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2√2 ap × 2√2 ap phase in superconducting ceramics
PHYSlCA ELSEVIER
Physica C 248 (1995) 317-327
2~- ap X
ap phase in superconducting ceramics
T. Krekels "'*, S. Kaesche b, G. Van Tendeloo a a EMAT, RUCA, UniversiteitAntwerpen, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium b Max-Planek-lnstitut far Metallforschung, Institut far Werkstoffwissenschaft, PML, Heisenbergstrasse 5, D-70569 Stuttgart, Germany
Received 29 March 1995
Abstract By means of electron diffraction the 2v/2 ap )< 2V~- ap phase, well-known in YBa2Cu30 7_ ~s, was observed in two other perovskite-based materials (Yo.75Ceo.25)2(Sro.85Yo.15)2AIfu209 and Bil.sPbo.4Sr2Ca2Cu3Olo+x. Highly correlated ordering is observed in the ab-plane, the correlation along the c-direction being weak. The plane group of the superstructure symmetry elements was determined on the basis of observed reflection conditions in diffraction patterns. Our results unambiguously rule out oxygen ordering as a possible origin of the superstructure. Experimental evidence points out that the superstructure is associated with the CuO 2 layers, that are the only structural elements common to the three compounds studied. A model is proposed where the CuO 2 sheet is displacively modulated. Experimental evidence suggests a correlation between adjacent CuO 2 sheets. Comparison of simulated and experimental [001] zone diffraction patterns strongly supports our model.
1. Introduction In YBa2Cu307_ ~ by electron [1-9], X-ray [ 1 0 12] and neutron [13-15] diffraction a variety of superstructures have been observed that are due to ordering of the oxygen vacancy system in the CuO x_ 8 layer. The oxygen atoms in the layer order into CuO chains along the b-direction. Due to the variable oxygen content in the CuO 1_ ~ layer various alternation sequences of " f u l l " CuO chains and " e m p t y " C u - v a c a n c y chains occur. The 2 v ~ ap × 2V2 ap phase (subindices " p " indicating indexing with reference to the basic tri-perovskite) was discussed in many studies of the Y B a 2 f u 3 0 7 _ ~ compound and
* Corresponding author.
considered as an oxygen ordered superstructure [2-4, 14-19] or as due to partial decomposition [1]. Previously [20] we have purported experimental evidence refuting both these hypotheses. As an alternative model, we have proposed a deformation modulated superstructure, the displacements taking place in the pair of CuO 2 layers. In this paper we discuss the additional observation of the 2V~- ap X 2v/2- ap phase in (Yo.75Ceo.25)2(Sro.85Yo.15)2AICu209 and in Bil.sPb0.4SrECa 2Cu3Olo+x. In each of the compounds the superstructure is reproducibly obtained by heating the compound to about 500°C at a rate of 5 ° C / m i n , in situ. The pattern of superstructure reflections in [001] zone diffraction patterns is the same in the three compounds, but experimental details differ. Therefore we choose to treat the three materials separately and to recombine the results in the conclusions. The
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former experimental results for the YBa2Cu307_ ~ compound are represented here in brief and completed with new data.
2. Materials and experiment Details on materials preparation of the compounds YBa2Cu30 7_ ~ and (Y0.75Ce0.25)2(Sr0.85Y0.15)2AICu20 9 can be found in Refs. [21] and [22], respectively. The sample with the nominal composition of Bil.84Pb0.36Sr2Ca2Cu3O10 +~ was prepared by the conventional solid state synthesis route starting from 99.9% pure oxides (Bi203, PbO, CuO) and carbonates (CaCO3, SrCO3). The powders were weighed and intimately mixed in an agate mortar. Calcination was performed in three steps: 12 h at 750°C, 24 h at 780°C and 12 h at 800°C. After calcination the powder was ground, pressed into pellets and sintered at 850°C for 240 h with two intermittant grinding and pressing steps. Electron microscopy was performed on JEOL 4000EX and Philips CM20 microscopes. In the Philips CM20 microscope, heating experiments were performed in situ, i.e. inside the vacuum of the electron microscope (10 -7 Torr). A Gatan Ta-heating holder was used to temperatures of about 600°C.
3. Observations and discussion 3.1. Y B a 2 C u 3 0 7_ 8
The 2x/-}- ap × 2v/2" ap phase is reproducibly obtained by heating the material in situ, by means of a heating sample holder, to temperatures of about 500°C at a rate of 5°C/min [20]. Alternatively, the phase can be obtained by heating through focusing the electron beam on a grain. It is well known that during a heating treatment under vacuum oxygen is lost from the YBa2Cu30 7_8 material. Only in the tetragonal ~ range the 2v~- ap × 2v~- ap phase appears. That is strongly suggested by the disappearance of twin-splitted reflections prior to the appearance of the 2v/2- ap × 2x/2- ap phase reflections. Observations of the 2v/-2 ap × 2vf2-ao phase in the orthorhombic stoichiometry range [2,3] are probably
Fig. 1. [00lip zone electron diffraction pattern for the compound YBa2Cu307_ ~ after inducing the transition to the 2v/2- ap × 2v~ap phase. Intense reflections are due to the basic perovskite structure. Weaker but sharp reflections are superstructure reflections. Note the intensity distribution: superstructure reflections h + ~1 k + z1 0 are most intense at basic reflections hkO = 440 or 400, less intense at hkO = 220, and weaker elsewhere.
erroneous, and must be attributed to sample inhomogeneities. An upper limit of the oxygen content was set by obtaining the superstructure in a sample with an initial oxygen deficiency of 8 = 0.75. Thermodynamical measurements [23] yield a value 8 = 0.85 at the temperatures and pressures of our experiment. The [001] zone diffraction pattern is shown in Fig. 1. The superstructure has a square unit mesh of dimensions 2v~- ap × 2v~- ap, diagonal with respect to the perovskite mesh. Characteristic for the [001] zone diffraction pattern is a high sharpness and intensity of the superstructure spots. There is no intermediate transition state [24,5] of diffuse or streaked scattering in the diffraction pattern. Sharp extra reflections are weak at their appearance and intensify rapidly. Note the intensity distribution of the superstructure reflections. Roughly: superstruc1 ture reflections h __+X k + ~1 0 are most intense at basic reflections hkO = 440 or 400, less intense at hkO = 220, and weaker elsewhere. The [ll0]p zone pattern is reproduced in Fig. 2. In the [hk0] zone diffraction patterns streaks appear along c *, indicating that, although the superstructure within a (001) plane is well-ordered, the correlation between the ordered planes is successive unit cells is weak. (Close inspection of the streaks reveals a fine structure of weak reflections, of which the origin
T. Krekels et al. / Physica C 248 (1995) 317-327
cannot be assessed on the basis of the available experimental data.) The streaks are observed to be intensity modulated, typical of a system in which the scattering elements are paired [25]. The period of the modulation is the reciprocal of the spacing between the paired elements, in casu, the modulated or ordered layers. In the present case the modulation was measured to be 3.5c o , corresponding to a real space spacing of 0.286c 0. The pair of layers that fits the distance well within the experimental error is the CuO 2 pair (spaced 0.278c 0 [26], see Fig. 3(a)), not the pair of BaO layers (spaced 0.389c 0 [26]). Thus, rather than the BaO pair (that surrounds the CuO~_ layer), the CuO 2 layer pair is involved in the formation of the superstructure. Tilting experiments with the c* axis as tilt axis have allowed to determine the reflection conditions in the [001] zone diffraction pattern. In the [li0]p zone diffraction pattern of Fig. 2 streaks at positions h + ~ 1 h + ~ 1 and h + z 1 h + ~ 3 are systematically absent. Indexed with respect to the superstructure mesh one obtains: streaks hOl with h odd and Okl with k odd are absent. Reflection conditions in the (001)* plane thus become h0 with h even and Ok with k even. Note that double diffraction fills in the absent reflections in the [001] zone diffraction pattern of Fig. 1. Knowledge of the reflection conditions allows to determine p4gm (No. 12) as the plane group of the superstructure. Since no information is obtained about the stacking of modulated layer pairs along c, a space group cannot be derived. Space groups compatible with the plane group p4gm are
Fig. 2. [ll0]p diffraction pattern. Basic perovskite reflections 1 1 occur at positions h, h, l; streaks occur at positions h + 2, h + ~-. 1 Streaks at positions h + ¼, h + ¼ and h + 7, h + ~3 are absent due to space group extinctions. The period of the intensity modulation of the streak can be measured on this pattern to be 3.5c o .
319
a~
1.3~.
Fig. 3. Schematic structures of the unmodulated CuO 2 layer configuration for the three compounds discussed. (a) CuO 2 layerpair in YBa2Cu307_~, (b) CuO 2 layer-pair in (Yo75Ce0.25)2(Sro 85Yo.15)2AlCu209. Note the offset of ~2[110]0 in between both layers due to the insertion of the fluorite-like layer (Y, Ce)202, (c) CuO 2 layer triplet in the compound Bil.sPbo.4Sr2Ca 2Cu3Olo+x. White atoms: O, black atoms: Cu, grey atoms: Y, Ce or Ca. In the three cases, the layer multiplets shown are sandwiched by BaO or SrO layers.
