New trends in the crystal chemistry of high-Tc superconductors

Materials Science and Engineering, A I 0 9 (1989) 281-288

281

New Trends in the Crystal Chemistry of High-Tc Superconductors* B. RAVEAU, M. HERVIEU, C. MICHEL and J. PROVOST Laboratoire de Cristallographie et Sciences des Mat&iaux (CRISMA 7), lnstitut des Sciences de la Mati&e et du Rayonnement, Bd du Marichal Juin, 14032 Caen COdex (France) (Received June 2, 1988)

Abstract

The La2CuO4-type superconductors have opened the route to the research of superconductors with higher critical temperatures. The 90 K superconductor YBa2CuL~07_~ is a complex oxygen-deficient perovskite whose crystal chemistry is presented here: the high-resolution electron microscopy study of different compositions allows orderdisorder and non-stoichiometry phenomena to be understood. The superconducting properties of this oxide are interpreted here in terms of disproportionation of Cu(II) into Cu(III) and Cu(1). Besides this latter oxide, two other families, bismuthcopper and thallium-copper oxides, are described whose T,. ranges from 85 to 125 K. These two recent series belong with the La2CuO4-type oxide to the same structural family which consists of intergrowths of distorted rock salt-type layers and oxygen-deficient perovskite layers. The possible dynamic transfer of holes from thallium layers towards copper layers in those oxides is discussed.

very complex oxygen-deficient perovskite whose crystal chemistry has been extensively studied and is presented here. The recent investigations of systems involving thallium and bismuth have allowed seven new high-T~ superconductors to be detected in less than six months. The crystal chemistry of these oxides is described here. The evolution of the critical temperature vs. oxygen deficiency 6 in the oxide YBa2Cu307_ 6 (Fig. 1) established by different authors [21-24], leads to the following comments. (a) T~ values generally differ from one author to another. This can partly be due to the method used to measure the value and to the accuracy of the measurement of the oxygen content. But another important feature which is likely to be at the origin of these differences deals with the method of synthesis which can influence drastic-

Tc CK;

• TOKUMOTO

The investigations carried out over the last two years concerning the mixed-valence copper oxides have shown that, in addition to the La2CuO4-type oxides which are superconductive below 40 K [1-3], three other series of oxides were superconducting: YBa2Cu307_ 6 oxides [4-8] with T~ ranging from 40 to 92 K, bismuth cuprates [9-13] with T~ranging from 85 to 110 K and thallium cuprates [14-20] with T~ ranging from 60 to 125 K. From the point of view of the solid state chemist the non-stoichiometry and order-disorder phenomena should have a great influence upon the superconducting properties of those materials. In this respect YBa2Cu307_ 6 is a *Paper presented at the Symposium on Ceramic Materials Research at the E-MRS Spring Meeting, Strasbourg, May 31-June 2, 1988. 0921-5093/89/$3.50

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Fig. 1. Evolution of Tc w'. 6 in the oxide YBa2Cu307 0. © Elsevier Sequoia/Printed in The Netherlands

282

ally the structure and the morphology of the samples. (b) T~ decreases as 6 increases. This suggests that the mixed-valence Cu(III)-Cu(II) drastically influences Tc not only from a qualitative point of view but more quantitatively by taking into account the Cu(III)-to-Cu(II) ratio. (c) Two regions must be distinguished in this diagram on both sides of 6 = 0.50 if one takes into account the mean oxidation state of copper deduced from the classical charge balance. The first, with 0 ~<6 <0.50, is characterized by the mixed valence Cu(II)-Cu(III) and is expected to be superconductive whereas the second, which corresponds to 6>/0.50, involves Cu(II)-Cu(1) and should not be superconductive but semi-conductive or insulating, contrary to observations. (d) A discontinuous evolution of Tc vs. 6 is observed, and especially existence of plateaux, which suggests the possibility of ordering of oxygen vacancies. Considering the two extreme structures of the superconductive phase YBa2Cu307 (Fig. 2(a)) and the insulating phase YBa2fu306 (Fig. 2(b)) it appears that, for the intermediate compositions where 0 < 6 < 1, the additional oxygen vacancies are expected to appear in the [CuO2] chains of corner-sharing CuO4 square planar groups. Nevertheless, a statistical distribution of the anionic vacancies in those chains would lead to a threefold coordination of Cu(II)-Cu(III) which is not likely. The recent investigations of these oxides by high-resolution electron microscopy allow these phenomena to be explained. The preliminary studies of YBa2Cu307 showing the existence of twins, and oxygen over-stoichio-

