o n , a n d the s e c o n d o n e (SL II) is s u p p o s e d h e r e to be due to the extra oxygen a t o m s in or b e t w e e n the B i O layers. T h e two types o f superlattice p r e s e n t e d a b o v e will cause satellites in the d i f f r a c t i o n patterns. T h e two

S. Tao et al. / HREM of bismuth-barium cuprates reciprocal spaces (SL I)* and (SL II)* are defined by the basic vectors a T = a * - b * / l l + c * / l l , bT=2b*/ll-2c*/ll, c T = 2 c * and a'~=a*+2b*/ 11-2c*/11, b~= - a * + 2 b * / 1 1 - 2 c * / 1 1 , c~=2c*, respectively, where l a*l = Ib*l = R L / l a l , Ic*l = R L / Icl, and RL is the camera length. A schematic drawing of the [ 100 ]-diffraction pattern based on the (SL I )*- and the (SL II)*-lattice is presented in fig. 4 (a). The satellites in fig. 4 ( a ) are arranged in a row parallel to the [ 01 T ]*-direction, which is different from that observed in fig. 1 (a). The reason for the deviation of the satellites from the [01 i ]*-direction will

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be discussed in the next paragraph. Figure 4 (c) presents a schematic drawing of the (011)*-plane, in which the basic vectors aT, b1', a~ and b~ have been indicated. The experimental results (see fig. 1 ( a ) ) indicate that the satellites at the positions defined by vectors a~ and b~ have rather strong intensities, and these can possibly act as a source o f double diffraction. The cross symbols in fig. 4 (c) represent the diffraction spots originating from double diffraction. The diffraction streaks appearing in fig. 1 (a) may be caused by the twin boundaries. The deviation of the satellites from the [01 i ]*-direction can be explained by the distortions of the SL I. Considering a superlattice named SL I', which has the same a and b vectors as that of SL I, but a different ¢ vector with increasing 6b along the b-axis. Consequently, the three basic vectors of the SL I' can be expressed as a'~ = a t , b'~ =b~ and C'l =c~ +6b. The corresponding reciprocal space (SL I ' ) * is defined by a ' ~ = a * - b * / 11+(1+26)c*/11, b ' ~ * = 2 b * / l l - 2 ( l + 2 6 ) c * / l l , and c'~*= 2c*. It is clear that the modulation directions can change due to different 6-values, for example, when 6=~o, a'~*=a*-b*/ll+c*/lO, b'l* = 2 b * / 1 1 - 2 c * / 1 0 , and c'~*=2c*. As a result, the modulation direction deviates from the [01i]*-direction. A random shift of the modulation layers thus causes the diffraction spots to be elongated parallel to the c*-axis. In order to visualise the result of a specific change in the 6-value, a schematic drawing of the b*-c* plane with 6 = ~ois presented in fig. 4 ( b ) .

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4. Conclusions

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The experimental results presented in this paper can be interpreted as follows: ( 1 ) The bismuth-based 2212-phase is present in the c o m p o u n d Bi2Ba2NdCu2Os+a and has cell parameters a = 5.60 A and c = 31.4 A; these are significantly larger than those o f Bi2Sr2CaCu2Os+a. (2) The modulations appearing in the specimen with composition Bi2BazNdCu2Os+a differ from those observed in the Bi-Sr or the T1-Ba cuprates. (3) Two different types of modulation can be distinguished in the Bi2Ba2NdCu2Os+a specimen by SAD and H R T E M techniques. One is a triclinic superstructure caused most probably by heavy cation substitution. The other one is also a triclinic super-

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structure, but o n e with different cell parameters. T h i s latter s u p e r s t r u c t u r e is s u p p o s e d to be c a u s e d by extra o x y g e n a t o m s .

Acknowledgements T h e a u t h o r s w o u l d like to express t h e i r g r a t i t u d e to R. W e s s i c k e n for his skilful t e c h n i c a l assistance in high-resolution t r a n s m i s s i o n electron microscopy. We are v e r y grateful to A. Schilling for his a d v i c e in the p r e p a r a t i o n o f s p e c i m e n s . T h i s w o r k was s u p p o r t e d by a grant f r o m S c h w e i z e r i s c h e VolkswirtschaftsStiftung, w h i c h is gratefully a c k n o w l e d g e d . It also f o r m s part o f a r e s e a r c h p r o g r a m on e l e c t r o n mic r o s c o p y o f real crystal s t r u c t u r e s s u p p o r t e d by the Swiss N a t i o n a l S c i e n c e F o u n d a t i o n .

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