P4212 (No. 90), P4bm (No. 100), P4b2 (No. 117) and P 4 / m b m (No. 127). The [010]p high resolution images of Fig. 4 allow a direct imaging of the locus of superstructure formation. Images obtained in two different grains are shown (grain 1: upper pair at defocus f = 300 ,~; grain 2: lower pair at f = - 2 0 0 .~.). For each grain a pair of images is shown, the left images taken in thin areas at the grain edges, the right images taken in thicker areas (of the same respective grains). Only
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Fig. 4. High-resolution image of YBa2Cu307_ ~ along the [010] ( = [100]) zone. The two pairs of images yield the same information but are obtained at different defocus (upper pair: f = 300 A, lower pair: .f = - 200 .~). The two left images are obtained at low thickness (upper left d = 40 ,~, lower left d = 10 ~,); only in thicker areas, the double period due to the superstructure is clearly visible (right images). The stacking along c[001] is irregular.
in the thicker areas, the superstructure contrast is pronounced. Comparison of the thin area images to simulated images (for the unmodulated structure; shown in the insets) allows to identify the different layers in the sequence. The identification can be extrapolated to the thicker areas, from which it is clear that the double period contrast does occur in the pair of C u O 2 layers, not in the CuO 1_ 0 layer. The images also reveal the stacking along c[001]. In the projection along b[010]p, domains are observed of vertical stacking, and others of staggered stacking (i.e. with an offset of a[100]p from layer to layer). Neither in the images along [010]p nor in images along the [310]p zone, long periodicities could be recognized.
Most models reported for the 2x/2- a v × 2vr2 ap phase are based on oxygen vacancy ordering in the CuO chain plane of the compound [2-4,14-19]. The absence of the transition state and the sharpness of the reflections are untypical of oxygen ordered phases, however [5]. We have constructed a wide variety of low oxygen content models based on an oxygen vacancy ordering in the CuO 1_ 8 plane. Some of these models included vacancies in the Ba layer, in order to reduce the oxygen content of the model structure below 1 - 8 = 0.25. The models constructed were all consistent with the determined plane group. Kinematical diffraction pattern simulations were carried out for these models as well as for those proposed in the literature, with unsatisfactory matches as a result. As an alternative model, a deformation modulated structure was proposed [20]. A plausible deformation is one of the CuO s pyramids in between the BaO and the Y layers. A possible mechanism could be of Jahn-Teller type [27]. A model based on the modulation of the CuO 2 pair is supported by the observation of the modulation of the streaks as described before. Simple models were constructed with displacements within the (001) plane. Both CuO 2 sheets of a pair are identically modulated and the displacement configurations coincide along c. The latter is suggested by the observation that the maxima of the modulations of the streak all occur at level l = 0. The CuO 1_ ~ layer is considered to contain only a low fraction of statistically filled oxygen sites, not necessarily critically related to the modulation in the CuO 2 layers. Details of the model are discussed in Section 4. 3.2. (Yo.75Ceo.25)2(Sro.s5Yo.15)2 A l C u 2 0 9
The
compound (Yo.75Ceo.25)2(Sro.85Yo.15)2layer sequence (Y, Ce)-OE-(Y, Ce)CuO2-(Sr, Y)O-AIO-(Sr, Y ) O - C u O 2 is similar to the compound YBazCu307_~, taking into account the following substitutions: the CuOl_ 8 layer has been replaced by an A10 layer with fixed stoichiometry, Ba has been substituted by Sr and the Y layer has been replaced by a fluorite-like layer (Y, Ce)O 2 - ( Y , C e ) [22]. The basic lattice is tetragonal and body centered due to the lateral shift over ½1110]p at the fluorite-like lamella. The unit cell dimensions are
AICu209 with
T. Krekels et al. / Physica C 248 (1995) 317-327
a 0 = b 0 = 3 . 9 .~ and c 0 = 2 8 . 2 .~. In a previous paper [28] we have described an oxygen vacancy ordered superstructure in the AIO layer, observed by
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means of electron diffraction and high resolution electron microscopy techniques. The oxygen vacancy ordering is of a different nature than that in the YBa2Cu3OT_ ~ compound. In the AIO layer meandering chains or closed loops are formed by corner linked A104 tetrahedra, on a two-dimensional lattice with a rectangular mesh with dimensions 2 a 0 × 4b 0. Two variants of the superstructure occur, which convert into each other by a mirror operation over the (ll0)p plane. The observed diffraction pattern is shown in Fig. 5(a). The body centering of the basic lattice yields reflection conditions for the basic reflections: h k 0 with h + k = even. Upon heating up to 500°C in the vacuum of the microscope, one finds that the diffraction pattern acquires the aspect of Fig. 5(b). The transition is sudden and occurs without the appearance of transitional diffuse intensity. Inspection of the diffraction patterns before (Fig. 5(a)) and after (Fig. 5(b)) the phase transition reveals that (1) the originally present 2 a 0 X 4b 0 superstructure has not disappeared; (2) weak extra reflections have appeared at positions h + ~ k1 0 ; h k + ~ 01 and intense ones at h + x 1 k 1 + 7 0, with h k 0 a basic reflection. The mesh onto which the configuration of new reflections must be indexed has dimensions 2 v ~ a 0 X 2 ~ - a 0, diagonal with respect to the perovskite mesh. (The subindex " 0 " indicates reference to the basic unit cell of the compound under discussion, in casu a 0 ~ ap.) The fact that the 2 a o X 4b o superstructure reflections coexist with the reflections of the new 2x/2- a 0 × 2v~-a 0 superstructure suggests that (1) also on a structural level two superstructures now coexist; (2) the new 2 v ~ a o X 2 v ~ a o modulation does not involve the S r O - A 1 0 - S r O triplet since the 2 a o × 4 b o superstructure remains unaffected. Since the reflection configuration (four most intense
Fig. 5. [001] zone electron diffraction patterns for the compound (Yo.75Ceo.25)2(Sro.85Y0.15)2AICu.209(a) before and (b) after inducing the transition to the 2¢2 ap x 2v/2 ap phase. The weak reflections in (a) are due to the oxygen ordering in the A10 layer. These reflections persist in (b). Extinctions of basic as well as superstructure reflections are due to the body centering of the lattice. (e) [001] zone diffraction pattern of the 2~- ap X2~- ap phase in YBa2Cu307_ ~ on which the reciprocal rows extinct in the Al-containing compound (h or k = 2(2n + 1); n integer) are indicated by double white lines.
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reflections around basic reflections) of the superstructure resembles that of the 2re2- ap × 2v~- ap superstructure in YBa2Cu30 7_ ~ and since a pair of CuO 2 layers is present in this compound as well (separated now by a fluorite layer instead of a single Y layer), we propose the following model for the
new phase: In the SrO-A10-SrO triplet the corner linked A104 tetraheder configurations persist; the CuO 2 layers undergo a displacive modulation, similar to that of the CuO 2 layers of the YBa2Cu30 7_ 8 compound. The two CuO 2 displacement configurations in a pair of CuO 2 layers are shifted over 1111010 due to the presence of the shift at the fluorite-like layer (see Fig. 3(b)). The difference in aspect between the patterns for the present compound and the YBa2Cu30 7_ 8 compound is due to this offset, as can be demonstrated by simple structure factor considerations. To avoid the necessity of a detailed knowledge of the displacement configuration in the CuO 2 layer, we assume the modulation in the CuO 2 layer of the YBa2Cu307_ 8 compound to be identical to that in a CuO 2 layer of the (Y0.75Ce0.25)2(Sr0.85Y0.15)2AlCu209 compound. In both compounds, the scattering into the (001)* plane, due to single modulated layers is then described by the same structure factor Fcu°2(h, k, 0). This factor we can consider as known, since its square is proportional to the diffraction pattern of the YBa2Cu30 7_ 8 compound: For the structure factor of the YBa2Cu3OT_ 8 compound, modulated by the 2vr}ap X 2v/2 ap phase, we can write F ( g ) = FCU°2(g) E exp(2-trig pj), J
(1)
where j = 0, 1 and Pl -- [0, 0, 0], P2 = R Irc --[0, 0, R]. And to describe the [001] pattern with g = (h, k, 0) s (indexed with respect to the supercell): F ( h , k, 0) = 2FC~°:(h, k, 0).