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metry and sub-stoichiometry will not be presented here [25-32]. Let us expose some recent high resolution electron microscopy (HREM) observations [33] which show the "semi-ordered" distribution of the oxygen vacancies for different compositions intermediate between 07 and 06. Two compositions were especially investigated for such a purpose: 06.55 and 06.63 which exhibit a sharp transition with a T~ value of 60 K. All the crystals of these samples exhibit [001] electron diffraction patterns characterized by the splitting of the hkO reflections: this phenomenon results from an orthorhombic distortion of the cell, i.e. the ordering of the oxygen atoms and oxygen vacancies is established or set up enough to involve the twinning mechanism. Moreover, all crystals exhibit local superstructures and modulations of the contrast which are related to local variations and ordering of oxygen but their actual nature is sometimes difficult to determine owing to the moir6 patterns resulting from their overlapping. Such an example is shown in Fig. 3 where, all over the crystal, superstructures are set up with shifting and modulated features which only imply the existence of new periodicities in tiny areas. However, the mean principal directions of the oxygen ordering can be found: they are [100]p, [110]p, [210]p and [310]p where p is an index referring to the perovskite subcell. In the zone of the crystal indicated by 1, three directions

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Fig. 3. High-resolution [001] image of YBa2CusOr.6~ sample: superstructures are set up all over the crystal.

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are enhanced ([100]p, [l]0]p and [2i0]p) so that contrast periodicities of aJ2xapf5 appear locally whereas in zones 3 and 3', the [310]p direction is combined with local 2ap superstructure. The ordering of the oxygen atoms appears to be well established in local areas such as those shown in Figs. 4(a) and 4(b); they are set up along [100]p and [110]p respectively with periodicities ranging from two to four. These features are indeed observed when the crystals are viewed along [010]o (o is the index referring to the orthorhombic cell of the ordered perovskite "123") as shown in Figs. 5(a) and 5(b); in both images a periodicity of 2ao is observed but in the second image (Fig. 5(b)) an ordering is established in the same way along the c axis leading to 2a,, x 2Co. Several hypothetical ordering models of the oxygen and the vacancies can be proposed to

Fig. 5. High-resolution [010]o images with periodicity (a) 2a,, x c,, and (b) 2a,, x ~c --

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explain such phenomena; they are established by only taking into account the possible coordinations of copper, i.e. a twofold coordination for Cu(I) and the coordinations 4, 5 and 6 for Cu(II)-Cu(III). Some idealized models are shown in Fig. 6 to illustrate the way the oxygen can settle along favoured directions. These models take into account only the medium plane of copper between the BaO layers where the oxygen atoms and oxygen vacancies are ordered: the circle corresponds to the oxygen atoms and the full dots to the copper atoms. This illustration is so far not exhaustive but shows the rules for setting up the superstructure. A perspective view of two models 2% x c o and 2ao x 2c ° is shown in Figs. 6(e) and 6(f). The first can be described as an intergrowth along the a o axis of one [CuO2]oo chain of Cu(III)O4 groups with one [CuO]oo chain of Cu(I) in twofold coordination; such a model corresponds to an ideal formulation YBa2Cu306.5, or [WBa2Cu2(ll)Cu([II)O7]0.5, [YBa2Cu2(II)Cu(l)O6]0,5.

284

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The second, which corresponds to the same formulation, differs from the previous one by the shifting of one row out of two along the c axis. Thus it can be seen that every one corresponds to a particular oxygen content. Among all these complex phenomena which take place in the crystal in a more or less aleatory way it appears necessary to emphasize the one which is quite systematical. It is the existence of contrast modulations along a,, which appear in the orthorhombic crystals and whose direction

varies, from one twinning domain to the adjacent ones (Fig. 7). However, these modulations do not appear as regularly as in the proposed model (Figs. 6(a) or 6(e)). These variations along the a, direction can be easily considered as a way to accommodate a particular oxygen content: YBa,Cu,O, _ d would correspond to [YBa,Cu,(llJ1l)O,] 1_ s, [YBa,Cu,Wu(‘)OJ6, i.e. of [CU(~‘*~‘~)O~]~ chain and a ratio ( 1 - 6)/6 [Cu(‘)O]m rows, distributed all over the crystal. It should be noted that the contrast variation

285

Fig. 7. Modulations of the contrast along [100L, observed in an orthorhombic crystal: high-resolution [001],, image.

