carbonate cuprates” An electron microscopy study

I11gll ELSEVIER

Physica C 231 (1994) 103-108

The superconducting "copper/carbonate cuprates" An electron microscopy study M . A . A l a r i o - F r a n c o a,., p . B o r d e t a, j . . j . C a p p o n i a, C. C h a i l l o u t a, j . C h e n a v a s ~, T . F o u r n i e r a, M . M a r e z i o a, B. S o u l e t i e b, A. S u l p i c e b, J . - L . T h o l e n c e b, C. C o U i e x c, R . A r g o u d ~, J . L . B a l d o n e d o d, M . F . G o r i u s a, M . P e r r o u x a • Laboratoire de Cristallographie, CNRS, BP 166x-38042, Grenoble, France b CRTBT, CNRS, BP 166x~38042. Grenoble, France c Laboratoire de Physique des Solides, Universit~ de Paris-Sud, Orsay 91405. France d Centro de Microscopia, Universidad Complutense, 28040, Madrid, Spain

Received 12 April 1994; revised manuscript received 12 July 1994

Abstract

By means of electron microscopy and diffraction together with EDX and EELS, we have confirmed the main characteristics of the structure of the recently discovered HTSC family of"copper/carbonate cuprates". These can be obtained at high pressure and have critical temperatures of the order of 117 K. It consists of a copper/carbonate ordered variant of the tetragonal "single X-O layered" cuprate structure. The unit cell is given by a= 2at, b= bt and c= 2ct, where subindex t corresponds to the tetragonal single layer cell.

I. Introduction

We have recently [ 1 ] found, by means of a highpressure synthesis, a novel superconducting family apparently related to the "single X - O layered cuprates", from here onwards abbreviated as "singlelayer structure". These have a general formula XBa2Ca,_ ,Cu,O2,+2+~, where X is thallium or mercury [2,3] and, in our case, copper, but see below; this formula is sometimes abbreviated to { 1 : 2: ( n - 1 ) :n}. For reasons which are not obvious, the value of n is 1, 2, 3, 4 and, perhaps, 5, in the case of thallium, n = 1-6 in the case of mercury and n = 3, 4, 5, 6 and, at least, 7 - even if, up to now, only as an intergrowth - in the case of copper. * Corresponding author. On leave from: Facultad de Ciencias Quimicas, Universidad Complutense, 28040, Madrid, Spain.

Similar results have also been obtained by Ihara et al. [4], Jin et al. [5] and Wu et al. [6]. These materials were actually obtained in attempting to replace mercury or thallium by silver in the single-layer cuprates. After this paper had been refereed, an article by Kawashima, Matsui and T a k a y a m a - M u r o m a c h i [ 7 ] on the same subject has appeared. These authors suggest that this novel HTSC family corresponds, in fact, to a mixed " c u p r a t e / c a r b o n a t e " with carbon and copper being ordered in the basal plane of the unit cell, and n = 3, 4 and, possibly, another member. We have then checked, and confirmed, the presence of carbon, which was not suspected in the first version of this paper. Although the samples prepared at high pressure and high temperature were usually multiphased, an electron microscopy and diffraction study has allowed us

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to confirm the main characteristics of the structure as well as the composition of this new family. The structure is, in fact, a copper-deficient ordered variant of the single-layer cuprate structure [ 8 ], with the copper alternating with carbon along the a-axis. However, the copper deficiency seems to be restricted to the single layer, i.e. the basal plane of the unit cell, so that the conducting Cu-O2 planes are apparently not affected and these materials are superconducting with rather high Tc values. In this way, critical temperatures up to 117.5 K have been observed.

2. Experimental The present samples were prepared following Refs. [1,8 ]. Essentially, a barium-calcium-copper oxide precursor of the desired composition was mixed and ground with 2/n moles of silver oxide, AgO, prepared as in Ref. [ 9 ], per mole of copper. The role of silver oxide is that of an in situ oxidizing agent. Essentially no silver was present in the superconducting samples as determined by X-ray microanalysis, XEDS, in the electron microscope. The mixture was treated at 40 kbar and 850°C for 1.5 h. After this, the sample was quenched to room temperature and the pressure was then slowly released. Electron microscopy and diffraction and X-ray microanalysis were performed on a Philips 400 T microscope, operated at 120 kV and fitted with a Kevex, ~-class, X-EDS spectrometer, and on a Jeol 200 FX microscope, operated at 200 kV. In these experiments, holey carbon-coated aluminum grids were used; the well-established mercury cuprates were employed as standard materials. As carbon is a difficult element to detect by X-EDS, we have used EELS to confirm its presence in the crystals. These experiments were performed on a STEM VG HBS01 microscope, operated at 100 kV, fitted with a Gatan PEELS 666 spectrometer. The critical temperatures and the apparent superconducting fraction were obtained from magnetization measurements on a SQUID magnetometer and/ or from AC susceptibility measurements as a function of the temperature.

