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Electron microscopy of recent high Tc superconductors
P#YSICA
@
Iq*>sica~ '~s 24()(1()'~4~ !~,i! i~,? N ~MJ/-Jh~ll:md
Electron microscopy of recent high Tc superconductors S. Amclinckx, (3. Van Tendel(x) University of Antwerp, EMAT, Grocnenborgerl~mn I 71 B-2020 Antwerp. Belgium Recently the use of high resolution electron microscopy (HREM) has contributed increasingly to elucidate the complex structures of high Tc superconductors. The observation of planar defects has suggested new homologous series. Substitution of copper by aliovalent cations such as Ga, Co .... and by complex anions such as S( )4, C()3, PO4 often leads to superstructures which are only revealed by HREM,
1. INTRODUCTION The complex structures of the high Tc cupralc superconductors often exhibit deviations from periL'el periodicity or superstructures which may interfere with die superconducting properties. Delect structures are usually not revealed in X- or n-diffraction studies, often conducted on powders. and which are only based on data collected in reciprocal space. However defect structures can conveniently be studied m direct space where defecl reformation remains strongly loc',dized, rather than in reciprocal space where this information is diluled ()vcr extended regions. Access to direct space is provided by lngh resolution electron microscopy (HREM) which allows to image individual atom columns. Moreover selected area electron diffraction offers the possibility Io move easily between direct and reciprocal space and establish a link with X- and n-diffraction in providing single crystal data, even from sub la-size powders. Different types of chemical microanalysis can furthermore bc performed on the same small cryslal fragment.
2. H O M O L O G O U S SERIES 2.1. The Y-Ba-Cu-O system HREM has been particularly successful m detecting singular layers which may locally change the composition and suggest possible new structures in which such layers occur periodically. By varying the period of occurrence via the composition. homologous series of compounds may be generated. The simplest example is the double (CuO)2 layer in (001) planes, replacing a single CuO layer in the 12-:l compound YBa2Cu307_ 8, which accommodates excess copper. By control of the composition it is possible to prepare YBa2Cu40g(1-2-4) in which truly {Cut)) 2 layers occur. HREM has produced evidence for a homologous series in which single 0921-4534/94/507.00 © 1994 - l;.lscvicr Science I't.V. All right', .S'5"t}/(N21-4534(94)(X)67S 4
and double layers lorm periodic sequences. [:ig. 1 shows the H.R. image of YBa2Cu3.507.5 in which single and double layers alternate compared with a similar image ol the 1-2-4 compound. More complicated periodic sequences have been discovered. In some cases n-lold 3<_n<_5(CuO)n layers have been detectcd.111
2.2. The Bi-Sr-Ca-Cu-O system The Bi series has likewise been inspired by the occurrence, already m the early images, o17 singular unit cells with a deviating number of CuO 2 (and Ca) layers. The preparation of compounds with a large and constant number of CuO 2 layers per uui! cell was motivated by a possible correlation between this number and Tc. Unl()rtunately the hope thai Tc would increase monotonously with the number ot CuO2 layers was not tx~rn out. The discovery' ol a new type ol delormation modulated structure has triggered a large volume ()1 research on the origin of this modulation.t21 II has led to the finding that the formation of single phase 2223 can be promoted by the substitution of Bi by' Pb: this is accompanied by a change in Ihc modulation. Whereas in the pure Bi compounds successive sinusoidal wavy' BiO layers are in "antiphase" they are "in-phase" in the lead stabilized compounds. The origin of the modulation is still a matter of debate, but the fundamental reason seems u> bc the lattice misfit between the perovskite block. and the bismuth oxide layers.