(2)
(We thus concentrate on the superstructure by only taking in account the CuO 2 layers. The other layers contribute to the basic spots only.) Eq. (1) describes the scattered amplitude by a pair of two identical layers, separated by a distance R, along the c-direction. In the case of the YBa2Cu307_ 8 phase, R = 3.4 A, the distance between the two layers in a CuO 2 bi-layer, that are separated by an Y layer. Eq. (1) describes equally well the (Y0.75Ce0.25)2(Sro.85Yoas)2AICu209 case. Although one unit cell contains four CuO2 layers, only the pair of CuO 2 layers in a CuO2-(Y, Ce)-O2-(Y, Ce)-CuO 2 quintuplet (Fig. 3(b)) are correlated. With Pl = [0, 0, 0], o
Fig. 6. [001]o zone electrondiffractionpatterns for the compound Bil.sPbo.4Sr2Ca2Cu3010+x. Image (a) is taken at 190°C where reflectionsdue to Bi and Pb modulations(indicatedby [1] respectively [2]) are both present. These reflectionspersist in (b), taken at 250°C, after the transition to the 2v~ ap X2v/2- ap phase was completed.
T. Krekels et al. / Physica C 248 (1995) 317-327
P2 = R LIc + R l c, the equation now describes a half unit cell containing two correlated bi-layers, displaced with respect to each other by the shift due to the (Y, Ce)-O2-(Y, Ce) fluorite-type layer. The displacement R blc now represents a separation of 6.1 ,~ along the c-direction between two CuO 2 layers of the pair, R± c = ½[ll0]p = l[100]s is the shift in the plane due to the fluorite layer. (Indexing " s " when referring to the 2x/2- ap × 2x/2- ap phase base.) To describe the [001] pattern:
F(h, k, 0) = Fcu°2(h, k, 0)[1 + exp('rri h / 2 ) ] ,
(3) where h and k now refer to the 2v~- ap × 2v/-2- ap cell. The superstructure spot positions (with respect to the reciprocal perovskite mesh) are identical to those for the 123 structure, but with altered intensities. In particular: F(h, k, 0) = 0 when h -- 4n + 2 (n integer). And because of the four-fold symmetry F(h, k, 0) = 0 when k = 4n + 2. The resulting diffraction pattern is that of the 2v~ap X 2V/2 ap phase of the Y B a 2 C u 3 O T _ 6 c o m p o u n d , with vanishing intensity for h = 4n + 2 or k = 4n + 2 as shown in Fig. 5(c). This pattern exactly matches the present observation, thus supporting the prior assumption that identical modulations deform the CuO 2 layers in the compounds YBa2Cu3078 and (Y0.75Ceo.25)2(Sr0.85Y0.15) 2AICH 209 .
3.3.
Bil. 8Pbo.4Sr2Ca2Cu301o
323
tures between 270°C and 500°C. Careful heating allows to obtain the 2a 0 × 2b 0 phase before the material starts decomposing. No diffuse rings are apparent in the diffraction patterns, neither are precipitations visible in the images. A [001] zone diffraction pattern is reproduced in Fig. 6(b). Inspection of the pattern yields that (1) the Bi modulation (period 4.5b 0) persists, and is unaffected by the heating; (2) the Pb modulation persists, but the real space period decreases with increasing temperature (12b 0 at room temperature to 7b 0 at 190°C); (3) extra reflections appear at positions n/4 m / 4 0 (n, m integer), the most intense ones at positions h +_ 1 1 k +__-~ 0 around a basic reflection h k 0. When indexed with respect to a perovskite basis one obtains a 2v~- ap × 2v~ ap mesh in the [001] zone view. The fact that the Bi modulation persists suggests that the superstructure is not effectuated in the BiO layers. The sharpness, intensity and intensity distribution of the extra reflections are similar to that in the compounds discussed above. Here also, the superstructure appears without the intermediate diffuse streaking in the diffraction patterns. In the [hk0] o zones streaks appear in the heated sample, as shown in Fig. 7. The intersection of these streaks with the (001)* plane yields the reflections in the [001] zone pattern (Fig. 6(b)). Tilting experiments with the c *-axis as tilt axis show that not all reflections in the [001] zone diffraction patterns are genuine but are in fact filled in by double diffraction.
+x
The structural characteristics of the Bil.sPb0.4Sr 2Ca2Cu3010+x compound at room temperature are well-known [29,30]. The perovskite-based unit cell is o B-centered and has dimensions 5.4 A × 5.4 A × 37.1 ,4, (reflection conditions are: for hkl: h + l --- 2n, for hkO: h, k = 2n, for Okl: k, l = 2n, for hOl: h + l= 2n). Two modulations are commonly observed in the compound, both along the b-direction, one modulation typical for the Bi compounds, the other due to the addition of Pb. In the [001] pattern of Fig. 6(a) reflections due to both these modulations can be observed. Upon heating at a rate of about 5°C/min, the Bit.sPb0.4Sr2Ca2Cu3Olo+x compound was observed to produce a 2a 0 x 2b 0 superstructure at tempera-
Fig. 7. [100] 0 zone diffraction pattern for Bil.8Pbo.4Sr2Ca2Cu 3O~o+x. The superstructure causes streaks at positions k +½. Streaks at positions k + ¼ and k + 43-are extinct because of space group symmetry requirements. The intensity modulation in the streaks is very prominent and has a period of 11.5c~.
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In the [100] 0 pattern of Fig. 7 the streaks at positions k + ~1 and k + Z3 are extinct. Reflection conditions in the (001)* plane, indexed with respect to the superstructure, are h0 for h even and Ok for k even, i.e. the same as those determined in the YBa~Cu307_ ~ case. The same plane group p4gm must thus be associated with this superstructure. As in the case of YBa2Cu307_ 8 the streaks in Fig. 7 are modulated. The period of modulation is now l l . 5 c d , corresponding to a real space interplanar distance of 0.087c 0. There are three layer sets that have a spacing close to the observed spacing: (1) The BiO-BiO layer pair is slightly more narrowly spaced (0.084c 0 [30]), and is, moreover, unaffected by the 2v~ ap × 2v~- ap superstructure, as suggested by the [001] zone diffraction pattern. (2) In the triple stack of CuO 2 layers (Fig. 3(c)) the Cu positions have a slightly larger spacing (0.090c 0 [30]) than the measured spacing. However, these layers are dimpled, in such a way that the O positions are spaced by 0.087c 0 [30]. (3) The pair of Ca layers is sepa-
rated by 0.087c 0. It follows that the modulation of the streaks suggests that the five-fold CuO2-CaCuO2-Ca-CuO 2 layer is involved in the formation of the superstructure. The intensity modulation of the streak in Fig. 7 is much more prominent than in the case of the YBa2Cu307_ 8 compound, presumably related to the fact that three layers are now correlated instead of two. High resolution images were recorded along the [100]0 and [010]0 (or [ll0]p and [110]) directions. The [100]0 view of Fig. 8 allows a direct imaging of the layer sequence in the c-direction. As in Fig. 4 for the YBa2CuaO 7_ 8 compound, only in thicker areas, the doubled period is well contrasted. The 2b 0 cell doubling can thus be observed in image 8(b), taken in the same grain as Fig. 8(a), but at higher thickness. As explained in the figure caption, the layers can be identified on the basis of symmetry elements in the image. The layer identification allows to determine the CuO 2 sheet triplet as the locus of the superstructure formation.
Fig. 8. (a) [010] 0 zone high-resolution image of the compound Bil.sPbo.4Sr2Ca2Cu3010+x. Different layers can be recognised by their symmetry: In thin areas one easily recognises the centering of the lattice, the centering being due to an offset between layers of the BiO-BiO pair (indicated by two short arrows). As these layers have been recognised, the other layers can be named making a direct layer-to-layer identification between model and image; (b) shows part of the image at higher thicknesses, showing the 2a 0 period of the superstructure, indicated by white arrows. The latter contrast occurs halfway between the BiO layers, or at the CuO2-Ca-CuOE-Ca-CuO 2 quintuplet (indicated by one long arrow).
