Diamagnetism and structural transition in the Ba2Cu3O4Cl2 compound

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Diamagnetism and Structural Transition in the Ba₂Cu₃O₄Cl₂ Compound

M. S. da Luz, C. A. M. dos Santos, A. J. S. Machado, and B. Ferreira Grupo de Supercondutividade, Departamento de Engenharia de Materiais - FAENQUIL

Polo Urbo-Industrial Gleba AI-6, CEP 12600-000, Lorena, SP, Brazil

Received on 28 February, 2002

This work presents a study of structural and magnetic properties of the Ba₂Cu₃O₄Cl₂ (Ba2342) compound. Doped samples with K or Sr in the Ba site and Zn or Mg in the Cu site of the Ba2342 were prepared using a solid state diffusion method. They were characterized by X-ray powder diffractometry, scanning electron microscopy and dc-magnetization. Samples with Ba2342 composition presented tetragonal structure of the I4/mmm space group when heat-treated at 750ºC. A structural transition was observed when the samples were doped with Zn or Mg or heat-treated at 700ºC. dc-Magnetization measurements performed in SQUID showed the existence of diamagnetic signal. A difference of the magnetic moments measured at zero field cooled (ZFC) and field-cooled (FC) procedures was observed suggesting the existence of localized superconductivity in this oxychloride system.

I Introduction

In the last years great attention has been given to the study of the magnetic properties in the Ba₂Cu₃O₄Cl₂ (Ba2342) compound, specially due to the presence of copper-oxygen planes with Cu₃O₄ stoichiometry and distance between Cu-O atoms similar to the high-critical temperature ceramic superconductors (1.95 Å) [1-5]. The Ba2342 compound is reported to have tetragonal structure of the I4/mmm space group and two kind of Cu atoms in the crystalline structure. Due to this structure several authors have reported different magnetic effects [2-7]. To obtain information about the influence of the La in the Ba site, recently, Jinhua Ye et al. [8] presented results showing a diamagnetic transition close to 20K and a metal like behavior to the electrical resistance in the normal state. They argued that this behavior was caused by the substitution with La^{3 +} .

Thus, in order to obtain more information about magnetic and structural properties of Ba2342, we have doped this compound with K or Sr in the Ba site and Zn or Mg in the Cu site and performed measurements of X-ray powder diffractometry, scanning electron microscopy, dc-magnetization and electrical resistance.

II Experimental procedure

Samples with Ba_{2 - x}K_xCu₃O₄Cl₂ (0 £ x £ 0.1), Ba_1.8Sr_0.2Cu₃O₄Cl₂, Ba₂Cu₂ZnO₄Cl₂ and Ba₂Cu₂MgO₄Cl₂ nominal composition were prepared by solid state diffusion method using SrCO₃, BaCO₃, SrCl₂.6H₂0, BaCl₂.2H₂0, CuO, K₂O, ZnO and MgO powder of high purity. These powders were grounded in the appropriated amounts and calcined at 650-700^ºC. After this procedure, the calcined powders were compacted into pellets at 500 MPa and sintered between 700 and 750^ºC depending on starting composition. All prepared samples were characterized by X-ray powder diffractometry, electrical resistance and dc-magnetization measurements performed in a SQUID (Quantum Design) in the zero field cooled (ZFC) and field cooled (FC) procedures. Some samples were also characterized by scanning electron microscopy accomplished with energy dispersive spectroscopy (EDS).

III Results and discussion

Fig. 1 presents X-ray diffractograms for the Ba_{2 - x}K_xCu₃O₄Cl₂ system. The (hkl) presented in the top of the Fig. 1 is in agreement to the Kipka and Buschbaum reference's [1]. We can see that the samples are single phase in the x range of K doping. At the same heat treatment, we also observe that an increase of K composition to x = 0.15 produces impurity phases in this system. Using these x-rays diffractograms we can calculate the lattice parameters. Fig. 2 shows a slow increasing of the a and b parameters, therefore, c axis decreases as a function of the K content, suggesting an approximation of the Cu-O planes. Sample with Ba_1.8Sr_0.2Cu₃O₄Cl₂ heat treated at 700^ºC for 48 hours presents a similar x-ray powder diffractogram such as non-doped sample. When, samples were doped with Zn or Mg and heat treated at 730^ºC, one can see an enlargement of some peaks. Fig. 3 shows results to the doping with ZnO for a sample with Ba₂Cu₂ZnO₄Cl₂ nominal composition. We observe that there is in this sample minor ZnO impurities (see  symbol). Therefore, we can relate the others peaks with the Ba2342 structure. Results of scanning electron microscopy and EDS provide that the Zn is effectively incorporated into the grains. Essentially, the same results were obtained to the Mg doping.

On the other hand, in order to verify if there is the same split of peaks to non-doped samples, we have performed heat treatments at 700^ºC. Fig. 4 presents a resume to the (103) and (114) peaks for several compositions heat treated at different temperatures. We can observe the presence of enlarged peaks for the samples heat treated at lowest temperatures. In Zn-doped sample treated at 730^ºC (Fig. 4 c), we can unambiguously observe two peaks for each (hkl). These results suggest that there is a phase transition in this oxy-chloride system. Preliminary results of powder diffraction simulation, suggest that this behavior is related to a tetragonal to orthorrhombic phase transition with decreasing temperature, which is similar to the YBa₂Cu₃O_{7 - d} transition [9].

Finally, in order to verify the magnetic behavior of this system, we have performed several dc-magnetization measurements in SQUID. In Fig. 5 is presented some ZFC and FC magnetization curves as a function of temperature measured at 20 Oe. Depending on the sample composition, we can observe a diamagnetic signal accomplished with two transitions close to 30 K and 100 K. The most important fact of these magnetization measurements is related to the existence of difference between ZFC and FC curves. This can indicate the presence of superconductivity in this system. To verify this possibility, transport properties were performed using standard four-probe method, which demonstrated that all samples are insulators below room temperature. These results suggest that if there is superconductivity in the samples it must localized in the crystalline structure [10].

IV Conclusion

This work presents the doping effects of K, Sr, Zn and Mg atoms on the Ba2342 compound. The doping studies provide a split of the peaks, which was interpreted as an experimental evidence of a phase transition in this oxy-chloride system. dc-Magnetization measured as a function of temperature presented diamagnetic signals in the some samples. A difference of

ZFC and FC procedures were interpreted as a evidence of localized superconductivity in the Ba2342 compound.

Acknowlegments

The authors are grateful to R. R. da Silva, Y. Kopelevich and C. A. Rodrigues for experimental support. This work has been supported by the FAPESP (Grants No. 00/08972-1 and 00/03610-4).

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Publication Dates

• Publication in this collection
  04 Dec 2002
• Date of issue
  Sept 2002

History

• Received
  28 Feb 2002