3. Results and discussion Magnetization measurements of a sample with a

nominal composition corresponding to the n = 3 term of the above general formula, i.e. CuBa2Ca2Cu308 +6 did show the presence of two superconducting transitions at ~ 117 K and ~ 80 K, with most of the superconducting sample corresponding to the lower one, Fig. 1. The X-ray powder pattern was rather complicated, having several overlapping lines, so that it was difficult to index. An electron microscopy and diffraction study was then attempted. Fig. 2 shows an electron diffraction pattern corresponding to the [001 ] axis of the single-layer structure. Although the strongest spots suggest a tetragohal structure, with a cell parameter a ~ 3.86 A, weaker spots, halving both a ° and b*, are also present. However, the absence of spots at the (h/2, k/2, O) positions for h, k odd indicates that only one of the real axes is doubled, the other also appearing doubled due to twinning (see below). Although all patterns along [001 ] were essentially identical, different c-axes were observed in different crystals. Fig. 3(a) shows a [ 1 0 0 ] / [ 0 1 0 ] pattern. From the ¢*-axis repeat, a c parameter of ~ 14.9 A is obtained. There exists a simple relationship between the c parameter and the n value of the corresponding family member in the single layer structure [ 8 ]. In the present case, c ~ = { 8 . 5 + 3 . 2 ( n - 1 ) } , so that the pattern of Fig. 3 (a) corresponds to n = 3. The observed doubling of the a-axis is reflected in 0.02 0 -0.02

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Fig. I. Magnetization as a function of temperature for a sample with nominalcomposition "CuBa2Ca2Cu30/'.

M.A. Alario-Franco et al. / Physica C 231 (1994) 103-108

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Fig. 2. Electrondiffractionpattern correspondingto the sample in Fig. 1; zone axis [001 ]. Fig. 3(a) in the extra rows appearing between the main spots. However, the fact that these extra rows only show spots with l = n + ½, (n = 0, l .... ) indicates that the c-axis is also doubled. The unit cell observed is then 2at X bt × 2ct, where at, b~ and ct correspond to the tetragonal cell of the single layer cuprates. The corresponding space group, as determined by electron diffraction, is Bmmm. This type of cell has already been found by Hervieu et al. [ 10] in materials of a similar kind, i.e. singlelayer cuprates, but having a mixed cationic occupancy of the single layer (the basal plane of the structure). This is the case in, for example, (Hgo.4Pro.6)Sr2(Srl _xPrx)Cu206+~, corresponding to n = 2 of the single-layer family. Streaking is also apparent along c*, in Fig. 3(a), suggesting the presence of some planar disorder along c in the real structure. This disorder is indeed rather obvious in the electron micrograph of Fig. 3(b), where, even if the main separation between the white fringes is often of ~ 15 A, i.e. c/2, as in regions marked N, there are many places where this separation is larger, as in the regions marked L. An intergrowth [ 11 ] of several family members having different nvalues, cf. b = a / 2 , is then a most plausible explanation of the disorder, and of the streaking. However, the most interesting feature of this pic-

105

ture resides in the structure within the white fringes. It can be seen that the white line is often (but not always) interrupted by short dark lines perpendicular to it and with a separation of ~ 7.6 A. Obviously, this corresponds to the a=2b-axis (cf. Fig. 3 ( a ) ) . Initially, from the X-EDS results we thought to have an approximate composition Ba2Ca,_lCU,+~/2Oy, or according to the single-layer formula: Cu~/2Ba2Ca,_ ~Cu,O2,+2+~. When this copper-"deficient" composition was tested in the synthesis, the most immediate consequence was a very marked increase in the superconducting fraction. Also, when the preparation of the n = 4 member was attempted, a single superconducting phase was observed, Fig. 4(a). It can be seen that there is a single transition, starting at ~ 70.0 K. However, somewhat to our surprise, electron diffraction of these new sampies only gave evidence of the n = 3 term of the above family. Even more, the composition of those crystals, as determined by EDX, was on average (six different crystals): Ba:Ca:Cu=27.3(8): 25.6(7):46.8(8), i.e. CUl/2Ba2Ca2Cu3Ox, clearly confirming the presence of the n = 3 term. On the other hand, the X-ray powder pattern showed the presence of the n = 3 term. It also showed that, besides silver, Ca6CuTOl4 was the main impurity. Nevertheless, it is to be noted that materials with more than 50% of superconducting sample, albeit multiphase, can already be obtained. Fig. 4(b) shows this, with Tc values of ~ 117 and 70 IC In this case, diffraction and analytical characterization in the electron microscope indicated the coexistence of crystals of the n = 3 and n = 4 terms; this was also deduced from the X-ray powder pattern. This, together with the above evidence, implies that the higher Tc corresponds to the n = 4 term. Since the report by Kawashima et al. [ 8 ] suggested the presence of carbon, EELS experiments were undertaken. Fig. 5 shows the spectra of two crystals that were partly lying on holes in the electron microscope grid- so as to prevent any possible interference of the carbon grid coating. The presence of barium, calcium, oxygen and carbon is obvious; copper, on the other hand, appears in a different region of the spectrum ( ~ 9 3 0 eV), but it was also present. The detailed fine structure of the carbon K edge is quite close to that of calcite, CaCO3, suggesting a similar environment for the carbon atoms.