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4. General discussion From the above we learn: (1) an identical treatment of in situ heating of the compounds YBa2Cu 30 7_ ~, (Yo.75Ceo25)2(Sro.ssYo.15)2AICu209 and Bil.8Pb0.4Sr2Ca2CUaOl0+x results in the formation of a superstructure which can be indexed on a 2 ~ ap × 2V~ ap mesh as observed along the [001] zone; (2) diffraction patterns show that the superstructure is well-ordered within the a-b-plane, and that a correlation between successive unit ceils along c is absent; (3) several factors (modulation of streaks in [h, k, 0] diffraction patterns and high resolution images) indicate that the CuO 2 layers are involved in the superstructure; (4) the formation of the 2 v ~ ap X 2x/2- ap modulation does not interfere with modulations (concentration or ordering) that are present in other layers of the compounds; (5) the formation of the 2 v ~ ap × 2V~- ap phase takes place before sample degradation due to heating or irradiation commences. Since the CuO 2 layer pairs are the only common feature of the three compounds and because of the small likelyhood of oxygen vacancies in the CuO 2 layer, and therefore also of oxygen vacancy ordering in this layer, the only reasonable model for the superstructure common to the compounds discussed is one of a displacive modulation in the CuO 2 layers. The symmetry of the displacement configuration within a layer is determined on the basis of the plane group derived above. Only one displacement configuration of the Cu framework is compatible with the plane group. It is shown in Fig. 9 and atom positions compatible to spacegroup 127 are given in Table 1. The Cu atoms at positions (+_ ¼, _ ¼, 0) are displaced over a distance v/2 Aa 0, other Cu positions are unchanged. The oxygen atoms remain positioned halfway between every two Cu atoms. Except for a translation, the displacement configuration does not depend on the choice of either one of the four space groups compatible with the plane group. The other layers in our model are considered to be unaffected, although the presence of an according relaxation of layers adjacent to the CuO 2 layers is likely. A correlation between two layers of a C u O 2 layer pair is necessarily introduced to explain the modulation of the streaks in [h, k, 0] zone diffraction patterns. For the Y B a 2 C u 3 0 7_ ~ compound, our model
Fig. 9. Schematic model of a single modulated CuO2 sheet. The Cu atoms at positions (+ 7,1 +a,l 0) are displaced over A in both x- and y-directions. Displacements of O atoms are A/2 in either direction. The symmetry of the displacement configuration is in correspondence with plane group p4gm. (Choice of origin according to plane group No. 12 or space groups No. 90 and 127.)
assumes a vertical (along c) coincidence of the displacement configuration. As shown in Fig. 3(b) in the (Yo.75Ceo.25)2(Sro.ssYo.15)eAICu20 9 compound the two CuO 2 layers are shifted over l [ l l 0 ] p . The model we propose assumes a similar shift over ½[ll0]p of the displacement configuration in a pair of CuO 2 layers, with respect to each other. The presence of the shift induces the presence of extinctions in the [001] zone diffraction pattern, as was shown above. For the compound Bil.sPbo.aSr2Ca 2CU3Olo+x the configurations in the three CuO 2 layers are again coincident along c. Although the latter Table 1 Positional parameters x, y, z for the modulated CuO2 layer in YBazCu307_ Site x y z Cu(2) Cu(2) Cu(2) 0(2/3) 0(2/3) 0(2/3)
4e 4f 8k 161 8k 8k
0 0 ~1 + g1 + g3 + ~+
A A/2
At/2 A/2
0 1/2 ~1 - A ~-- A/2 g1 - A/2 3 _ A/2
0.359
0.379
Space groupP4/mbm (N° 127). Lattice parameters a = b = 10.75 A, c = 11.7 A. Parameter d = 0.015, yields the simulated dynamical diffraction pattern best matching the experiment.
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5. Conclusion
Fig. 10. Calculated dynamical[001]p zone diffractionpattern for the compoundYBa2Cu307_8, based on the model shown in Fig. 8 with atom positionsas listed in Table 1. Microscopeparameters for the JEOL 4000EX 400kV electron microscope, thickness t = 15 .A. This image can readily be compared to the observed [001]p zone pattern of Fig. 1. Note that the intensity distribution of superstructurereflectionsin experimentalimage and simulation are identical. Large font: indexingwith respect to tri-perovskite, small font: indexingwith respectto supercell. compound is also centered, this centering does not influence the modulation (and thus does not introduce extinctions) as the centering is taking place between the BiO layers (between unrelated triplets of CuO 2 layers) and not between the CuO 2 layers of the correlated triplet (see Fig. 3(c)). Dynamical diffraction pattern calculations were performed by the MacTempas programme [33] with JEOL 4000EX microscope parameters. The calculated diffraction pattern of Fig. 10 is based on the model shown in Fig. 9. The parameter A was roughly optimised to A = 0.015 by trial-and-error, to give the best matching superstructure reflection intensity distribution. Attempts to produce the 2x/2 ap × 2v~ ap structure in other compounds were unsuccessful. The compounds Hg-2212 and YSr2CoCu20 7 desintegrate under the heating treatment before transforming; the 124 compound transforms to the 123 compound prior to forming the 2v~- ap X 2v~- ap modulation; in the Bi-2212 compound a resembling superstructure occurs under the same experimental conditions, with a slightly different, incommensurate mesh parameters. A 2v~" ap × 2v~- ap phase in this compound was reported previously in Refs. [31,32], however. It is very likely that the deformation model proposed here is valid for these compounds as well.
The 2vt2 ap × 2v~ ap phase, well-known in YBa2Cu307_8, has been found in two other perovskite-based materials (Yo.75Ce0.25)2(Sr0.85Yo.15)2A1Cu20 9 and Bil.sPb0.4Sr2Ca2Cu3010+x. Experimental evidence points out that the superstructure is associated with the CuO 2 layers, that are the only structural elements common to the compounds studied. Only one plane group and a limited number of space groups are in agreement with the systematic extinctions in electron diffraction patterns. A model was proposed where the CuO 2 layers are displacively modulated, the displacement vectors of Cu and O lying within the (001) plane. Only one displacement scheme for the Cu framework can correspond to the derived plane or space groups. The amplitude of Cu displacement is the only parameter. Comparison of the experimental [001] zone diffraction pattern to a dynamically calculated pattern strongly supports the model and allows to determine the Cu displacement. Experimental data show that within a unit cell the displacement configurations in CuO 2 layer pairs or triplets are correlated. The displacement configurations coincide along c in the CuO 2 pair in YBa2Cu30 7_ ~ and in the CuO~ triplet in Bil.8Pb0.aSr2Ca2Cu3Olo+x, an offset of l[ll0]p is present between both configurations in the CuO 2 pairs in (Y0.75Ceo.25)2(Sro.85Y0.15)2AlCu2Og. This offset leads to additional extinctions in the [001] zone diffraction pattern, that are understood on the basis of a simple structure factor argument. Finally, as a dispute has been going on in literature for a long time about the origin of the 2 ~ ap × 2 ~ - ap phase in YBa2fu307_8, we believe that these results unambiguously rule out the possibility of oxygen ordering in the CuO 1_ 8 layer.
Acknowledgements The authors kindly acknowledge C. Greaves and H. Verweij for materials synthesis. J. Reyes-Gasga and S. Amelinckx are acknowledged for their contribution to the work on the YBa2Cu307_ 8 compound. One of the authors would like to thank the German Academic Exchange Service (DAAD) for a grant.
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References [1] D.J. Werder, C.H. Chen and G.P. Espinoza, Physica C 173 (1990) 285. [2] M.A. Alario-Franco, C. Chaillout, J.J. Capponi and J. Chenavas, Mater. Res. Bull. 22 (1987) 1685. [3] M.A. Alario-Franco, C. Chaillout, J.J. Capponi, J. Chenavas and M. Marezio, Physica C 156 (1988) 455. [4] Y. Ueda and K. Kosuge, Physica C 156 (1988) 281. [5] J. Reyes-Gasga, T. Krekels, G. Van Tendeloo, J. Van Landuyt, S. Amelinckx, W.H.M. Bruggink and H. Verweij, Physica C 159 (1989) 831. [6] C. Chaillout, M.A. Alario Franco, J.J. Capponi, J. Chenavas, J.L. Hodeau and M. Marezio, Phys. Rev. B 36 (1987) 7118. [7] R.J. Cava, A.W. Hewat, E.A. Hewat, B. Batlogg, M. Marezio, K.M. Rabe, J.J. Krajewski, W.F. Peck Jr. and L.W. Rupp Jr., Physica C 165 (1989) 419. [8] R. Beyers, B.T. Alan, G. Gorman, V.Y. Lee, S.S.P. Parkin, M.L. Ramirez, K.P. Roche, J.E. Vazquez, T.M. Giir and R.A. Huggings, Nature 340 (1989) 6]9. [9] M. Hervieu, B. Domeng[s, B. Raveau, M. Post, W.R. McKinnon and J.M. Tarascon, Mater. Lett. 8 (1989) 73. [10] R.M. Fleming, L.F. Schneemeyer, P.K. Galagher, B. Batlogg, L.W. Rupp and J.V. Waszczak, Phys. Rev. B 37 (1988) 7920. [11] H. You, J.D. Axe, X.B. Kan, S. Hashimoto, S.C. Moss, J.Z. Liu, G.W. Crabtree and D.J. Lam, Phys. Rev. B 38 (1988) 9213. [12] T. Zeiske, D. Hohlwein, R. Sonntag, F. Kubanek and T. Wolf, Physica C 194 (1992) 1. [13] Y.P. Lin, J.E. Greedan, A.H. O'Reilly, J.N. Reimers, C.V. Stager and M.L. Post, J. Solid State Chem. 84 (1990) 226. [14] R. Sonntag, D. Hohlwein, T. Biickei and G. Collin, Phys. Rev. Lett. 66 (1990) 1497. [15] R. Sonntag, Th. Zeiske and D. Hohlwein, Physica B 180 181 (1992) 374. [16] A.A. Aligia, Europhys. Lett. 18 (1992) 181.
