The influence of Tl and O content from the starting mixture on phase formation in (Cu,Tl)-1234 system

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Abstract

Superconducting samples have been synthesized from starting compositions Cu0.5TlxBa2Ca3Cu4O12+y, xTl=0.35–1 in the sealed Au-tubes. Starting oxygen content was y=0 and/or y=±0.225. Samples contain non-superconducting phases and only one main superconducting phase, 1234. The energy dispersive spectroscopy grain composition was Tl0.87–1.3Ba1.9–2.05Ca2.55–3.1Cu4Ox (normalized to Cu=4). The starting Tl and oxygen content do not influence the occurrence of the phases, grain size, morphology and composition. On the other hand the phase content strongly depends on Tl content and almost does not depend on oxygen content. Both Tl and oxygen content are important for the transport properties of the superconducting grains and it seems that also of the non-superconducting matrix. Tc of the samples was in the range 119.1–111.3 K.

Introduction

In both Tl double layer and Tl single layer series several phases exhibit critical temperature above 100 K. Other superconducting parameters are also among the best. Tl toxicity and volatility remains however a problem for the future large-scale technologies and applications. Tl-free, single layer type, derived compounds in which Tl was totally substituted by another element e.g. Cu [1], Ca [2], have shown remarkable properties. Unfortunately, these materials are difficult to synthesize, usually under high pressure. Substitution for the Cu in CuBa2Can−1CunOx (n>1) by different elements as C, Tl, Cr, V, S, P ([3], [4]) can enhance the phase formation as well as superconducting properties. Jc of 2×107 A/cm2 at 77 K [5] and Tc of 133.5 K [3] have been attained on (Cu,Tl)-1223 while, Tc of (Cu,Tl)-1234 has been of 127 K [3].

The above results motivated us to investigate the possibility of ambient pressure synthesis of (Cu,Tl)-1234 starting from mixtures with different Tl content and to characterize the obtained samples. This work is also useful for understanding some aspects of phase formation and properties control of the superconducting material.

Section snippets

Experimental

Samples with starting composition Cu0.5TlxBa2Ca3Cu4O12+y, xTl=0.35–1 were prepared from C-free oxides: Tl2O3, BaOw (87% BaO2+13% BaO), CaO and CuO. For samples A–D yO2=0 while for A1 and C1 yO2=−0.225 and yO2=0.225 respectively. Oxygen content has been controlled by using a certain amount of Cu2O instead of CuO in the starting mixture or by placing in the Au-tube a pellet of Ag2O together with the sample. Samples sealed in the Au-tubes were heated by applying a constant heating rate of 0.11

Results and discussion

XRD data of the A–D samples are presented in Fig. 1. As one can see, all samples consist of 1234 phase as the main phase. Secondary phases were Ba–Cu–O phases, Ba(Ca,Tl)–Cu–O, CaO, CuO, Ca2CuO3 and traces of 1223 or 1245 superconducting phases. The XRD peak at 2θ∼29.2° can be ascribed to the crystallized residual liquid phase [Ba(Ca,Tl)]0.73–0.95CuOx as well as for the Ba2Cu3O5 phase (330 plane, JCPDS 40-0312) or BaCuO2 (600 plane, JCPDS 38-1402). The occurrence of the residual liquid phase

Conclusions

Our results demonstrate that it is possible to synthesize 1234 superconducting phase starting from (Cu,Tl)-compositions, and the reaction is taking place since all Tl is consumed in the reaction to form grains with composition Tl0.87–1.3Ba1.9–2.05Ca2.55–3.1Cu4Ox (normalized to Cu=4), or is escaping from the capsule. The occurrence of different phases in the samples obtained for our synthesis conditions does not depend on Tl and oxygen content, but their content is strongly influenced by Tl

Acknowledgements

P.B and A.C gratefully acknowledge STA fellowships. Authors also thank A. Iyo for helpful discussions, Y. Ishiura for FT-IR measurements of C-content in the raw materials and H. Kito for help with EDAX measurements.

References (10)

  • N.L Wu et al.

    Physica C

    (1999)
  • S Yu et al.

    Physica C

    (2000)
  • P Majewski et al.

    Physica C

    (2000)
  • H Ihara

    Solid State Phys.

    (2000)
  • A Iyo et al.

    Supercond. Sci. Technol.

    (2001)
There are more references available in the full text version of this article.

Cited by (3)

  • Modified synthesis route to achieve Sr substituted Cu<inf>0.5</inf>Tl<inf>0.5</inf>-1234 superconductor phase

    2016, Materials Chemistry and Physics
    Citation Excerpt :

    This problem has been overcome by substituting thallium at Cu site in the charge reservoir layer (CRL). The vapor pressure of thallium in a closed system helps to achieve CuTl-1234 phase at ambient pressure [3,4]. However, some impurity phases were still present in that modified compound of Cu-12(n-1)n series [5,6].

1

Permanent address: Laboratory of Low Temperature Physics and Superconductivity, National Institute for Materials Physics, Bucharest-Magurele, P.O. Box MG-7, RO-76900, Romania.

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