Physica C 156 (1988) 133-136 North-Holland, Amsterdam
H I G H - T E M P E R A T U R E SUPERCONDUCTIVITY IN LaBaCaCu306.ss W.T. FU, H.W. ZANDBERGEN ~, C.J. VAN DER BEEK and L.J. DE J O N G H Kamerlingh Onnes Laboratory and Gorlaeus Laboratories~, Leiden University, P.O. Box 9506, 2300RA Leiden, The Netherlands
Received 17 June 1988
A new superconductingcompoundwith compositionLaBaCaCu306s5 has been prepared. It crystallizesin a pseudo-tetragonal structure related to YBa2CU3OT.The onset superconductingtemperature is 78 K. Electrondiffraction deafly shows the presence of a superstructure in this material. Conductivityand superconductivitydata are presented and possible structures of the unit cell are discussed.
1. Introduction The observation of superconductivity above 90 K in the YBa2Cu307 system [1,2] has triggered an enormous amount of research for other structures that could display high-temperature superconductivity. The crystal structure of YBa2Cu307 itself has been well determined. It can be regarded as a modified perovskite structure with a perfect B a - Y - B a ordering. It is reported that in the related L a - B a - C u - O system, lanthanum can replace barium forming a solid solution La~+xBa2_xCU3OT+~ with 0
2. Experimental Samples were prepared by thoroughly mixing and grinding La203, BaCO3, CaCO3 and CuO powder in the stoichiometric ratio and firing at 950 °C in air for 24 hours. The samples were reground and retired at this same temperature to improve the homogeneity.
The final annealing was in oxygen flow at 400 °C for 2 hours. The resulting materials were characterized by Xray diffraction and electron diffraction. Samples prepared in this way appeared to be tetragonal. X-ray powder diffraction showed the presence of only small amounts of CaCuO2 as an impurity phase. The oxygen content was determined by a gravimetric procedure by heating the samples in hydrogen at 950°C, and x was found to be 6.85+0.05. Resistance measurements were carried out in a closed-cycle refrigerator in the temperature range between 12 and 300 K. A standard DC, four-probe technique was used. Data were corrected for thermovoltages in wiring and contacts. To determine the resistivity, rectangular bars were cut out of the ceramic pellets with a diamond saw. AC-susceptibility measurements were performed in a 4He-cryostat using a mutual inductance technique. A driving field frequency of 87 Hz and amplitudes of 0.43 and I. 1 0 e were used. Data were taken between 4.2 and 80 K. Electron microscopy and electron diffraction were carried out with a Jeol 200CX electron microscope, equipped with a top entry _+10 ° double tilt holder and operating at 200 kV and with a Siemens Elmiscop 102 electron microscope with a top entry +_40 ° double tilt-lift cartridge operating at 100 kV. Specimens for electron microscopy were prepared by grinding, suspending in methanol and putting a few
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W.T. Fu, et al. /High-temperature superconductivity in LaBaCaCu30~85
droplets on carbon coated holey Triafol films supported by Cu grids [ 5 ].
3. Results and discussion
Figure 1 gives the DC-resistivity as a f u n c t i o n of temperature. The resistivity shows metallic behaviour from 300 K down to the onset of the superconducting transition. Zero resistance is o b t a i n e d at 75 K, which is considerably higher t h a n for the corn . p o u n d s La 1+ xBa2 _.~Cu307 + a prepared in the similar way [3,4 ]. The temperature dependence of the magnetic susceptibility is shown in fig. 2, a n d confirms the superconducting transition. Electron diffraction clearly shows that a superstructure is present. Diffraction patterns along [ 001 ]* a n d [ 010 ]* are shown in fig. 3. They show a doubling of all three axes with respect to those of the Y B a 2 C u 3 0 7 structure. Consequently all diffraction spots can be indexed with a u n i t cell in which all axes are doubled. However, u p o n doing so one obtains systematic absences of the reflections h k l with h, k v ~ 2 n a n d h + k = 2 n .
any space group, suggesting that another u n i t cell should be taken. The electron diffraction patterns can also be indexed with a 2a, b, 2c u n i t cell in c o m b i n a t i o n with [001 ] 90 ° rotation t w i n n i n g whereby the a a n d b axes are interchanged across the twin boundary. The superstructure spots are not as sharp as those of thebasiclattice. Additionally some streaking along
. J _ _
Fig. 2. Temperature dependence of the differential (AC) magnetic susceptibility of LaBaCaCu3Of.85.
c* is often observed whereby the degree of streaking was f o u n d to vary strongly from crystal to crystal within one specimen. All this points to a rather short-
o o o o o • o • o 9-~ • o • i J o ° b* o , o o , o o • o • o • o • o • o o o o o • o • o • o • o •
.: ffzt .o.
,o o 0 o o o
O o O O
Fig. 1. DC-resistivity of LaBaCaCu306.s~ as a function of temperature.