2.3. The Hg-Ba-Ca-Cu-O system The recently discovered mercury based cupratcs are isostructural with the homologous single layer Tl-polytypoids. The succession of layers along the cdirection in the calcium tree comtx)und is .../HgOs-BaO-CuO2-BaO-/HgOs...: for short 1201 (the digits refer respectively to the numbers of Hg(TI), Ba(Sr), Ca(Y) and Cu atoms in the unit cell. The mercury layers are always oxygen
S. Amelinckx, G. Van Tendeloo/Physica C 235~40 (1994) 162-165
deficient 8 << 0,4) Copper is octahedrally coordinated by oxygen whereas B a t and HgO 8 layers are rocksalt like. Mercury forms a linear arrangement with the apical oxygen ions of C u t 6 octahedra (lattice parameters : a : ap = 0,38 nm, c1201 = 0,95 nm).[3] By Ca(Y) doping it is possible to insert additional copper oxyde layers in the unit cell. The succession of layers in the 1212 compound then becomes for instance : .../HgOs-BaO-CuO2-Ca(Y)-CuO2-BaOPAgOs... Copper is now five fold pyramidally coordinated by oxygen. The Ca(Y) layers remain oxygen free and separate two C u t 5 pyramids.[4] Structures containing up to seven copper layers have been detected in HREM.[5] The chemical composition of this homologous series can be formulated as : HgBa2Can-lCunO2n+2+8 (in short 1 2 n-1 n). It is analogous to the corresponding Tl-homologous series T1Ba2Can-lCunO2n+3. This analogy suggests that perhaps like in the T1 compounds also in the mercury based compounds a second family of phases might occur containing a double HgO8 layer with a rocksalt structure, leading to a composition 2 2 n-1 n. HREM images have in fact occasionally revealed isolated double layers of (HgOs) 2 but so far it has not been possible to prepare samples exhibiting systematically double HgO8 layers; presumably because such a double layer can only be stabilized by an oxygen content close to 8 = 1.[6] Like in the T1 and Bi compounds intergrowth structures containing lamellae with n -values differing by one unit (occasionally two) units frequently occur in samples with nominal compositions with n _>3.[6]
3. S U P E R S T R U C T U R E S ORDERING 3.1. The Y-Ba-Cu-O system
DUE
TO
HREM and e-diffraction have been used extensively to detect superstructures. Because of the weakness of superstructure reflections, often due to the small coherence length such reflections are sometimes missed in n- or X-diffraction. HREM makes it possible to determine in which layer(s) the superstructure is localized in a complex structure, providing the basis for a model of the superstructure. Many different substitutions, have been performed in the C u t layers of the 1-2-3 type compounds. They are motivated by the consideration that via these layers the carrier density can be optimized. Of particular structural interest is the
163
substitution of Cu by cations such as Co,Ga,AI .... which favour a tetrahedral coordination with oxygen instead of the square planar one of copper. Also complex ions such as sulphate, carbonate or phosphate groups have been incorporated in the C u t layers. However the simplest modification of the C u t layers consists in oxygen deficiency, which gives rise to the 2at (ortho II) and 3at (ortho III) vacancy ordered structures, in which along the a t direction Cu-O-Cu-O chains parallel to bo and parallel Cu-~-Cu-~ chains form arrangements with periods 2ao, 3ao .... Also in YBa2Cu3.507.5 with alternating single and double C u t layers the vacancy ordered superstructures occur [6]; they are localized in the single C u t layers only. Computer simulated images show that relaxation around the vacancy rows is significant (fig. 2).
3.2. The (Hg,Pr)-Sr-Cu-O system Samples of the 1201 type with a composition HgxPrl_xSr2CuO4+8 where 0,4 < x < 0,5 exhibit an orthorhombic structure with lattice parameters a = 0,76 n m , = 2at, b = 0,368 nm and c=0,888nm.[7] This phase is a superstructure of the 1201 structure of HgBa2CuO4+8. Comparing the images along the [010] and [100] zones with those of the corresponding praesodymum free phase shows that the superperiod must be associated with the arrangement in the mercury containing layers since these are the only layers exhibiting period doubling, alternating dots having different brightness. The fact that the cations in the other layers exhibit their usual dot configurations and the consideration that the only modification in the chemical composition is the addition of Pr and the corresponding reduction in Hg content, lead to the assumption that Hg and Pr become ordered and give rises to the observed superlattice. A model in which the Hg rows parallel to the b-direction are replaced by the regular alternation of Hg and Pr chains is consistent with all observations. Computer simulated images based on this model reproduce well the observed images. The model is also consistent with the Hg/Pr ratio. The Hg-Pr ordering not only occurs in the 1201 compound but also in the 1212 and the 1222 compound.J9]. An image of the latter and the defects occurring in the ordering is shown in figure 3. The superstructure also causes a slight orthorhombicity of the basic lattice. The superstructure clearly allows for coherent twinning on { l l 0 } p planes, the rows of Hg and Pr being mutually perpendicular in the two parts of the twin. Such twins have been observed in HREM along the [001] zone.[7,8] In the diffraction pattern along this
164
S. Amelinc~:r, G. Van Tendeloo/Physica C 235 240 H994) 162 /69
zone the twins simulate fourfold symmetry provided the orthorhombic deformation of the basic lattice is ignored; the latter causes spot splitting of all hko spots except for those in the [hho] row perpendicular to the twin interface. The orthorhombic deformation of the basic structure is clearly a consequence of the loss of fourfold symmetry in the (Hg, Pr) layers.