along one row in these modulations is difficult to interpret if we only consider a direct correlation of the brightness with the oxygen content. We must keep in mind that in such oxygen-deficient perovskites the contrast is considerably more sensitive to the cation displacements resulting especially from oxygen vacancies than to the vacancies themselves. In fact, the variations of parameters and Cu-O distances due to the change of oxidation state of copper from Cu(II)-Cu(III) to Cu(I) are large enough to induce strong distortions in such mixed crystals and to explain why a regular contrast is never observed all over the matrix. These observations confirm that copper keeps a normal coordination for particular ordering whatever the deviation from stoichiometry may be for 0 < 6 < 1, and explain that ordered compositions, which appear in the form of microdomains, may be at the origin of particular T~. values. Thus, for all these intermediate compositions, a model of disproportionation of Cu(II) into Cu(III) and Cu(1) can be proposed which corresponds to the coexistence in the same crystal of insulating regions or chains alternating with superconductive regions or chains. This disproportionation is confirmed by the X-ray absorption studies of these oxides [34, 35] which show without any ambiguity the presence of Cu(I) and of (Cu 2÷ - O - ) (3dgL). The discovery of superconductivity in the bis-

muth cuprates [9] and the thallium cuprates [14] opened the route to the synthesis of eight new oxides. The structures of these oxides which are schematically presented in Figs. 8(a)-8(e) are all closely related to the L%CuO4-type superconductors. They can be described as intergrowths of insulating rock salt-type layers and superconductive oxygen-deficient perovskite layers. The rock salt-type layers which are almost regular and single in the La2CuO4-type structure (Fig. 8(f)) are distorted and either double or triple in the case of bismuth and thallium compounds. In the same way the perovskite layers are multiple so that these oxides can be formulated [AO],,[A'Ca,,,_ ICumO2m+ 1] with A= Bi/Sr or T1/ Ba or La and A' = S r or Ba or La. The oxides Bi2Sr2CuO 6 [9, 36] and TI2Ba2CuO ~ [16, 36], which are superconductive below 22 K and 75 K respectively, represent like La2CuO4 the members m = 1 of the series, involving single perovskite layers, but contrary to La2CuO4 which contains only single rock salt-type layers (n = 1), they exhibit triple rock salt-type layers ( n = 3 ) formed of a bismuth or thallium bilayer sandwiched by strontium or barium monolayers (Fig. 8(a)). The members m = 2 are represented by Bi2Sr2CaCu20 ~ [10-13] and TI2Ba2CaCu20 s [36-38] which are superconductive below 85 K and 107 K respectively. In those oxides, which also exhibit triple rock salt-type layers (n=3), the double oxygen-deficient perovskite layers are formed of two simple layers of cornersharing pyramids interleaved with a plane of calcium ions (Fig. 8(b)). The 60 K superconductor T1Ba2CaCu207 [171 is also a member m = 2 of this series but differs from the other two oxides by the multiplicity of the rock salt-type layer (n = 2) which is formed from a thallium monolayer sandwiched by two barium monolayers (Fig. 8(d)). Two m = 3 members are known: the 125 K superconductor Tl2Ba2Ca2Cu301o [39-41] and the 120 K superconductor TIBa2Ca2Cu30, ~ [18, 19] (Figs. 8(c) and 8(e)). Both of them have their triple oxygen-deficient perovskite layers formed from two [CuO2.5]~ pyramidal layers and one [CuO2]~ layer of corner-sharing CuO4 groups interleaved with calcium ions; they only differ by the multiplicity of their rock salt-type layer which is triple for the first one (Fig. 8(c)) and double for the second (Fig. 8(e)). The detailed structure of those compounds is in fact more complex than described here especially in the case of bismuth [42-46] for which problems of incommensura-

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tion are observed which prevent an accurate structural determination to be achieved so that the question of localization of oxygen remains open: the formation of (Bi2Oj 2+ layers as in Aurivillius phases or of an excess of oxygen by forming distorted perovskite layers cannot be

ruled out. In all cases a distortion of the rock salttype layers and a possibility of cationic disorder and non-stoichiometry in those layers is observed. Moreover, numerous extended defects due to intergrowth phenomena are observed by H R E M [38, 40, 45]. Thus it is difficult to appreci-

287

ate the role of the mixed valence of copper in those oxides for which the problems of nonstoichiometry are not resolved. Nevertheless the formulation of several thallium oxides (TI2Ba2CaCuEO s, T12Ba2CuO6 and T1EBa2Ca2Cu3Olo ) suggests from the charge balance that superconductivity in those oxides would result from a dynamic transfer of the holes from the thallium layers towards the copper layers according to the equilibrium TI(III) + 2Cu(II) ~ - 2Cu(llI)+ TI(I) X-ray absorption investigations will be necessary to check the validity of this model.

