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Physica C 467 (2007) 192–197

Crystal structure analysis of Co-based layered cupratesCoSr2(Y1�xCax)Cu2O7+d by transmission electron

microscopy techniques

T. Nagai a,*, Y. Baba a,b, M. Karppinen c,d, T. Asaka a, S. Alam a, H. Yamauchi c,A. Yamazaki b, K. Kimoto a, Y. Matsui a

a Advanced Electron Microscopy Group, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japanb Department of Resources and Environmental Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku, Tokyo 169-8555, Japan

c Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japand Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, FI-02015 TKK, Finland

Received 15 October 2007; accepted 17 October 2007Available online 25 October 2007

Abstract

Crystal structures of Co-based 1212-type layered cuprates, CoSr2(Y1�xCax)Cu2O7+d (x = 0, 0.4; d = 0, 0.3) have been investigated bymeans of transmission electron microscopy (TEM). For the non-superconductive CoSr2YCu2O7.0 parent phase (x = 0, d = 0), electrondiffraction (ED) patterns revealed superlattice reflections due to an ordered arrangement of two kinds of chain of CoO4 tetrahedra thatare mirror images of each other, along the b direction. In the non-superconductive CoSr2(Y0.6Ca0.4)Cu2O7.0 phase (x = 0.4, d = 0), thesuperlattice reflections were found to be streaked along the b* direction, indicating that the partially disordered arrangement is inducedby aliovalent CaII-for-YIII substitution. In the superconductive CoSr2(Y0.6Ca0.4)Cu2O7.3 phase (x = 0.4, d = 0.3), the superlattice reflec-tions are not seen anymore, suggesting that the distinction of the two CoO4 chains has disappeared. Furthermore, high-resolution TEMobservation revealed that the introduced excess oxygen atoms interrupt the regular shift of the chain position along the c direction.� 2007 Elsevier B.V. All rights reserved.

PACS: 74.72.Jt; 61.14.�x; 61.16.Bg

Keywords: Co-based cuprates; Superconductor; Crystal structure; Transmission electron microscopy

1. Introduction

The well-known high-Tc superconductive layeredcuprate YBa2Cu3O7 (CuBa2YCu2O7 or Cu-1212) acceptsvarious elemental substitutions. A number of MA2Q-Cu2O7±d phases (A = Ba, Sr, La, etc.; Q = Ca, Y, etc.)have been successfully synthesized by means of such substi-tutions. The M-1212 phases with M = Ga, Al, and Co, par-ticularly, have attracted considerable interest as potential

0921-4534/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.physc.2007.10.012

* Corresponding author. Tel.: +81 29 851 3354X8612; fax: +81 29 8604700.

E-mail address: NAGAI.Takuro@nims.go.jp (T. Nagai).

candidates to show superconductivity and also due to thecomplicated structure of their MO1±d charge reservoirs[1–6]. In the charge reservoir, the M cations are tetrahe-drally coordinated by oxygen atoms to form chains of cor-ner-sharing MO4 tetrahedra that run diagonally relative tothe perovskite base. Further, it has been reported that theMO4 tetrahedra are arranged into two kinds of chain, L

and R, in which the tetrahedra rotate in different ways[4,7]. Quite recently, the first indication of superconductiv-ity was reported for Co-1212 (CoA2QCu2O7+d) indepen-dently by two groups [8,9]. Moreover, the roles of excessoxygen introduction and aliovalent Q-site cation substitu-tion were discussed in the process of superconducterizingthe CoSr2(Y1�xCax)Cu2O7+d phase [9].

T. Nagai et al. / Physica C 467 (2007) 192–197 193

In this paper, we present the crystal structure analysis ofthree Co-1212-phase samples of CoSr2(Y1�xCax)Cu2O7+d

(x = 0, 0.4; d = 0, 0.3) by transmission electron microscopy(TEM) techniques. Careful investigation by means ofelectron diffraction (ED) and high-resolution electronmicroscopy (HREM) revealed a change in the CoO1+d

charge-reservoir structure upon the aliovalent CaII-for-YIII

substitution and the excess oxygen introduction. Theimportance of the change in the process of superconduc-terizing the Co-1212 phase is discussed.

2. Experimental procedures

First two samples with nominal compositions of CoSr2-YCu2O7+d and CoSr2(Y0.6Ca0.4)Cu2O7+d were synthesizedin air through a conventional solid-state reaction route.The starting materials, Co3O4, SrCO3, Y2O3, CaCO3

and CuO, were mixed to the nominal ratios and calcinedin air at 975 �C and 1000 �C with an intermediate grinding.Chemical analyses showed that both the as-air-synthesizedsamples were stoichiometric in terms of oxygen, i.e. d � 0[9]. To introduce excess oxygen (d � 0.3), a portion of theCoSr2(Y0.6Ca0.4)Cu2O7.0 sample was high-pressure-oxygen(HPO) annealed in a cubic-anvil-type ultra-high-pressureapparatus at 5 GPa and 500 �C using Ag2O2 as an excessoxygen source [9]. Among the three samples, only thehigh-pressure oxygenated CoSr2(Y0.6Ca0.4)Cu2O7.3 samplewas found to be superconducting (Tc � 40 K) [9].