3.3. Co a n d Ga substituted 1-2-3 In the Co and Ga substituted 1-2-3 type compounds YSr2CoCuO7 and YSr2GaCu207 the layer sequence along c is SrO-CuO2-Y-CuO2-SrOMO (M=Co, Ga). Chains of corner linked CoO4(GaO4) tetrahedra are formed in the MO layers along { l l 0 } p directions. The Ga-structure was described with reference to an orthorhombic unit cell with lattice parameters ao : b o : ap ",/2; co = 2Cp (ap = 0,38nm, Cp ~ 1,14nm). The chains are parallel to bo and they arc assumed to be all of the same kind; successive layers being stacked in a staggered fashion. It was found by electron microscopy that a superstructure with a unit cell as=2ao, bs=b o and cs=co is present in two variants differing by a 90 ° rotation about the c-axis.[9] Fig. 4 shows the two variants in a single HREM image; one variant is viewed along [100]o the other one along [0101o i.e. ~dong the chain direction. The positions of the cobalt containing columns are indicated by open dots. A model whereby successive chains in the same layer are assumed to be mirror images i.e. left (L) and right (B) chains is consistent with the superstructure. In fig. 4 the two types of chains cannot be distinguished since their projections are the same along [010]o. However along the [ l l 0 ] s zone it is possible to distinguish between a sequence ol identical chains and one consisting of alternating L and R chains. (fig. 5) It reveals the period doubling and images a number of defects in the stacking of the chains. The lateral shift over 1/4 of the interdot spacing between successive rows of dots is consistent with the model.
3.4. S O 4 substituted 1-2-3 In ( Y S r C a ) S r 2 C u 2 , 7 8 ( S O 4 ) 0 , 2 2 0 7 _ 6 the sulphate groups substitute in part for copper in the Cud layers; the 1-2-3 structure being conserved. The diffraction patterns of this material exhibit incommensurate satellite sequences associated with the basic reflections m the [010] and [001] zones; they reveal a modulated structure. The most informative HREM image is obtained along the [010] zone; it exhibits a quasi-lattice of prominently bright dots forming small crosses
situated along the traces of C u d layers (fig. 0); they image the chains parallel to bo, which contain a significant concentration of sulphur atoms. The arrangement is not strictly periodic and it depends on the SO4 content. It can be described as resulting from a concentration wave with a wave vector enclosing an angle c~ = 30 ° with the [100] direction and a length q - l n m - l . l l 0 ] The positions of the SO4 containing chains are determined by the maxima of this concentration wave and by the requirement to be situated in C u d layers and on chain sites. (fig. 6). The bright lines represent the maxima of the concentration waves. Along any C u d layer the spacings between successive S-bearing chains form a non-periodic sequence such as 3 3 4 3 4 ... (in units of ao), ACKNOWLEDGEMENTS The authors are grateful to T. Krekels, (). Milat, M. Hervieu, B. Raveau, C. Greaves, C. Chaillout, H.W. Zandbergen, E. Kaldis for the use of results from common research.