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Fig. 3. (a) Electron diffraction pattern, zone axis [ 100]/[010]; (b) electron micrograph along the same orientation. Sample: same as Figs. 1 & 2 (for an explanation of the symbols see text). (c) Micrograph of another crystal clearly showing the two orientations of the unit cell as well as regions where the c-axis is not doubled.

M.A. Alario-Franco et ad. / Ph),sica C 231 (1994) 103-108

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Fig. 4. (a) Magnetic susceptibility as a function o f temperature for a sample o f Cul/2Ba,Ca2Cu3Os+6; a single transition is observed with To~ 70 K. (b) Magnetic susceptibility of a mixed, n = 3 and n = 4, sample. Two transitions with Tc ~ 117 K and ~ 70 K are observed.

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A copper/carbon ordering, along a, as suggested in Ref. [ 7 ] is then most likely to be present. Although this situation is then geometrically similar to that observed by Hervieu et al. [ 10 ], it is quite different in crystallochemical terms, due to the substantial differences existing between copper and carbon, which is likely to be included within a carbonate ion. Also, in these "copper cuprates', the terms that have been prepared so far have n values higher than 2. Clearly, if this a ordering alternates by ½[ 100 ] along c, both a and c will be doubled.

107

Although the resolution of the picture in Fig. 3 (b) is not sufficient to determine the crystal structure in detail - contrarily to the pictures at 1000 kV (see Fig. 3(b) in Ref. [8] ) we cannot distinguish the carbon atoms, which appear here as ordered vacancies- there are three more features worthy of comment: ( l ) In the first instance the fact that, although the white fringes have a "fine structure", the contrast that we have attributed to the a-axis doubling, sometimes there is no contrast; it is then continuous. This means that, in those fringes, the structure is oriented along [100] and the b-axis, ~ 3.8 A, is not resolved. The crystal is then twinned at a very fine scale (this is sometimes called disordered polysynthetic twinning [ 12] ): the [010] and [ 100] projections randomly alternate along c. Along [ 010 ], a ~ 7.7 A is resolved, while along [100], b~3.SA is not. Fig. 3(c), at somewhat higher resolution, confirms this point, since there both d~ooand d0~oare clearly resolved in different regions. (2) Another interesting feature of Fig. 3(b) is the fact that, sometimes, we observe a much wider white fringe. This is again sometimes interrupted by a mottled contrast, such as in D, while in other occasions there is no contrast, such as in E. The most obvious interpretation of these wider lines is to attribute them to the presence of two consecutive copper layers along c. This will then be observed either along [ 010 ] when the copper atoms are alternating with the carbon atoms, or along [ 100 ] when they are not. This is then reminiscent of the YzBa4Cu7Ol5 and YBa2Cu408 structures [ 13,14 ] of the so-called YBCO family, where two consecutive copper-oxygen chains are present between the barium planes. In fact, up to six consecutive copper-oxygen chains have been found, i.e. YBa2Cu60~o+6 [ 15 ]. More generally, this corresponds to regions of the double-layer cuprates, similar, then, to the bismuth or the thallium family. Yet, as already pointed out, the stoichiometry here is different. (3) A final interesting feature observed in Fig. 3(b) is the occasional presence of a parallel mottled contrast in consecutive (001) planes, as seen in the regions marked P. This is indicative of yet another type of structure, where the Cu/C ordering does not alternate along c, so that the unit cell is, in those regions, 2a,×b~×c~. This is quite obvious in Fig. 3(c). Interestingly enough, this type of unit cell has also been

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M.A. Alario-Franco et al. / Physica C 231 (1994) 103-108

observed in the multinary oxides studied by Hervieu etal. [10].