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[17] S. Semenovskaya and A.G. Gotchaturyan, Phys. Rev. B 46 (1992) 6511. [18] A.A. Aligia, J. Garces and H. Bonadeo, Physica C 190 (1992) 234. [19] S. Semenovskaya and A.G. Khachaturyan, Phil. Mag. Lett. 66 (1992) 105. [20] T. Krekels, T.S. Shi, J. Reyes-Gasga, G. Van Tendeloo, J. Van Landuyt and S, Amelinckx, Physica C 167 (1990) 677. [21] H. Verweij and L.F. Feiner, Philips J. Res. 44 (1989) 99; H. Verweij and W.H.M. Bruggink, J. Phys. Chem. Solids 49 (1988) 1063. [22] P.R. Slater, C. Greaves, Physica C 180 (1991) 299. [23] M. Tetenbaum, L.A. Curtiss, B. Tani, B. Czech and M. Blander, HTS-Physics and Materials Science, NATO Advanced Study Institute, 13-26 August (1989) Bad Windesheim, Germany. [24] D. Van Dyck, R. De Ridder, G. Van Tendeloo and S. Amelinckx, Phys. Stat. Sol. (a) 43 (1977) 541. [25] M. Verwerft, G. Van Tendeloo, J. Van Landuyt and S. Amelinckx, Appl. Phys. A 51 (1990) 332. [26] J.D. Jorgensen, M.A. Beno, D.G. Hinks, L. Soderholm, K.J. Volin, R.L. Hitterman, J.D. Grace, I.K. Schuller, C.U. Segre, K. Zhang and M.S. Kleefisch, Phys. Rev. B 36 (1987) 3608. [27] W.B. Pearson, The Crystal Chemistry and Physics of Metals and Alloys (Wiley-Interscience, New York, 1972) p. 273. [28] O. Milat, T. Krekels, G. Van Tendeloo, S. Amelinckx, C. Greaves and A.J. Wright, Physica C 217 (1993) 444. [29] A. Sequeira, J.V. Yakhmi, R.M. Iyer, H. Rajagopal and P.V.O.S.S. Sastry, Physica C 167 (1990) 291. [30] W. Carrilo-Cabrera and W. G/~pel, Physica C 161 (1989) 373. [31] H.W. Zandbergen and W.A. Groen, Physica C 166 (1990) 282. [32] J.G. Wen, Y.F. Yan and K.K. Fung, Phys. Rev. B 45 (1992) 561. [33] R. Kilaas and M.A. O'Keefe, in: Computer Simulation of Electron Microscope Diffraction and Images, eds. W. Krakow and M.A. O'Kcefe, TMMMS (1989) 171.
Physica C 248 (1995) 317-327
2~- ap X
ap phase in superconducting ceramics
T. Krekels "'*, S. Kaesche b, G. Van Tendeloo a a EMAT, RUCA, UniversiteitAntwerpen, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium b Max-Planek-lnstitut far Metallforschung, Institut far Werkstoffwissenschaft, PML, Heisenbergstrasse 5, D-70569 Stuttgart, Germany
Received 29 March 1995
Abstract By means of electron diffraction the 2v/2 ap )< 2V~- ap phase, well-known in YBa2Cu30 7_ ~s, was observed in two other perovskite-based materials (Yo.75Ceo.25)2(Sro.85Yo.15)2AIfu209 and Bil.sPbo.4Sr2Ca2Cu3Olo+x. Highly correlated ordering is observed in the ab-plane, the correlation along the c-direction being weak. The plane group of the superstructure symmetry elements was determined on the basis of observed reflection conditions in diffraction patterns. Our results unambiguously rule out oxygen ordering as a possible origin of the superstructure. Experimental evidence points out that the superstructure is associated with the CuO 2 layers, that are the only structural elements common to the three compounds studied. A model is proposed where the CuO 2 sheet is displacively modulated. Experimental evidence suggests a correlation between adjacent CuO 2 sheets. Comparison of simulated and experimental [001] zone diffraction patterns strongly supports our model.
1. Introduction In YBa2Cu307_ ~ by electron [1-9], X-ray [ 1 0 12] and neutron [13-15] diffraction a variety of superstructures have been observed that are due to ordering of the oxygen vacancy system in the CuO x_ 8 layer. The oxygen atoms in the layer order into CuO chains along the b-direction. Due to the variable oxygen content in the CuO 1_ ~ layer various alternation sequences of " f u l l " CuO chains and " e m p t y " C u - v a c a n c y chains occur. The 2 v ~ ap × 2V2 ap phase (subindices " p " indicating indexing with reference to the basic tri-perovskite) was discussed in many studies of the Y B a 2 f u 3 0 7 _ ~ compound and
* Corresponding author.
considered as an oxygen ordered superstructure [2-4, 14-19] or as due to partial decomposition [1]. Previously [20] we have purported experimental evidence refuting both these hypotheses. As an alternative model, we have proposed a deformation modulated superstructure, the displacements taking place in the pair of CuO 2 layers. In this paper we discuss the additional observation of the 2V~- ap X 2v/2- ap phase in (Yo.75Ceo.25)2(Sro.85Yo.15)2AICu209 and in Bil.sPb0.4SrECa 2Cu3Olo+x. In each of the compounds the superstructure is reproducibly obtained by heating the compound to about 500°C at a rate of 5 ° C / m i n , in situ. The pattern of superstructure reflections in [001] zone diffraction patterns is the same in the three compounds, but experimental details differ. Therefore we choose to treat the three materials separately and to recombine the results in the conclusions. The
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former experimental results for the YBa2Cu307_ ~ compound are represented here in brief and completed with new data.
2. Materials and experiment Details on materials preparation of the compounds YBa2Cu30 7_ ~ and (Y0.75Ce0.25)2(Sr0.85Y0.15)2AICu20 9 can be found in Refs. [21] and [22], respectively. The sample with the nominal composition of Bil.84Pb0.36Sr2Ca2Cu3O10 +~ was prepared by the conventional solid state synthesis route starting from 99.9% pure oxides (Bi203, PbO, CuO) and carbonates (CaCO3, SrCO3). The powders were weighed and intimately mixed in an agate mortar. Calcination was performed in three steps: 12 h at 750°C, 24 h at 780°C and 12 h at 800°C. After calcination the powder was ground, pressed into pellets and sintered at 850°C for 240 h with two intermittant grinding and pressing steps. Electron microscopy was performed on JEOL 4000EX and Philips CM20 microscopes. In the Philips CM20 microscope, heating experiments were performed in situ, i.e. inside the vacuum of the electron microscope (10 -7 Torr). A Gatan Ta-heating holder was used to temperatures of about 600°C.
3. Observations and discussion 3.1. Y B a 2 C u 3 0 7_ 8
The 2x/-}- ap × 2v/2" ap phase is reproducibly obtained by heating the material in situ, by means of a heating sample holder, to temperatures of about 500°C at a rate of 5°C/min [20]. Alternatively, the phase can be obtained by heating through focusing the electron beam on a grain. It is well known that during a heating treatment under vacuum oxygen is lost from the YBa2Cu30 7_8 material. Only in the tetragonal ~ range the 2v~- ap × 2v~- ap phase appears. That is strongly suggested by the disappearance of twin-splitted reflections prior to the appearance of the 2v/2- ap × 2x/2- ap phase reflections. Observations of the 2v/-2 ap × 2vf2-ao phase in the orthorhombic stoichiometry range [2,3] are probably
Fig. 1. [00lip zone electron diffraction pattern for the compound YBa2Cu307_ ~ after inducing the transition to the 2v/2- ap × 2v~ap phase. Intense reflections are due to the basic perovskite structure. Weaker but sharp reflections are superstructure reflections. Note the intensity distribution: superstructure reflections h + ~1 k + z1 0 are most intense at basic reflections hkO = 440 or 400, less intense at hkO = 220, and weaker elsewhere.