REFERENCES l 1]
T. Krekels, G. Van Tendeloo, S. Amelinckx, J. Karpinski, S. Rusiecki, E. Kaldis and E. Jilek, Physica C 178 (1991) 383. [2] H.W. Zandbergen, W.A. Groen, F.C. Mijlhoff. G. Van Tendeloo and S. Amelinckx. Physica (7 156 (1988) 325. 13] S.N. Putilin, E.V. Antipov, O. Chmaissem and M. Marezio, Nature 362 (1993) 226. [41 E.V. Antipov, SM. Loureiro, C. Chaillout, J.J. Capponi, P. Bordet, J.L. Tholence, S.N. Putilm and M. Marezio, Physica C 215 (1993) 1. [5] G. Van Tendeloo, C. Chaillout, J.J. Capponi, M. Marezio and E.V. Antipov, Physica C 223 (1994) 219. [ 61 T. Krekels, G. Van Tendeloo, J. Karpinkski, E. Kaldis and S. Rusiecki, Appl. Phys. Lett. 59 i1991) 3048. 17] F. Goutenoire, P. Daniel. M. Hervieu, G. Van Tendeloo, C. Michel, A. Maignau and B. Raveau, Physica C 216 (1993) 243. [8] G. Van Tendeloo, M. Hervieu, X.F. Zhang and B. Raveau. J. Sol. State Chem. (1994) in the press. [91 T. Krekels, O. Milat, G. Van Tendeloo, S. Amelinckx, T.G.N. Babu, A.J. Wright and C. Greaves, J. Sol. State Chem. 105 (1993) 313. [ 10] T. Krekels, O. Milat, G. Van Tendeloo, J. Van Landuyt, S. Amelinckx, R.P. Slater and C. Greaves, Physica C 210 (1993) 439.
S. Amelinckx, G. Van Tendeloo/Physica C 235-240 (1994) 162-165
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165
I
Figure 1. HREM image of the 1-2-3,5 and the 1-2-4 structure along the [010] zone. The elongated bright dots correspond to channels between CuO4 square planar arrangements in the double (CuO)2 layers.
Figure 4. Co (1-2-3) containing two orientation variants with corresponding diffraction patterns
m-rrmrm'ere,rmm
Figure 2. HREM image of vacancy ordering in the single CuO layers of the 1-2-3.5 structure as compared with a simulated image.
~~. •
'
•
C
~
~ - : 2 . ~
f
~
..... ". . . . . . . . . . . . . ~".....
. . . . 27"
Figure 5 : HREM of Co(1-2-3) imaged along [lOlls for one variant or along [410] s for a second orientation variant.
Figure 3. HREM image of the (Hg-Pr) 1222 compound, antiphase lines are indicated by arrows.
Figure 6 • [010] HREM image of SO4 (1-2-3).
@
Iq*>sica~ '~s 24()(1()'~4~ !~,i! i~,? N ~MJ/-Jh~ll:md
Electron microscopy of recent high Tc superconductors S. Amclinckx, (3. Van Tendel(x) University of Antwerp, EMAT, Grocnenborgerl~mn I 71 B-2020 Antwerp. Belgium Recently the use of high resolution electron microscopy (HREM) has contributed increasingly to elucidate the complex structures of high Tc superconductors. The observation of planar defects has suggested new homologous series. Substitution of copper by aliovalent cations such as Ga, Co .... and by complex anions such as S( )4, C()3, PO4 often leads to superstructures which are only revealed by HREM,
1. INTRODUCTION The complex structures of the high Tc cupralc superconductors often exhibit deviations from periL'el periodicity or superstructures which may interfere with die superconducting properties. Delect structures are usually not revealed in X- or n-diffraction studies, often conducted on powders. and which are only based on data collected in reciprocal space. However defect structures can conveniently be studied m direct space where defecl reformation remains strongly loc',dized, rather than in reciprocal space where this information is diluled ()vcr extended regions. Access to direct space is provided by lngh resolution electron microscopy (HREM) which allows to image individual atom columns. Moreover selected area electron diffraction offers the possibility Io move easily between direct and reciprocal space and establish a link with X- and n-diffraction in providing single crystal data, even from sub la-size powders. Different types of chemical microanalysis can furthermore bc performed on the same small cryslal fragment.