4. Concluding remarks Electron microscopy and diffraction confirm that the structure of this new family [ 1 ] of superconducting "copper/carbonate cuprates", is related to, but different from, the thallium and mercury "singlelayer" cuprates. It is characterized by a different, copper-"deficient" stoichiometry, and shows an ordering between copper and carbon in the metal positions in the single-layer plane. This originates a doubling of the unit cell in both a and c. Whether this stoichiometry also exists in the cases of the mercury and thallium single-layer cuprates remains to be seen. As we did not use carbonates in the synthesis of these materials, it must be concluded that the carbon comes out of the manipulation process; it is to be remembered that barium oxides are particularly prone to this type of contamination. Obviously, more work needs to be done concerning the synthesis conditions to master the corresponding phase diagram. In any event, confirmation of the copper/carbon ordering requires a crystal structure determination. On the other hand, it is worth pointing out that the well-known YBCO is formally related to the n = 2 member of this family; so far, however, the n = 2 term has not been found in the present Cul/2 (CO3) l/2Ba2Can_ ~CunOy system. Nevertheless, Kaldis et al. [ 16 ] and Bordet et al. [ 17 ] have observed a copper-deficient YBCO (YBa2Cu2.7807) that gave indications of critical temperatures of the same order as those found here. However, no indication of the presence of carbon, or of any extra ordering, appears to have been found in those solids. A further consequence of the present work is the observation that, contrarily to previous experience in the HTSC cuprates, the term with the highest Tc does not correspond to n = 3, but to n = 4, an intriguing result worth theoretical study. For this to happen, though, more should be known about the oxygen content, the coordination of the copper and carbon atoms, as well as the interatomic distances.

Acknowledgements We thank Dr. Barba and staff at the Centro de Microscopia Electr6nica of Complutense University. MAAF also thanks CICYT for financial support (Program MAT 92/0374).

References [I]M.A. Alario-Franco, C. ChaiUout, J.-J. Capponi, J.L. Tholence and B. Souletie, Physica C 222 (1994) 52. [2] R. Beyers, S.S.P. Parkin, V.Y. Lee, A.I. Nazzal, R. Savoy, G. Gorman, T.C. Huang and S. LaPlaca, Appl. Phys. Left. 53 (1988) 432. [3] S.N. Putilin, E.V. Antipov, O. Chmaissem and M. Marezio, Nature (London) 362 (1993) 226. [ 41 H. lhara, K. Tokiwa, H. Ozawa, M. Hirabayashi, A. Negishi, H. Natahuta and Y.S. Song, Jpn. J. Appl. Phys. 33 (1994) L503. [5] C.-Q. Jin, S. Adachi, X.J. Wu, H. Yamauchi and S. Tanaka, Physica C 223 (1994) 238. [6] X.J. Wu, S. Adachi, C.-Q. Jin, H. Yamauchi and S. Tanaka, Physica C 223 (1994) 243. [7] T. Kawashima, Y. Matsui and E. Takayama-Muromachi, Physica C 224 (1994) 69. [8] E.V. Antipov, S.M. Loureiro, C. Chailiout, J.-J. Capponi, P. Bordet, J.L. Tholence, S.N. Putilin and M. Marezio, Physica C215 (1993) I. [ 9 ] R.N. Hammer and J. Kleinberg, Inorganic Synthesis, Vol. 4 (1953) p. 12. [10] M. Hervieu, G. van Tendeloo, A. Maignan, C. Michel, F. Goutenoire and B. Raveau, Physica C 216 (1993) 264. [ 11 ] M. Vallet, J.M.G. Calbet, M.A. Alario-Franco, J.-C. Grenier and P. Hagenmullerr, J. Solid State Chem. 55 (1984) 251. [12] L.A. Bursill and B.G. Hyde, Prog. Solid State Chem. 7 (1972) 177. [13]P. Border, C. Chaillout, J. Chenavas, J.L. Hodeau, M. Marezio, J. Karpinski and E. Kaldis, Nature (London) 334 (1988) 596. [14] P. Marsh, R.M. Fleming, M.L. Mandich, A.M. de Santolo, J. Kwo, M. Hong and J.L. Martinez-Heredia, Nature (London) 334 (1988) 141. [ 15 ] M.A. S¢fiaris-Rodriguez, A.M. Chippindale, A. Vlirez, E. Morlin and M.A. Alatio-Franco, Physica C 172 ( 1991 ) 477. [ 16 ] E. Kaldis, J. Karpinski, S. Rusiecki, B. Bucher, K. Konder and E. Jilek, Physica C 185-189 ( 1991 ) 190. [ 17 ] P. Bordet, C. Chaillout, T. Fournier, M. Marezio, E. Kaldis, J. Karpinski and E. Jilek, Phys. Rev. B 47 (1993) 3465.