erroneous, and must be attributed to sample inhomogeneities. An upper limit of the oxygen content was set by obtaining the superstructure in a sample with an initial oxygen deficiency of 8 = 0.75. Thermodynamical measurements [23] yield a value 8 = 0.85 at the temperatures and pressures of our experiment. The [001] zone diffraction pattern is shown in Fig. 1. The superstructure has a square unit mesh of dimensions 2v~- ap × 2v~- ap, diagonal with respect to the perovskite mesh. Characteristic for the [001] zone diffraction pattern is a high sharpness and intensity of the superstructure spots. There is no intermediate transition state [24,5] of diffuse or streaked scattering in the diffraction pattern. Sharp extra reflections are weak at their appearance and intensify rapidly. Note the intensity distribution of the superstructure reflections. Roughly: superstruc1 ture reflections h __+X k + ~1 0 are most intense at basic reflections hkO = 440 or 400, less intense at hkO = 220, and weaker elsewhere. The [ll0]p zone pattern is reproduced in Fig. 2. In the [hk0] zone diffraction patterns streaks appear along c *, indicating that, although the superstructure within a (001) plane is well-ordered, the correlation between the ordered planes is successive unit cells is weak. (Close inspection of the streaks reveals a fine structure of weak reflections, of which the origin
T. Krekels et al. / Physica C 248 (1995) 317-327
cannot be assessed on the basis of the available experimental data.) The streaks are observed to be intensity modulated, typical of a system in which the scattering elements are paired [25]. The period of the modulation is the reciprocal of the spacing between the paired elements, in casu, the modulated or ordered layers. In the present case the modulation was measured to be 3.5c o , corresponding to a real space spacing of 0.286c 0. The pair of layers that fits the distance well within the experimental error is the CuO 2 pair (spaced 0.278c 0 [26], see Fig. 3(a)), not the pair of BaO layers (spaced 0.389c 0 [26]). Thus, rather than the BaO pair (that surrounds the CuO~_ layer), the CuO 2 layer pair is involved in the formation of the superstructure. Tilting experiments with the c* axis as tilt axis have allowed to determine the reflection conditions in the [001] zone diffraction pattern. In the [li0]p zone diffraction pattern of Fig. 2 streaks at positions h + ~ 1 h + ~ 1 and h + z 1 h + ~ 3 are systematically absent. Indexed with respect to the superstructure mesh one obtains: streaks hOl with h odd and Okl with k odd are absent. Reflection conditions in the (001)* plane thus become h0 with h even and Ok with k even. Note that double diffraction fills in the absent reflections in the [001] zone diffraction pattern of Fig. 1. Knowledge of the reflection conditions allows to determine p4gm (No. 12) as the plane group of the superstructure. Since no information is obtained about the stacking of modulated layer pairs along c, a space group cannot be derived. Space groups compatible with the plane group p4gm are
Fig. 2. [ll0]p diffraction pattern. Basic perovskite reflections 1 1 occur at positions h, h, l; streaks occur at positions h + 2, h + ~-. 1 Streaks at positions h + ¼, h + ¼ and h + 7, h + ~3 are absent due to space group extinctions. The period of the intensity modulation of the streak can be measured on this pattern to be 3.5c o .
319
a~
1.3~.
Fig. 3. Schematic structures of the unmodulated CuO 2 layer configuration for the three compounds discussed. (a) CuO 2 layerpair in YBa2Cu307_~, (b) CuO 2 layer-pair in (Yo75Ce0.25)2(Sro 85Yo.15)2AlCu209. Note the offset of ~2[110]0 in between both layers due to the insertion of the fluorite-like layer (Y, Ce)202, (c) CuO 2 layer triplet in the compound Bil.sPbo.4Sr2Ca 2Cu3Olo+x. White atoms: O, black atoms: Cu, grey atoms: Y, Ce or Ca. In the three cases, the layer multiplets shown are sandwiched by BaO or SrO layers.
P4212 (No. 90), P4bm (No. 100), P4b2 (No. 117) and P 4 / m b m (No. 127). The [010]p high resolution images of Fig. 4 allow a direct imaging of the locus of superstructure formation. Images obtained in two different grains are shown (grain 1: upper pair at defocus f = 300 ,~; grain 2: lower pair at f = - 2 0 0 .~.). For each grain a pair of images is shown, the left images taken in thin areas at the grain edges, the right images taken in thicker areas (of the same respective grains). Only
320
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Fig. 4. High-resolution image of YBa2Cu307_ ~ along the [010] ( = [100]) zone. The two pairs of images yield the same information but are obtained at different defocus (upper pair: f = 300 A, lower pair: .f = - 200 .~). The two left images are obtained at low thickness (upper left d = 40 ,~, lower left d = 10 ~,); only in thicker areas, the double period due to the superstructure is clearly visible (right images). The stacking along c[001] is irregular.
in the thicker areas, the superstructure contrast is pronounced. Comparison of the thin area images to simulated images (for the unmodulated structure; shown in the insets) allows to identify the different layers in the sequence. The identification can be extrapolated to the thicker areas, from which it is clear that the double period contrast does occur in the pair of C u O 2 layers, not in the CuO 1_ 0 layer. The images also reveal the stacking along c[001]. In the projection along b[010]p, domains are observed of vertical stacking, and others of staggered stacking (i.e. with an offset of a[100]p from layer to layer). Neither in the images along [010]p nor in images along the [310]p zone, long periodicities could be recognized.
Most models reported for the 2x/2- a v × 2vr2 ap phase are based on oxygen vacancy ordering in the CuO chain plane of the compound [2-4,14-19]. The absence of the transition state and the sharpness of the reflections are untypical of oxygen ordered phases, however [5]. We have constructed a wide variety of low oxygen content models based on an oxygen vacancy ordering in the CuO 1_ 8 plane. Some of these models included vacancies in the Ba layer, in order to reduce the oxygen content of the model structure below 1 - 8 = 0.25. The models constructed were all consistent with the determined plane group. Kinematical diffraction pattern simulations were carried out for these models as well as for those proposed in the literature, with unsatisfactory matches as a result. As an alternative model, a deformation modulated structure was proposed [20]. A plausible deformation is one of the CuO s pyramids in between the BaO and the Y layers. A possible mechanism could be of Jahn-Teller type [27]. A model based on the modulation of the CuO 2 pair is supported by the observation of the modulation of the streaks as described before. Simple models were constructed with displacements within the (001) plane. Both CuO 2 sheets of a pair are identically modulated and the displacement configurations coincide along c. The latter is suggested by the observation that the maxima of the modulations of the streak all occur at level l = 0. The CuO 1_ ~ layer is considered to contain only a low fraction of statistically filled oxygen sites, not necessarily critically related to the modulation in the CuO 2 layers. Details of the model are discussed in Section 4. 3.2. (Yo.75Ceo.25)2(Sro.s5Yo.15)2 A l C u 2 0 9
The
compound (Yo.75Ceo.25)2(Sro.85Yo.15)2layer sequence (Y, Ce)-OE-(Y, Ce)CuO2-(Sr, Y)O-AIO-(Sr, Y ) O - C u O 2 is similar to the compound YBazCu307_~, taking into account the following substitutions: the CuOl_ 8 layer has been replaced by an A10 layer with fixed stoichiometry, Ba has been substituted by Sr and the Y layer has been replaced by a fluorite-like layer (Y, Ce)O 2 - ( Y , C e ) [22]. The basic lattice is tetragonal and body centered due to the lateral shift over ½1110]p at the fluorite-like lamella. The unit cell dimensions are
AICu209 with
T. Krekels et al. / Physica C 248 (1995) 317-327
a 0 = b 0 = 3 . 9 .~ and c 0 = 2 8 . 2 .~. In a previous paper [28] we have described an oxygen vacancy ordered superstructure in the AIO layer, observed by
321
means of electron diffraction and high resolution electron microscopy techniques. The oxygen vacancy ordering is of a different nature than that in the YBa2Cu3OT_ ~ compound. In the AIO layer meandering chains or closed loops are formed by corner linked A104 tetrahedra, on a two-dimensional lattice with a rectangular mesh with dimensions 2 a 0 × 4b 0. Two variants of the superstructure occur, which convert into each other by a mirror operation over the (ll0)p plane. The observed diffraction pattern is shown in Fig. 5(a). The body centering of the basic lattice yields reflection conditions for the basic reflections: h k 0 with h + k = even. Upon heating up to 500°C in the vacuum of the microscope, one finds that the diffraction pattern acquires the aspect of Fig. 5(b). The transition is sudden and occurs without the appearance of transitional diffuse intensity. Inspection of the diffraction patterns before (Fig. 5(a)) and after (Fig. 5(b)) the phase transition reveals that (1) the originally present 2 a 0 X 4b 0 superstructure has not disappeared; (2) weak extra reflections have appeared at positions h + ~ k1 0 ; h k + ~ 01 and intense ones at h + x 1 k 1 + 7 0, with h k 0 a basic reflection. The mesh onto which the configuration of new reflections must be indexed has dimensions 2 v ~ a 0 X 2 ~ - a 0, diagonal with respect to the perovskite mesh. (The subindex " 0 " indicates reference to the basic unit cell of the compound under discussion, in casu a 0 ~ ap.) The fact that the 2 a o X 4b o superstructure reflections coexist with the reflections of the new 2x/2- a 0 × 2v~-a 0 superstructure suggests that (1) also on a structural level two superstructures now coexist; (2) the new 2 v ~ a o X 2 v ~ a o modulation does not involve the S r O - A 1 0 - S r O triplet since the 2 a o × 4 b o superstructure remains unaffected. Since the reflection configuration (four most intense
Fig. 5. [001] zone electron diffraction patterns for the compound (Yo.75Ceo.25)2(Sro.85Y0.15)2AICu.209(a) before and (b) after inducing the transition to the 2¢2 ap x 2v/2 ap phase. The weak reflections in (a) are due to the oxygen ordering in the A10 layer. These reflections persist in (b). Extinctions of basic as well as superstructure reflections are due to the body centering of the lattice. (e) [001] zone diffraction pattern of the 2~- ap X2~- ap phase in YBa2Cu307_ ~ on which the reciprocal rows extinct in the Al-containing compound (h or k = 2(2n + 1); n integer) are indicated by double white lines.