16

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References 1 J. G. Bednorz and K. A. Muller, Z. Phys. B, 64 (19861 189. 2 N. Nguyen, J. Choisnet, M. Hervieu and B. Raveau, J. Solid State Chem., 39 ( 1981 ) 120. 3 C. Michel and B. Raveau, Rev. Chim. Mineral., 21 (1984) 407. 4 M. K. Wu, J. R. Ashburn, C. D. Torng, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang and C. W. Chu, Phys. Rev. Lett., ,58(1987)98. 5 C. Michel, E Deslandes, J. Provost, P. Lejay, R. Tournier, M. Hervieu and B. Raveau, C.R. Acad. Sci., 304 (19871 1059. 6 R.J. Cava, B. Battlog, R. B. van Dover, D. W. Murphy, S. Sunshine, T. Siegrist, J. P. Remeika, E. A, Rietman, S. Zahurak and G. P. Espinosa, Phys. Rev. l+ett., 58 (19871 1676. 7 Y. Lepage, W. R. McKinnon, J. M. Tarascon, L. H. Greene, G. W. Hull and D. M. Huang, Phys. Rev. B, 35 (198717245. 8 J. J. Capponi, C. Chaillout, A. W. Hewat, P. Lejay, M. Marezio, N. Nguyen, B. Raveau, J. L. Soubeyroux, J. L. Tholence and R. Tournier. Europhys. Lett., 3 (1987)

23

24

25 26 27 28 29 30 31 32 33

13O 1.

9 C. Michel, M. Hervieu, M. M. Borel, A. Grandin, F. Deslandes, J. Provost and B. Raveau, Z. Phys. B, 68 (19871421. 10 A. H. Maeda, Y. Tanaka, M. Fukutomi and T. Asano, Jpn. J. Appl. Phys., 27 (1988) L209. l l M. A. Subramanian, C. C. Torardi, J. C. Calabrese, J. Gopalakrishnan, K. J. Morrissey, T. R. Askew, R. B. Flippen, U. Chowdhw and A. W. Sleight, Science, 239 (1988) 1015. 12 R. M. Hazen, C. T. Prewitt, R. J. Angel, N. L. Ross, L. W. Fingen, C. G. Hadidiacos, D. R. Veblen, P. J. Heaney, P. H. Hor, R. L. Meng, Y. Y. Sun, Y. Q. Wang, Y. Y. Xue, Z. J. Huang, L. Gao, J. Bechtold and C. W. Chu, Phys. Rev. Lett., 60 (198811174. 13 C. Politis, Appl. Phys., A45 (19881261. 14 Z. Z. Sheng and A. M. Hermann, Nature (London), 332 (19881 55, 138. 15 Z. Z. Sheng, A. M. Hermann, A. El All, C. Almasan, J.