The ED patterns and HREM images were obtainedusing an ultra-high-voltage transmission electron micro-scope (Hitachi H-1500) operated at an accelerating voltageof 820 kV. The TEM specimens were prepared by crushingthe synthesized samples into fine fragments, which wereultrasonically dispersed in CCl4 and transferred to carbonmicrogrids.

Fig. 1. (a) An ED pattern taken along the [001] direction for the as-synthesizlinked CoO4 tetrahedra chains, L-chain and R-chain.

3. Results and discussion

Fig. 1a shows an ED pattern taken along the [001]direction for the as-synthesized non-superconductive CoSr2-YCu2O7.0 sample. The main hk0 (h + k = even) spots arederived from the basic perovskite structure. Weak super-lattice reflections are observed at (h, k ± 1/2, 0), whichindicates double periodicity along the b direction. The(0, k ± 1/2, 0) reflections are induced by multiple scatter-ing of electron beam, which is judged by another zone-axis pattern. As discussed above, in Co-1212 the Co andO atoms form two (L and R) kinds of chain of corner-linked CoO4 tetrahedra that are mirror images of eachother, as shown in Fig. 1b [7]. The superlattice reflectionsare indicative of an alternate arrangement of the L- andR-chains.

An HREM image taken for the same sample along the[100] direction and the corresponding ED pattern areshown in Fig. 2a. The 0kl (k, l = even) fundamental reflec-tions and 0kl (k, l = odd) ones are observed in the ED pat-tern. The former reflections are derived from the basic 1212structure, whereas the latter ones indicate the presence ofthe CoO4-tetrahedra chains. If the superlattice reflectionsrelated to the chain type are ignored, the reciprocal latticeis face-centered and therefore the real lattice is body-cen-tered, indicating a relative shift of the CoO4-chain positionby [1/21/21/2] between the neighboring CoO layers. Therelative shift is clearly seen in the HREM image. In thisimage, the white dot corresponds to a chain of oxygenvacancies between neighboring CoO4 chains in a CoOlayer. Based on these results for the as-synthesized CoSr2Y-Cu2O7.0 sample, we have proposed a superstructure modelwith P2cm (No. 28) orthorhombic symmetry and latticeparameters, as = a, bs = 2b, and cs = c, as illustrated inFig. 2b [7].

ed non-superconductive CoSr2YCu2O7.0 sample. (b) Two kinds of corner-

194 T. Nagai et al. / Physica C 467 (2007) 192–197

Fig. 3a shows an ED pattern taken along the [00 1] direc-tion for the non-superconductive CoSr2(Y0.6Ca0.4)Cu2O7.0

sample. The superlattice reflections are streaked along theb* direction. Hence, it is estimated that the arrangementof the L- and R-chains along the b direction becomes par-tially disordered, such as –L–R–L–R–R–L–, upon the alio-valent CaII-for-YIII substitution. From our previous X-rayabsorption near-edge structure (XANES) spectroscopy

Fig. 2. (a) An ED pattern and an HREM image taken along the [100] direction28) orthorhombic symmetry proposed for the CoSr2YCu2O7.0 sample.

Fig. 3. ED patterns taken along the (a) [001] and (b) [100] directions for themodel proposed for the CoSr2(Y0.6Ca0.4)Cu2O7.0 sample.

data for the same sample [10], it could be concluded thatthe holes produced by the CaII-for-YIII substitution arerather trapped on Co in the charge reservoir (in accordancewith the fact that the sample is non-superconductive). Pro-vided that the holes are randomly distributed among the Coatoms, it seems likely that the partial disorder of the chainarrangement is induced to maintain the local charge neu-trality within the CoO layers. The [100] zone-axis ED pat-

for the CoSr2YCu2O7.0 sample. (b) Superstructure model with P2cm (No.

non-superconductive CoSr2(Y0.6Ca0.4)Cu2O7.0 sample. (c) Superstructure

T. Nagai et al. / Physica C 467 (2007) 192–197 195

tern is very similar to that for the CoSr2YCu2O7.0 sample, asshown in Fig. 3b. It shows no disorder in the fundamentalstructure. Based on these results, we propose a superstruc-ture model, which is displayed in Fig. 3c.

Fig. 4. (a) An ED pattern and an HREM image taken along the [1–10] direcpattern taken along the [001] direction for the CoSr2(Y0.6Ca0.4)Cu2O7.3 samp

Fig. 5. (a) An ED pattern and an HREM image taken along the [100] directio[100] zone-axis HREM image.