2. H O M O L O G O U S SERIES 2.1. The Y-Ba-Cu-O system HREM has been particularly successful m detecting singular layers which may locally change the composition and suggest possible new structures in which such layers occur periodically. By varying the period of occurrence via the composition. homologous series of compounds may be generated. The simplest example is the double (CuO)2 layer in (001) planes, replacing a single CuO layer in the 12-:l compound YBa2Cu307_ 8, which accommodates excess copper. By control of the composition it is possible to prepare YBa2Cu40g(1-2-4) in which truly {Cut)) 2 layers occur. HREM has produced evidence for a homologous series in which single 0921-4534/94/507.00 © 1994 - l;.lscvicr Science I't.V. All right', .S'5"t}/(N21-4534(94)(X)67S 4
and double layers lorm periodic sequences. [:ig. 1 shows the H.R. image of YBa2Cu3.507.5 in which single and double layers alternate compared with a similar image ol the 1-2-4 compound. More complicated periodic sequences have been discovered. In some cases n-lold 3<_n<_5(CuO)n layers have been detectcd.111
2.2. The Bi-Sr-Ca-Cu-O system The Bi series has likewise been inspired by the occurrence, already m the early images, o17 singular unit cells with a deviating number of CuO 2 (and Ca) layers. The preparation of compounds with a large and constant number of CuO 2 layers per uui! cell was motivated by a possible correlation between this number and Tc. Unl()rtunately the hope thai Tc would increase monotonously with the number ot CuO2 layers was not tx~rn out. The discovery' ol a new type ol delormation modulated structure has triggered a large volume ()1 research on the origin of this modulation.t21 II has led to the finding that the formation of single phase 2223 can be promoted by the substitution of Bi by' Pb: this is accompanied by a change in Ihc modulation. Whereas in the pure Bi compounds successive sinusoidal wavy' BiO layers are in "antiphase" they are "in-phase" in the lead stabilized compounds. The origin of the modulation is still a matter of debate, but the fundamental reason seems u> bc the lattice misfit between the perovskite block. and the bismuth oxide layers.
2.3. The Hg-Ba-Ca-Cu-O system The recently discovered mercury based cupratcs are isostructural with the homologous single layer Tl-polytypoids. The succession of layers along the cdirection in the calcium tree comtx)und is .../HgOs-BaO-CuO2-BaO-/HgOs...: for short 1201 (the digits refer respectively to the numbers of Hg(TI), Ba(Sr), Ca(Y) and Cu atoms in the unit cell. The mercury layers are always oxygen
S. Amelinckx, G. Van Tendeloo/Physica C 235~40 (1994) 162-165
deficient 8 << 0,4) Copper is octahedrally coordinated by oxygen whereas B a t and HgO 8 layers are rocksalt like. Mercury forms a linear arrangement with the apical oxygen ions of C u t 6 octahedra (lattice parameters : a : ap = 0,38 nm, c1201 = 0,95 nm).[3] By Ca(Y) doping it is possible to insert additional copper oxyde layers in the unit cell. The succession of layers in the 1212 compound then becomes for instance : .../HgOs-BaO-CuO2-Ca(Y)-CuO2-BaOPAgOs... Copper is now five fold pyramidally coordinated by oxygen. The Ca(Y) layers remain oxygen free and separate two C u t 5 pyramids.[4] Structures containing up to seven copper layers have been detected in HREM.[5] The chemical composition of this homologous series can be formulated as : HgBa2Can-lCunO2n+2+8 (in short 1 2 n-1 n). It is analogous to the corresponding Tl-homologous series T1Ba2Can-lCunO2n+3. This analogy suggests that perhaps like in the T1 compounds also in the mercury based compounds a second family of phases might occur containing a double HgO8 layer with a rocksalt structure, leading to a composition 2 2 n-1 n. HREM images have in fact occasionally revealed isolated double layers of (HgOs) 2 but so far it has not been possible to prepare samples exhibiting systematically double HgO8 layers; presumably because such a double layer can only be stabilized by an oxygen content close to 8 = 1.[6] Like in the T1 and Bi compounds intergrowth structures containing lamellae with n -values differing by one unit (occasionally two) units frequently occur in samples with nominal compositions with n _>3.[6]
3. S U P E R S T R U C T U R E S ORDERING 3.1. The Y-Ba-Cu-O system
DUE
TO
HREM and e-diffraction have been used extensively to detect superstructures. Because of the weakness of superstructure reflections, often due to the small coherence length such reflections are sometimes missed in n- or X-diffraction. HREM makes it possible to determine in which layer(s) the superstructure is localized in a complex structure, providing the basis for a model of the superstructure. Many different substitutions, have been performed in the C u t layers of the 1-2-3 type compounds. They are motivated by the consideration that via these layers the carrier density can be optimized. Of particular structural interest is the
163
substitution of Cu by cations such as Co,Ga,AI .... which favour a tetrahedral coordination with oxygen instead of the square planar one of copper. Also complex ions such as sulphate, carbonate or phosphate groups have been incorporated in the C u t layers. However the simplest modification of the C u t layers consists in oxygen deficiency, which gives rise to the 2at (ortho II) and 3at (ortho III) vacancy ordered structures, in which along the a t direction Cu-O-Cu-O chains parallel to bo and parallel Cu-~-Cu-~ chains form arrangements with periods 2ao, 3ao .... Also in YBa2Cu3.507.5 with alternating single and double C u t layers the vacancy ordered superstructures occur [6]; they are localized in the single C u t layers only. Computer simulated images show that relaxation around the vacancy rows is significant (fig. 2).