Fig. 3. Schematic drawing of the diffraction patterns along [ 001 ] (top left) and  (top right). The superreflections are given as small circles, being full and open respectively when they are and are not at the same height as the main reflections. The two twin variants are given by a full drawn line and a dashed line. The original diffraction patterns are given in the lower part of the figure.
W.T. Fu, et al. / High-temperature superconductivity in LaBaCaCu~Or.s5
range order of the superstructure, with the shortest correlation length along the c axis. It is clear that the structure of LaBaCaCu306.85 is closely related to that of YBa2Cu307. An interesting question is which positions the Ca and Ba atoms occupy. In the series LaBaECUaOx, LaBaSrCu30~ and LaBaCaCu3Ox the value of Tc is lowest for the Srcompound [6 ]. This can be interpreted as arising from the replacement of Ba by Sr when comparing LaBa2Cu3Ox and LaBaSrCu3Ox, and a replacement of La by Ca and Ba by La for LaBaCaCuaOx. Also the Ca 2+ ion will fit much better in the 8-coordination of the Y site than in the 10-coordinated Ba site. The superstructure can be caused by an oxygenvacancy ordering, a cation ordering (Ca, Ba and La) or by a combination of both. The superreflections were found to become streaked upon intense heating with the electron beam, but they do not disappear. Also the intensities of the superreflections of a given crystal were found to become very weak after prolonged exposure to the beam during an extensive high resolution electron microscopy study [ 7 ]. This suggests that the superreflections are at least partly due to an oxygen-vacancy ordering. Oxygen-vacancy ordering will certainly lead to a superstructure. In fact the observed doubling of the a axis is also found for YBa2Cu3OT_~ with 0 . 2 < 8 < 0 . 6 [8,9]. In the latter compound strong streaking is very often observed along c*, and in the case of ordering along the c axis a superstructure occurs with a doubled a axis but, contrary to LaBaCaCu306.85, without a doubling of the c axis. When the superstructure is only due to oxygen-vacancy ordering one would expect a similar type of ordering of the YBa2Cu307_~ structure. This is evidently not observed, both the correlation along the c axis and the stacking sequence along the c axis are different. Such a difference can be understood if one assumes that the superstructure is also partly caused by an ordering between Ca, Ba and La. High resolution electron microscopy has shown that the ordering of the large cations, Ca and (Ba, La) * is close to random . However, the crystal used for this study did not show any superstructure * Ba and La can not be distinguished.
in the diffraction pattern after long exposure to the electron beam. Since electron diffraction is a much more sensitive technique to observe a superstructure than high resolution electron microscopy one would not expect to see any order in the large cation positions. A small deviation from a random distribution of Ca, Ba and La would be invisible by high resolution electron microscopy as is more fully discussed in ref. [ 7 ]. A local relatively weak ordering of Ca, Ba and La could however strongly enhance the correlation of the oxygen-vacancy ordering in the CuO planes (the planes with the O - C u - O chains) along the c axis. This correlation in YBa2Cu3OT_6 is weak because of the invariable BaO-CuO2-Y-CuO2-BaO block sandwiched between the CuO planes. An ordering in the block AO-CuO2-A-CuO2-AO (A : Ca, Ba and La) would enhance the correlation along the c axis and could also lead to a different kind of the oxygenvacancy lattice in the CuO planes. The absence of a clear difference between the a and b axes is unexpected. However, local ordering of the large cations can explain this. A high density of structural features, which tend to pin twin boundaries, will lead to a high density of twin boundaries. An example is the system YBa2Cua_xFexOT_6 which (with x = 0.23) is locally orthorhombic although Xray diffraction suggests it to be tetragonal . Clustering of Fe in this compound leads to a high density of twin boundaries because many pinning centers are present. A similar effect can occur in LaBaCaCu3Or.85 because of the ordering of Ca, Ba and La. It should be pointed out that the preparation procedure of LaBaCaCu306.85 was found to be much more delicate than for YBa2Cu3Ov. The question arises whether the formation of the superstructure is an important factor in this respect. Our research in the near future will be focussed on answering this question.
Note added in proof When preparing this manuscript, we learned that the same material LaBaCaCu3Ox has also been synthesized and studied by D.M. de Leeuw and C.A.H.A. Mutsaers of Philips Research Laboratories, Ein-
W.T. Fu, et al. / High-temperature superconductivity in LaBaCaCu306.s5
dhoven. Our results appear to agree with their findings.
Acknowledgements We wish to thank Ir. A.A. Verheijen for the thermogravimetric measurements. The continuous interaction with the members of the Leiden Materials Science Centre engaged in the research of superconductors is gratefully acknowledged, as well as the support for this research by the "Stichting FOM" (Foundation for Fundamental Research on Matter) which is sponsored by ZWO (Netherlands Organization for the Advancement of Pure Research).
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