Next, let us focus on the crystal structure of the HPO-annealed superconductive CoSr2(Y0.6Ca0.4)Cu2O7.3 sam-ple. It is expected that both the excess oxygen introductionand the aliovalent CaII-for-YIII substitution may have an

tion for the superconductive CoSr2(Y0.6Ca0.4)Cu2O7.3 sample. (b) An EDle.

n for the CoSr2(Y0.6Ca0.4)Cu2O7.3 sample. (b) An enlarged portion of the

196 T. Nagai et al. / Physica C 467 (2007) 192–197

influence on the structure. Fig. 4a shows the ED patternand the HREM image taken along the [1–10] direction.The 1212-type stacking of layers along the c-axis, SrO–CoO1+d–SrO–CuO2–(Y, Ca)–CuO2, is observed in theHREM image. The [001] zone-axis ED pattern shows nosuperlattice reflections, see Fig. 4b. It indicates that theL–R distinction of the CoO4 chains disappears due to theexcess oxygen atoms introduced into the vacant oxygensites in the CoO1+d layers. Previous synchrotron X-ray dif-fraction (SXRD) and neutron diffraction (ND) analyses ofthe phase have indicated a tetragonal P4/mmm structurewith a � 3.82 A and c � 11.30 A [10]. This result is in goodagreement with the disappearance of the L–R distinction,considering that the SXRD and ND measurements couldnot detect the very weak reflections due to the oxygen-vacancy chains in the sample. In fact, the [100] zone-axisED pattern and the HREM image, displayed in Fig. 5a,reveal the presence of oxygen-vacancy chains, i.e., CoOi

chains. The 0kl (k, l = odd) fundamental reflections in theED pattern are due to the presence of the CoOi chains.The HREM image clearly shows white dots which corre-spond to the chains of the vacancies. Using the reportedCu-valence value of V(Cu) = +2.17 in CoSr2(Y0.6Ca0.4)-Cu2O7.3 [10], the valence of Co may be calculated atV(Co) = +3.66. It is deduced that the high valence accom-panies the small average ionic radius, and that the shortCo–O bond length gives rise to no rotation of the CoO4 tet-

Fig. 6. A schematic illustration of the local crystal structure of theCoSr2(Y0.6Ca0.4)Cu2O7.3 sample. The relative shift of the CoOi chainalong the c direction is partially disordered and described as a mixture ofthe main shift by [1/21/21/2] (�A) and a partial shift by [001/2] (�B).

rahedra about the c-axis. This should be the reason for thedisappearance of the L–R distinction in the HPO-annealedsuperconductive sample.

The HREM image also provides another piece of infor-mation on the introduction of excess oxygen. In the image,the white dots corresponding to the chains of vacancieslocally disappear as shown in Fig. 5b. It suggests that asmall number of the chains accommodate most of theexcess oxygen atoms are therefore essentially saturatedwith oxygen. Moreover, these oxygen atoms shift the posi-tion of the neighboring CoOi chain by b/2 along the b

direction. A schematic illustration of the situation is shownin Fig. 6. Accordingly, the relative shift of the CoOi chainalong the c direction is partially disordered and describedas a mixture of the main shift by [1/21/21/2] (�A) and apartial shift by [001/2] (�B) such as –A–A–A–B–A–A–.The 0kl (k, l = odd) reflections in the [100] zone-axis EDpattern are streaked along the c* direction, which is consis-tent with the partially disordered arrangement of the CoOi

chains. In the HPO-annealed superconductive sample, theintroduced excess oxygen atoms interrupt the regular shiftof the position of the chains.

4. Conclusions

The change of the crystal structure in the process ofsuperconducterizing Co-based layered cuprates, CoSr2-(Y1�xCax)Cu2O7+d, was investigated by TEM techniques.The non-superconductive CoSr2YCu2O7.0 parent phasehas no structural disorder. In the phase, two kindsof chain of CoO4 tetrahedra, L and R, are alternatelyarranged along the b direction. Aliovalent CaII-for-YIII

substitution to the parent phase induces a partially disor-dered arrangement in CoSr2(Y0.6Ca0.4)Cu2O7.0. Finally,superconductorization of the phase by means of introduc-tion of excess oxygen in CoSr2(Y0.6Ca0.4)Cu2O7+d (d = 0.3)causes the disappearance of the L–R distinction. The super-conductive CoSr2(Y0.6Ca0.4)Cu2O7.3 phase consists of apartially disordered fundamental structure. A small num-ber of the oxygen-vacancy chains are responsible foraccommodating the excess oxygen atoms, which interruptsthe regular shift of the position of the CoOi chains.

Acknowledgements

The authors are grateful to C. Tsuruta for helpful dis-cussion and technical assistance with the present experi-ments. This work was supported by the NanotechnologySupport Project of the MEXT, Japan and also by aGrant-in-aid for Scientific Research (No. 15206002) fromthe Japan Society for the Promotion of Science and byAcademy of Finland (Decision No. 116254).

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