3.2. The (Hg,Pr)-Sr-Cu-O system Samples of the 1201 type with a composition HgxPrl_xSr2CuO4+8 where 0,4 < x < 0,5 exhibit an orthorhombic structure with lattice parameters a = 0,76 n m , = 2at, b = 0,368 nm and c=0,888nm.[7] This phase is a superstructure of the 1201 structure of HgBa2CuO4+8. Comparing the images along the [010] and [100] zones with those of the corresponding praesodymum free phase shows that the superperiod must be associated with the arrangement in the mercury containing layers since these are the only layers exhibiting period doubling, alternating dots having different brightness. The fact that the cations in the other layers exhibit their usual dot configurations and the consideration that the only modification in the chemical composition is the addition of Pr and the corresponding reduction in Hg content, lead to the assumption that Hg and Pr become ordered and give rises to the observed superlattice. A model in which the Hg rows parallel to the b-direction are replaced by the regular alternation of Hg and Pr chains is consistent with all observations. Computer simulated images based on this model reproduce well the observed images. The model is also consistent with the Hg/Pr ratio. The Hg-Pr ordering not only occurs in the 1201 compound but also in the 1212 and the 1222 compound.J9]. An image of the latter and the defects occurring in the ordering is shown in figure 3. The superstructure also causes a slight orthorhombicity of the basic lattice. The superstructure clearly allows for coherent twinning on { l l 0 } p planes, the rows of Hg and Pr being mutually perpendicular in the two parts of the twin. Such twins have been observed in HREM along the [001] zone.[7,8] In the diffraction pattern along this
164
S. Amelinc~:r, G. Van Tendeloo/Physica C 235 240 H994) 162 /69
zone the twins simulate fourfold symmetry provided the orthorhombic deformation of the basic lattice is ignored; the latter causes spot splitting of all hko spots except for those in the [hho] row perpendicular to the twin interface. The orthorhombic deformation of the basic structure is clearly a consequence of the loss of fourfold symmetry in the (Hg, Pr) layers.
3.3. Co a n d Ga substituted 1-2-3 In the Co and Ga substituted 1-2-3 type compounds YSr2CoCuO7 and YSr2GaCu207 the layer sequence along c is SrO-CuO2-Y-CuO2-SrOMO (M=Co, Ga). Chains of corner linked CoO4(GaO4) tetrahedra are formed in the MO layers along { l l 0 } p directions. The Ga-structure was described with reference to an orthorhombic unit cell with lattice parameters ao : b o : ap ",/2; co = 2Cp (ap = 0,38nm, Cp ~ 1,14nm). The chains are parallel to bo and they arc assumed to be all of the same kind; successive layers being stacked in a staggered fashion. It was found by electron microscopy that a superstructure with a unit cell as=2ao, bs=b o and cs=co is present in two variants differing by a 90 ° rotation about the c-axis.[9] Fig. 4 shows the two variants in a single HREM image; one variant is viewed along [100]o the other one along [0101o i.e. ~dong the chain direction. The positions of the cobalt containing columns are indicated by open dots. A model whereby successive chains in the same layer are assumed to be mirror images i.e. left (L) and right (B) chains is consistent with the superstructure. In fig. 4 the two types of chains cannot be distinguished since their projections are the same along [010]o. However along the [ l l 0 ] s zone it is possible to distinguish between a sequence ol identical chains and one consisting of alternating L and R chains. (fig. 5) It reveals the period doubling and images a number of defects in the stacking of the chains. The lateral shift over 1/4 of the interdot spacing between successive rows of dots is consistent with the model.