34

35

36

37

38 39

Estrada, T. Datta and R. J. Matson, Phys. Rev. Lett., 60 (19881937. R. M. Hazen, L. W. Fingen, R. J. Angel, C. T. Prewitt, N. L. Ross, C. G. Hadidiacos, P. J. Heaney, D. R. Veblen, Z. Z. Sheng, A. E1 Ali and A. M. Hermann, Phys. Rev. Lett., 60 (1988) 1657. M. Hervieu, A. Maignan, C. Martin, C. Michel, J. Provost and B. Raveau, J. Solid State Chem,, 75 (1988) 212. C. Martin, C. Michel, A. Maignan, M. Hervieu and B. Raveau, C.R. Acad. Sci., 307 (1988) 27. S.S.P. Parkin, V. Y. Lee, A. I. Nazzal, R. Savoy, R, Beyers and J. La Placa, t'hys. Rev. Lett., 61 (1988) 750. M. Hervieu, A. Maignan, C. Martin, C. Michel, J. Provost and B. Raveau, Mod. Phys., 2 (1988111(/3. J. M. Tarascon et al., Novel superconductivity, in S. A. Wolf and V. Z. Krezin (eds.), Proc. Int. Workshop on Novel Mechanism of Superconductivity, Berkeley, 1987, Plenum Press, New York, 1987. M. Monod et al., Orsay High-T~ Superconductors, 2 June, 1987. R. 1. Cava, B. Battlog, C. H. Chen, E. A. Rietman, S. M. Zahurak and D. Werder, Nature (London), 32 c) (1987) 423. M. Tokumoto, M. lhara, T. Matsubara, M. Hirabayashi, N. Terada, H. Oyanagi, K. Murata and Y. Kimur, Jpn. J. Appl. I'hys., 26 (1987) 1566. M. Hervieu, B. Domeng~s, C. Michel and B. Raveau, Europhys. Lett., 4 (1987) 205. B. Domeng/zs, M. Hervieu, C. Michel and B. Raveau, Europhys. Left.. 4 (1987) 211. M. Hervieu. B. Domeng6s, C. Michel, G. Heger, J. Provost and B. Raveau, Phys. Rev. B, 36 (19871392(t. M. Hervieu, B. Domengbs, C. Michel, J. Provost and B. Raveau, J. Solid State Chem., 71 (1987) 263. G. Van Tendeloo, N. W. Zandbergen and S. Amelynckx, Solid State ('ommun., 63 (1987) 389. N. W. Zandbergen, G. Van Tendeloo,, T. O. Kabe and S. Amelynckx, Phys., Status Solidi A, 103 ( 1987) 45. E.A. Hewat, M. Dupuy, A. Bourret, J. J. Capponi and M. Marezio, Nature (London), ,727 (1987). A. Ourmazd and J. C. Spence, Nature (London), 329 (19871. M. Hervieu, B. Domeng6s, B. Raveau, J. M. Tarascon, M. Post and W. R. McKinnon, J. Solid State Chem., submitted for publication. F. Baudelet, G. Collin, E. Dartyge, A. Fontaine, J. P. Kappler, G. Krill, J. P. ltie, J. Jegoudez, M. Maurer, Ph. Monod, A. Revcolevschi, H. Tolentino, G. Tourillon and M. Verdagues, Z. Phys. B, 69 (1988) 141. H. Oyanagi, M. lhara, T. Matsubara, M. Takumoto, T. Matsuhita, M. Hirabayashi and K. Murata, Jpn. J. Appl. l'hys., 26 (1987) 1561. C. C. Torardi, M. A. Subramanian, J. C. Calabrese, J. Gopalakrishnan, E. M. McCarron, K. J. Morrissey, T. R. Askew, R. B. Flippen, U. Chowdhry and A. W. Sleight, Phys. Rev. B, 38 (19881 225. M. A. Subramanian, J. C. Calabrese, C. C. Torardi, J. Gopalakrishnan, T. R. Askew, R. B. Flippen, K. J. Morrissey, U. Chowdhry and A. W. Sleight, Nature (London), 332 (1988) 420, A. Maignan, C. Michel, M. Hervieu, C. Martin, D. Groult and B. Raveau, Mod. Phys. Lett. B, 2 (1988)681, S. S. P. Parkin, V. Y. Lee, E. M. Engler, A. I. Nazzal, T. C.

288 Huang, G. Gorman, R. Savoy and R. Beyers, Phys. Rev. Left., 60 (1988) 2539. 40 M. Hervieu, C. Michel, A. Maignan, C. Martin and B. Raveau, J. Solid State Chem., 74 (1988) 428. 41 C. C. Torardi, M. A. Subramanian, J. C. Calabrese, J. Gopalakrishnan, K. J. Morrissey, T. R. Askew, R. B. Flippen, U. Chowdhry and A. W. Sleight, Science, 240 (1988)631. 42 J. M. Tarascon, W. R. McKinnon, P. Barboux, D. M. Hwang, B. G. Bagley, L. H. Greene, G. W. Hull, Y. Le Page, N. Stoffet and M. Giroud, Phys. Rev. B, 38 (1988) 8885.

43 N. Kijma, H. Eudo, J. Tsuchiya, A. Sumiyama, M. Mizuno and Y. Oguri, Jpn. J. Appl. Phys., 27 (1988) L821. 44 J. M. Tarascon, Y. Le Page, P. Barboux, B. G. Bagley, L. H. Greene, W. R. McKinnon, G. W. Hull, M. Giroud and D. H. Hwang, Phys. Rev. B, 37(1988) 9382, 45 M. Hervieu, C. Michel, B, Domengbs, Y. Laligant, A. Lebail, G. Ferey and B. Raveau, Mod. Phys. Lett. B, 2 (1988) 491. 46 H.G. Von Schnering, L. Walz, M. Schwarz, W. Becker, M. Hartweg, T. Popp, B. Hettlich, P. Miiller and G. Kampf, Angew. Chem., 27(1988) 574.