phys. stat. sol. (a) 163, 177 (1997)

Subject classification: 74.72.Jt; S10.15

A New Family of Cuprates with 1212 Structure:SnBa2CaCu2Oy

Y. Yu (a), X. N. Xu (a), X. Jin (a), S. Y. Ding (a), Z. Y. Zheng (a),X. X. Yao (a), and G. J. Shen (b)

(a) Department of Physics and Laboratory of Microstructure of Solid States,Nanjing University, Nanjing 210093, People's Republic of China

(b) Analysis and Test Center of Southeast University, Nanjing 210018,People's Republic of China

(Received November 28, 1996; in revised form June 16, 1997)

A new layered cuprate, SnBa2CaCu2Oy, which has a perovskite-related structure, has been discov-ered. The crystal structure is investigated by powder X-ray diffraction. It has a tetragonal symme-try with lattice parameters a � 4:09 �A and c � 12:65 �A. The space group is P4/mmm, which issimilar to that of Hg-1212. The transport measurement showed that the compound possesses semi-conductor characteristics.

1. Introduction

Since the discovery of high-Tc superconducting cuprates, much attention has been paidto layered cuprates which contain CuO2 sheets. This is because CuO2 sheets in the high-Tc superconductors have been thought to play an important role for the occurrence ofsuperconductivity. All the known p-type superconductors have a ªcharge reservoirº tosupply holes to the CuO2 sheets, e.g. Bi2 ±O2 [1, 2], Tl±O [3, 4] and Hg±Od [5, 6]. Themaximum value of Tc depends on the sort of the ªcharge reservoirº, the highest transi-tion temperatures so far have been found for mercury-based compounds [6]. Therefore, itis important to find a new ªcharge reservoirº for developing new superconducting cup-rates with higher Tc values.

Here we report the synthesis of a new type of layered cuprate SnBa2CaCu2Oy. Thecareful structural analysis reveals that this compound has a tetragonal 1212 structure,which is similar to that of Hg-1212. The compound has the potential to be a supercon-ductor if appropriate injection of carriers into the CuO2 sheets is achieved.

2. Experimental

The powders of SnBa2CaCu2Oy were prepared by solid state reaction starting fromSnO, BaCO3, CuO. The mixtures were preheated at 900 �C in air for 24 h. Then theywere sintered in air for 12 h at 960 �C followed by furnace cooling to room temperature.After that the samples were reground and cold pressed into pellets, calcined at 950 �Cfor 48 h in air and then cooled to room temperature in the furnace. Phases formed in thesintered specimen were analyzed by means of powder X-ray diffraction (XRD) usingCuKa radiation. The electrical resistivity of specimen was measured by following thestandard four-probe method, down to 20 K at a cooling rate 1 K/min.

Y. Yu et al. : A New Family of Cuprates with 1212 Structure: SnBa2CaCu2Oy 177

3. Results and Discussion

Table 1 lists the X-ray diffraction experimental data of the sample. The results of thetheoretical calculation based on the 1212 structure are listed for comparison. Almost allthe peaks are successfully indexed to a tetragonal unit cell with the lattice parametersa � 4:09 �A and c � 12:65 �A. There has been no report on oxide superconductors havingsuch a long a-axis. It is about 0.2 �A larger than 3.9 �A, the typical a-axis value of othercopper-oxide superconductors. The c-axis is approximately three times as long as thea-axis, and its value is similar to that of the Hg-1212 phase [7].

The crystal structure was refined from the tetragonal 1212 structure. Table 2 lists thefinal refinement of the atomic positions. Fig. 1 schematically shows the idealized crystal

178 Y. Yu, X. N. Xu, X. Jin, S. Y. Ding, Z. Y. Zheng, X. X. Yao, and G. J. Shen

T a b l e 1

The XRD data of SnBa2CaCu2Oy.The theoretical calculation is based on space groupP4/mm, a � 4:09 �A and c � 12:65 �A. (* represents impurity phase)

observed data calculated data

hkl 2q d ��A� intensity(arb. units)

2q d ��A� intensity(arb. units)

* 9.414� 9.382 5.960 ±± ±± ±±* 28.975� 3.079 7.526 ±± ±± ±±

103, 013 30.448� 2.933 62.504 30.425� 2.936 69.972110 30.881� 2.893 100 30.892� 2.894 100111 32.049� 2.790 11.018 31.980� 2.805 8.108