3.4. S O 4 substituted 1-2-3 In ( Y S r C a ) S r 2 C u 2 , 7 8 ( S O 4 ) 0 , 2 2 0 7 _ 6 the sulphate groups substitute in part for copper in the Cud layers; the 1-2-3 structure being conserved. The diffraction patterns of this material exhibit incommensurate satellite sequences associated with the basic reflections m the [010] and [001] zones; they reveal a modulated structure. The most informative HREM image is obtained along the [010] zone; it exhibits a quasi-lattice of prominently bright dots forming small crosses
situated along the traces of C u d layers (fig. 0); they image the chains parallel to bo, which contain a significant concentration of sulphur atoms. The arrangement is not strictly periodic and it depends on the SO4 content. It can be described as resulting from a concentration wave with a wave vector enclosing an angle c~ = 30 ° with the [100] direction and a length q - l n m - l . l l 0 ] The positions of the SO4 containing chains are determined by the maxima of this concentration wave and by the requirement to be situated in C u d layers and on chain sites. (fig. 6). The bright lines represent the maxima of the concentration waves. Along any C u d layer the spacings between successive S-bearing chains form a non-periodic sequence such as 3 3 4 3 4 ... (in units of ao), ACKNOWLEDGEMENTS The authors are grateful to T. Krekels, (). Milat, M. Hervieu, B. Raveau, C. Greaves, C. Chaillout, H.W. Zandbergen, E. Kaldis for the use of results from common research.
REFERENCES l 1]
T. Krekels, G. Van Tendeloo, S. Amelinckx, J. Karpinski, S. Rusiecki, E. Kaldis and E. Jilek, Physica C 178 (1991) 383. [2] H.W. Zandbergen, W.A. Groen, F.C. Mijlhoff. G. Van Tendeloo and S. Amelinckx. Physica (7 156 (1988) 325. 13] S.N. Putilin, E.V. Antipov, O. Chmaissem and M. Marezio, Nature 362 (1993) 226. [41 E.V. Antipov, SM. Loureiro, C. Chaillout, J.J. Capponi, P. Bordet, J.L. Tholence, S.N. Putilm and M. Marezio, Physica C 215 (1993) 1. [5] G. Van Tendeloo, C. Chaillout, J.J. Capponi, M. Marezio and E.V. Antipov, Physica C 223 (1994) 219. [ 61 T. Krekels, G. Van Tendeloo, J. Karpinkski, E. Kaldis and S. Rusiecki, Appl. Phys. Lett. 59 i1991) 3048. 17] F. Goutenoire, P. Daniel. M. Hervieu, G. Van Tendeloo, C. Michel, A. Maignau and B. Raveau, Physica C 216 (1993) 243. [8] G. Van Tendeloo, M. Hervieu, X.F. Zhang and B. Raveau. J. Sol. State Chem. (1994) in the press. [91 T. Krekels, O. Milat, G. Van Tendeloo, S. Amelinckx, T.G.N. Babu, A.J. Wright and C. Greaves, J. Sol. State Chem. 105 (1993) 313. [ 10] T. Krekels, O. Milat, G. Van Tendeloo, J. Van Landuyt, S. Amelinckx, R.P. Slater and C. Greaves, Physica C 210 (1993) 439.
S. Amelinckx, G. Van Tendeloo/Physica C 235-240 (1994) 162-165
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165
I
Figure 1. HREM image of the 1-2-3,5 and the 1-2-4 structure along the [010] zone. The elongated bright dots correspond to channels between CuO4 square planar arrangements in the double (CuO)2 layers.
Figure 4. Co (1-2-3) containing two orientation variants with corresponding diffraction patterns
m-rrmrm'ere,rmm
Figure 2. HREM image of vacancy ordering in the single CuO layers of the 1-2-3.5 structure as compared with a simulated image.
~~. •
'
•
C
~
~ - : 2 . ~
f
~
..... ". . . . . . . . . . . . . ~".....
. . . . 27"
Figure 5 : HREM of Co(1-2-3) imaged along [lOlls for one variant or along [410] s for a second orientation variant.
Figure 3. HREM image of the (Hg-Pr) 1222 compound, antiphase lines are indicated by arrows.
Figure 6 • [010] HREM image of SO4 (1-2-3).