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reflections around basic reflections) of the superstructure resembles that of the 2re2- ap × 2v~- ap superstructure in YBa2Cu30 7_ ~ and since a pair of CuO 2 layers is present in this compound as well (separated now by a fluorite layer instead of a single Y layer), we propose the following model for the
new phase: In the SrO-A10-SrO triplet the corner linked A104 tetraheder configurations persist; the CuO 2 layers undergo a displacive modulation, similar to that of the CuO 2 layers of the YBa2Cu30 7_ 8 compound. The two CuO 2 displacement configurations in a pair of CuO 2 layers are shifted over 1111010 due to the presence of the shift at the fluorite-like layer (see Fig. 3(b)). The difference in aspect between the patterns for the present compound and the YBa2Cu30 7_ 8 compound is due to this offset, as can be demonstrated by simple structure factor considerations. To avoid the necessity of a detailed knowledge of the displacement configuration in the CuO 2 layer, we assume the modulation in the CuO 2 layer of the YBa2Cu307_ 8 compound to be identical to that in a CuO 2 layer of the (Y0.75Ce0.25)2(Sr0.85Y0.15)2AlCu209 compound. In both compounds, the scattering into the (001)* plane, due to single modulated layers is then described by the same structure factor Fcu°2(h, k, 0). This factor we can consider as known, since its square is proportional to the diffraction pattern of the YBa2Cu30 7_ 8 compound: For the structure factor of the YBa2Cu3OT_ 8 compound, modulated by the 2vr}ap X 2v/2 ap phase, we can write F ( g ) = FCU°2(g) E exp(2-trig pj), J
(1)
where j = 0, 1 and Pl -- [0, 0, 0], P2 = R Irc --[0, 0, R]. And to describe the [001] pattern with g = (h, k, 0) s (indexed with respect to the supercell): F ( h , k, 0) = 2FC~°:(h, k, 0).
(2)
(We thus concentrate on the superstructure by only taking in account the CuO 2 layers. The other layers contribute to the basic spots only.) Eq. (1) describes the scattered amplitude by a pair of two identical layers, separated by a distance R, along the c-direction. In the case of the YBa2Cu307_ 8 phase, R = 3.4 A, the distance between the two layers in a CuO 2 bi-layer, that are separated by an Y layer. Eq. (1) describes equally well the (Y0.75Ce0.25)2(Sro.85Yoas)2AICu209 case. Although one unit cell contains four CuO2 layers, only the pair of CuO 2 layers in a CuO2-(Y, Ce)-O2-(Y, Ce)-CuO 2 quintuplet (Fig. 3(b)) are correlated. With Pl = [0, 0, 0], o
Fig. 6. [001]o zone electrondiffractionpatterns for the compound Bil.sPbo.4Sr2Ca2Cu3010+x. Image (a) is taken at 190°C where reflectionsdue to Bi and Pb modulations(indicatedby [1] respectively [2]) are both present. These reflectionspersist in (b), taken at 250°C, after the transition to the 2v~ ap X2v/2- ap phase was completed.
T. Krekels et al. / Physica C 248 (1995) 317-327
P2 = R LIc + R l c, the equation now describes a half unit cell containing two correlated bi-layers, displaced with respect to each other by the shift due to the (Y, Ce)-O2-(Y, Ce) fluorite-type layer. The displacement R blc now represents a separation of 6.1 ,~ along the c-direction between two CuO 2 layers of the pair, R± c = ½[ll0]p = l[100]s is the shift in the plane due to the fluorite layer. (Indexing " s " when referring to the 2x/2- ap × 2x/2- ap phase base.) To describe the [001] pattern:
F(h, k, 0) = Fcu°2(h, k, 0)[1 + exp('rri h / 2 ) ] ,
(3) where h and k now refer to the 2v~- ap × 2v/-2- ap cell. The superstructure spot positions (with respect to the reciprocal perovskite mesh) are identical to those for the 123 structure, but with altered intensities. In particular: F(h, k, 0) = 0 when h -- 4n + 2 (n integer). And because of the four-fold symmetry F(h, k, 0) = 0 when k = 4n + 2. The resulting diffraction pattern is that of the 2v~ap X 2V/2 ap phase of the Y B a 2 C u 3 O T _ 6 c o m p o u n d , with vanishing intensity for h = 4n + 2 or k = 4n + 2 as shown in Fig. 5(c). This pattern exactly matches the present observation, thus supporting the prior assumption that identical modulations deform the CuO 2 layers in the compounds YBa2Cu3078 and (Y0.75Ceo.25)2(Sr0.85Y0.15) 2AICH 209 .
3.3.
Bil. 8Pbo.4Sr2Ca2Cu301o
323
tures between 270°C and 500°C. Careful heating allows to obtain the 2a 0 × 2b 0 phase before the material starts decomposing. No diffuse rings are apparent in the diffraction patterns, neither are precipitations visible in the images. A [001] zone diffraction pattern is reproduced in Fig. 6(b). Inspection of the pattern yields that (1) the Bi modulation (period 4.5b 0) persists, and is unaffected by the heating; (2) the Pb modulation persists, but the real space period decreases with increasing temperature (12b 0 at room temperature to 7b 0 at 190°C); (3) extra reflections appear at positions n/4 m / 4 0 (n, m integer), the most intense ones at positions h +_ 1 1 k +__-~ 0 around a basic reflection h k 0. When indexed with respect to a perovskite basis one obtains a 2v~- ap × 2v~ ap mesh in the [001] zone view. The fact that the Bi modulation persists suggests that the superstructure is not effectuated in the BiO layers. The sharpness, intensity and intensity distribution of the extra reflections are similar to that in the compounds discussed above. Here also, the superstructure appears without the intermediate diffuse streaking in the diffraction patterns. In the [hk0] o zones streaks appear in the heated sample, as shown in Fig. 7. The intersection of these streaks with the (001)* plane yields the reflections in the [001] zone pattern (Fig. 6(b)). Tilting experiments with the c *-axis as tilt axis show that not all reflections in the [001] zone diffraction patterns are genuine but are in fact filled in by double diffraction.
+x
The structural characteristics of the Bil.sPb0.4Sr 2Ca2Cu3010+x compound at room temperature are well-known [29,30]. The perovskite-based unit cell is o B-centered and has dimensions 5.4 A × 5.4 A × 37.1 ,4, (reflection conditions are: for hkl: h + l --- 2n, for hkO: h, k = 2n, for Okl: k, l = 2n, for hOl: h + l= 2n). Two modulations are commonly observed in the compound, both along the b-direction, one modulation typical for the Bi compounds, the other due to the addition of Pb. In the [001] pattern of Fig. 6(a) reflections due to both these modulations can be observed. Upon heating at a rate of about 5°C/min, the Bit.sPb0.4Sr2Ca2Cu3Olo+x compound was observed to produce a 2a 0 x 2b 0 superstructure at tempera-
Fig. 7. [100] 0 zone diffraction pattern for Bil.8Pbo.4Sr2Ca2Cu 3O~o+x. The superstructure causes streaks at positions k +½. Streaks at positions k + ¼ and k + 43-are extinct because of space group symmetry requirements. The intensity modulation in the streaks is very prominent and has a period of 11.5c~.
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In the [100] 0 pattern of Fig. 7 the streaks at positions k + ~1 and k + Z3 are extinct. Reflection conditions in the (001)* plane, indexed with respect to the superstructure, are h0 for h even and Ok for k even, i.e. the same as those determined in the YBa~Cu307_ ~ case. The same plane group p4gm must thus be associated with this superstructure. As in the case of YBa2Cu307_ 8 the streaks in Fig. 7 are modulated. The period of modulation is now l l . 5 c d , corresponding to a real space interplanar distance of 0.087c 0. There are three layer sets that have a spacing close to the observed spacing: (1) The BiO-BiO layer pair is slightly more narrowly spaced (0.084c 0 [30]), and is, moreover, unaffected by the 2v~ ap × 2v~- ap superstructure, as suggested by the [001] zone diffraction pattern. (2) In the triple stack of CuO 2 layers (Fig. 3(c)) the Cu positions have a slightly larger spacing (0.090c 0 [30]) than the measured spacing. However, these layers are dimpled, in such a way that the O positions are spaced by 0.087c 0 [30]. (3) The pair of Ca layers is sepa-
rated by 0.087c 0. It follows that the modulation of the streaks suggests that the five-fold CuO2-CaCuO2-Ca-CuO 2 layer is involved in the formation of the superstructure. The intensity modulation of the streak in Fig. 7 is much more prominent than in the case of the YBa2Cu307_ 8 compound, presumably related to the fact that three layers are now correlated instead of two. High resolution images were recorded along the [100]0 and [010]0 (or [ll0]p and [110]) directions. The [100]0 view of Fig. 8 allows a direct imaging of the layer sequence in the c-direction. As in Fig. 4 for the YBa2CuaO 7_ 8 compound, only in thicker areas, the doubled period is well contrasted. The 2b 0 cell doubling can thus be observed in image 8(b), taken in the same grain as Fig. 8(a), but at higher thickness. As explained in the figure caption, the layers can be identified on the basis of symmetry elements in the image. The layer identification allows to determine the CuO 2 sheet triplet as the locus of the superstructure formation.