* 33.126� 2.702 6.891 ±± ±± ±±005 35.267� 2.543 12.347 35.410� 2.509 15.237113, 104 38.760� 2.321 9.269 37.683� 2.386 4.774006 42.974� 2.103 14.061 42.857� 2.109 13.313200, 020 44.244� 2.045 25.446 44.253� 2.046 26.785115 47.434� 1.915 7.341 47.720� 1.905 12.470016 48.659� 1.868 7.485 48.538� 1.875 4.756116 53.902� 1.699 12.627 53.758� 1.704 14.572123 54.636� 1.678 24.114 54.647� 1.679 22.946206, 026 63.372� 1.466 8.368 63.299� 1.469 3.657220 64.355� 1.446 17.349 64.371� 1.447 14.815310 73.060� 1.293 10.209 73.102� 1.294 6.804226 80.495� 1.192 8.360 80.468� 1.193 3.3231011 88.648� 1.103 8.604 88.640� 1.102 3.857323 89.583� 1.093 13.390 89.362� 1.096 10.560

T a b l e 2

Refined atomic coordinates for SnBa2CaCu2Oy

atom site x y z occupancy

Ca 1d 1/2 1/2 1/2 1Ba 2h 1/2 1/2 0.2117 1Sn 1a 0 0 0 1Cu 2g 0 0 0.3500 1O(1) 2i 0 1/2 0.3800 �0.6O(2) 2g 0 0 0.2200 1O(3) 1c 1/2 1/2 0 1

structure of SnBa2CaCu2Oy. The coordination ofSn is six, with four oxygen atoms O(3) and twooxygen atoms O(2). Ba is nine fold coordinatedby four oxygen atoms O(1), four oxygen atomsO(2) and one oxygen atom O(3). The coordina-tion polyhedron can be considered to be a cappedcubo-octahedron. The coordination of Ca is pris-matic eight fold, as in the similar homologous se-ries of Y in the 123 superconductor. Copper is co-ordinated by five oxygen atoms and thecoordination polyhedron is a pyramid. The CuO2

sheets may act as conduction paths for the carriers.The lengths of the metal±oxygen bonds derived from the refined atomic positions

are listed in Table 3. There are several characters which are different from that foundin the 123 superconductor and in the corresponding mercury compounds. The mostimportant one is the position of the apical oxygen atom in the Cu±O pyramid. Theapical distance (Cu±O(2) bond) is longer than the in-plane distances (Cu±O(1)bonds) in the 123 superconductor. In SnBa2CaCu2Oy, however, the Cu±O(2) bond(1.644 �A) is shorter than the Cu±O(1) bond (2.079 �A�. The angle of O(1)±Cu±O(1) is159�. The occupancy of O(1) is about 0.6, smaller than unity. Another difference isthe BaO plane. Comparing with the 123 and Hg-1212 structure, we note that Ba is onthe other side of the O(2) plane. These characters may result in a low carrier concen-tration in the CuO2 sheets. So the resistivity versus temperature �R±T � curve (Fig. 2)exhibits semiconductor behavior. If the structure is changed to that of Hg-1212, thecarrier concentration may increase to an appropriate value, and this compound will bea superconductor.

A New Family of Cuprates with 1212 Structure: SnBa2CaCu2Oy 179

Fig. 1. Schematic representation of the ideal crystalstructure of SnBa2CaCu2Oy

T a b l e 3

The lengths of the M±O bonds (in �A) of SnBa2CaCu2Oy

M±O bond length (�A) M±O bond length (�A)

Sn±O(3) 1.446(0) Ba±O(3) 2.804(5)Sn±O(2) 2.783(0) Cu±O(1) 2.079(9)Ba±O(1) 2.573(6) Cu±O(2) 1.644(5)Ba±O(2) 1.449(8) Ca±O(1) 2.546(8)

4. Conclusion

A new family of SnBa2CaCu2Oy was synthesized. The crystal structure of the specimenis similar to that of the Hg-1212 phase. A preliminary measurement shows that thiscompound exhibits semiconductor character in the temperature range of 283 to 20 K.

Acknowledgements The authors thank The Center of Material Analysis of NanjingUniversity for the X-ray diffraction analysis. This project was supported by the NationalCenter for Research and Development on Superconductivity of China and the Founda-tion for Doctoral Education.

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Fig. 2. Temperature dependence ofelectrical resistivity forSnBa2CaCu2Oy

180 Y. Yu et al.: A New Family of Cuprates with 1212 Structure: SnBa2CaCu2Oy