Fig. 8. (a) [010] 0 zone high-resolution image of the compound Bil.sPbo.4Sr2Ca2Cu3010+x. Different layers can be recognised by their symmetry: In thin areas one easily recognises the centering of the lattice, the centering being due to an offset between layers of the BiO-BiO pair (indicated by two short arrows). As these layers have been recognised, the other layers can be named making a direct layer-to-layer identification between model and image; (b) shows part of the image at higher thicknesses, showing the 2a 0 period of the superstructure, indicated by white arrows. The latter contrast occurs halfway between the BiO layers, or at the CuO2-Ca-CuOE-Ca-CuO 2 quintuplet (indicated by one long arrow).
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4. General discussion From the above we learn: (1) an identical treatment of in situ heating of the compounds YBa2Cu 30 7_ ~, (Yo.75Ceo25)2(Sro.ssYo.15)2AICu209 and Bil.8Pb0.4Sr2Ca2CUaOl0+x results in the formation of a superstructure which can be indexed on a 2 ~ ap × 2V~ ap mesh as observed along the [001] zone; (2) diffraction patterns show that the superstructure is well-ordered within the a-b-plane, and that a correlation between successive unit ceils along c is absent; (3) several factors (modulation of streaks in [h, k, 0] diffraction patterns and high resolution images) indicate that the CuO 2 layers are involved in the superstructure; (4) the formation of the 2 v ~ ap X 2x/2- ap modulation does not interfere with modulations (concentration or ordering) that are present in other layers of the compounds; (5) the formation of the 2 v ~ ap × 2V~- ap phase takes place before sample degradation due to heating or irradiation commences. Since the CuO 2 layer pairs are the only common feature of the three compounds and because of the small likelyhood of oxygen vacancies in the CuO 2 layer, and therefore also of oxygen vacancy ordering in this layer, the only reasonable model for the superstructure common to the compounds discussed is one of a displacive modulation in the CuO 2 layers. The symmetry of the displacement configuration within a layer is determined on the basis of the plane group derived above. Only one displacement configuration of the Cu framework is compatible with the plane group. It is shown in Fig. 9 and atom positions compatible to spacegroup 127 are given in Table 1. The Cu atoms at positions (+_ ¼, _ ¼, 0) are displaced over a distance v/2 Aa 0, other Cu positions are unchanged. The oxygen atoms remain positioned halfway between every two Cu atoms. Except for a translation, the displacement configuration does not depend on the choice of either one of the four space groups compatible with the plane group. The other layers in our model are considered to be unaffected, although the presence of an according relaxation of layers adjacent to the CuO 2 layers is likely. A correlation between two layers of a C u O 2 layer pair is necessarily introduced to explain the modulation of the streaks in [h, k, 0] zone diffraction patterns. For the Y B a 2 C u 3 0 7_ ~ compound, our model
Fig. 9. Schematic model of a single modulated CuO2 sheet. The Cu atoms at positions (+ 7,1 +a,l 0) are displaced over A in both x- and y-directions. Displacements of O atoms are A/2 in either direction. The symmetry of the displacement configuration is in correspondence with plane group p4gm. (Choice of origin according to plane group No. 12 or space groups No. 90 and 127.)
assumes a vertical (along c) coincidence of the displacement configuration. As shown in Fig. 3(b) in the (Yo.75Ceo.25)2(Sro.ssYo.15)eAICu20 9 compound the two CuO 2 layers are shifted over l [ l l 0 ] p . The model we propose assumes a similar shift over ½[ll0]p of the displacement configuration in a pair of CuO 2 layers, with respect to each other. The presence of the shift induces the presence of extinctions in the [001] zone diffraction pattern, as was shown above. For the compound Bil.sPbo.aSr2Ca 2CU3Olo+x the configurations in the three CuO 2 layers are again coincident along c. Although the latter Table 1 Positional parameters x, y, z for the modulated CuO2 layer in YBazCu307_ Site x y z Cu(2) Cu(2) Cu(2) 0(2/3) 0(2/3) 0(2/3)
4e 4f 8k 161 8k 8k
0 0 ~1 + g1 + g3 + ~+
A A/2
At/2 A/2
0 1/2 ~1 - A ~-- A/2 g1 - A/2 3 _ A/2
0.359
0.379
Space groupP4/mbm (N° 127). Lattice parameters a = b = 10.75 A, c = 11.7 A. Parameter d = 0.015, yields the simulated dynamical diffraction pattern best matching the experiment.
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5. Conclusion
Fig. 10. Calculated dynamical[001]p zone diffractionpattern for the compoundYBa2Cu307_8, based on the model shown in Fig. 8 with atom positionsas listed in Table 1. Microscopeparameters for the JEOL 4000EX 400kV electron microscope, thickness t = 15 .A. This image can readily be compared to the observed [001]p zone pattern of Fig. 1. Note that the intensity distribution of superstructurereflectionsin experimentalimage and simulation are identical. Large font: indexingwith respect to tri-perovskite, small font: indexingwith respectto supercell. compound is also centered, this centering does not influence the modulation (and thus does not introduce extinctions) as the centering is taking place between the BiO layers (between unrelated triplets of CuO 2 layers) and not between the CuO 2 layers of the correlated triplet (see Fig. 3(c)). Dynamical diffraction pattern calculations were performed by the MacTempas programme [33] with JEOL 4000EX microscope parameters. The calculated diffraction pattern of Fig. 10 is based on the model shown in Fig. 9. The parameter A was roughly optimised to A = 0.015 by trial-and-error, to give the best matching superstructure reflection intensity distribution. Attempts to produce the 2x/2 ap × 2v~ ap structure in other compounds were unsuccessful. The compounds Hg-2212 and YSr2CoCu20 7 desintegrate under the heating treatment before transforming; the 124 compound transforms to the 123 compound prior to forming the 2v~- ap X 2v~- ap modulation; in the Bi-2212 compound a resembling superstructure occurs under the same experimental conditions, with a slightly different, incommensurate mesh parameters. A 2v~" ap × 2v~- ap phase in this compound was reported previously in Refs. [31,32], however. It is very likely that the deformation model proposed here is valid for these compounds as well.
The 2vt2 ap × 2v~ ap phase, well-known in YBa2Cu307_8, has been found in two other perovskite-based materials (Yo.75Ce0.25)2(Sr0.85Yo.15)2A1Cu20 9 and Bil.sPb0.4Sr2Ca2Cu3010+x. Experimental evidence points out that the superstructure is associated with the CuO 2 layers, that are the only structural elements common to the compounds studied. Only one plane group and a limited number of space groups are in agreement with the systematic extinctions in electron diffraction patterns. A model was proposed where the CuO 2 layers are displacively modulated, the displacement vectors of Cu and O lying within the (001) plane. Only one displacement scheme for the Cu framework can correspond to the derived plane or space groups. The amplitude of Cu displacement is the only parameter. Comparison of the experimental [001] zone diffraction pattern to a dynamically calculated pattern strongly supports the model and allows to determine the Cu displacement. Experimental data show that within a unit cell the displacement configurations in CuO 2 layer pairs or triplets are correlated. The displacement configurations coincide along c in the CuO 2 pair in YBa2Cu30 7_ ~ and in the CuO~ triplet in Bil.8Pb0.aSr2Ca2Cu3Olo+x, an offset of l[ll0]p is present between both configurations in the CuO 2 pairs in (Y0.75Ceo.25)2(Sro.85Y0.15)2AlCu2Og. This offset leads to additional extinctions in the [001] zone diffraction pattern, that are understood on the basis of a simple structure factor argument. Finally, as a dispute has been going on in literature for a long time about the origin of the 2 ~ ap × 2 ~ - ap phase in YBa2fu307_8, we believe that these results unambiguously rule out the possibility of oxygen ordering in the CuO 1_ 8 layer.
Acknowledgements The authors kindly acknowledge C. Greaves and H. Verweij for materials synthesis. J. Reyes-Gasga and S. Amelinckx are acknowledged for their contribution to the work on the YBa2Cu307_ 8 compound. One of the authors would like to thank the German Academic Exchange Service (DAAD) for a grant.
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