Fabrication of superconducting HgBa2CaCu2Ox thick films using cation exchange process

Xing, Zhongwen; Xie, Yiyuan; Wu, Judy; Cardona, A. H.

doi:10.1016/j.physc.2003.07.008

Cited by 9 publications

(5 citation statements)

References 23 publications

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“…It should be pointed out that the residual Tl-2212 phase with a volume fraction typically around 10-20 % is not uncommon when the film thickness is above 0.15-0.2 lm, and it can be reduced with a longer cation exchange processing time. [16] Since the volume portion of the residual Tl-2212 phase remained constant in this whole process, the Tl-Hg cation exchange probably occurred within the "1212" phase. The sample remained predominantly in the "1212" structure with the c-axis oriented along the normal of the substrate during the three successive cation exchanges.…”

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confidence: 93%

Reversible Exchange of Tl and Hg Cations on the Superconducting “1212” Lattice

Xing

Zhao

2006

Advanced Materials

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A cation exchange method [1] has been developed recently for the synthesis and epitaxy of highly volatile Hg-based hightemperature superconductors (Hg-HTSs) that have high superconducting transition temperatures (T c ) up to 135 K [2] and are in great demand for both fundamental research and practical applications. [3,4] In the cation exchange process, Hg-HTS films are obtained from precursor matrices of Tl y Ba 2 Ca z Cu z+1 O x (y = 1 or 2, z = 1 or 2, and x = 2z + 5 for y = 1 or x = 2z + 6 for y = 2) by thermally deflecting the weakly attached Tl cations on the precursor lattices and replacing them with Hg cations, allowing the transfer of an epitaxial lattice from the precursor matrices to the Hg-HTS films. This circumvents the major difficulties in the epitaxy of Hg-HTS films that result from the instability of Hg-related compounds [5] and the attack of Hg cations on the film/substrate interfaces (Hg reacts with most metals and oxides). Using the cation exchange method, epitaxial thin films of HgBa 2 CaCu 2 O 6 (Hg-1212) exhibiting superior physical properties have been obtained from epitaxial precursor films of Tl-1212 (y = 1 and z = 1) or Tl-2212 (y = 2 and z = 1). [4,[6][7][8][9][10] Despite the exciting results obtained, the mechanism of the cation exchange has been the subject of debate ever since it was developed. Understanding this mechanism is important not only because cation exchange has provided a vital scheme for the epitaxy of Hg-HTS films, but also because it may be developed into a generic "atomic surgery" scheme for the synthesis and epitaxy of other volatile materials. One possible mechanism for cation exchange is that the targeted material (for example Hg-1212) must be energetically more preferable than the precursors (Tl-2212 or Tl-1212 in this case). This means that cation exchange is unidirectional and the direction of the process is determined by the relative values of the Gibbs free energies of the target material and the precursors. In order to achieve the targeted material via cation exchange, the precursor selected must have a higher Gibbs energy than the target material. This argument seems to be consistent with the observed crystal lattice decomposition temperatures (U th ) of around 610-630°C, 670-690°C, and 770-790°C, respectively, for Tl-1212, Tl-2212, and Hg-1212 films in oxygen. [8,11] The higher U th for Hg-1212 suggests that the Hg-1212 lattice is more energetically favorable than Tl-1212. This, on the other hand, implies that conversion from Hg-1212 back to Tl-1212 is not allowed. Another argument is that cation exchange is bidirectional since Tl (or Hg) cations are only weakly attached to the "1212" lattice [12] (see Fig. 1).The lattice decomposition temperature for Tl-1212 (or Hg-1212) may represent the binding energy of Tl (or Hg) cations to the "1212" lattice. This suggests that the cation exchange could be completely reversible and the direction of the process is controlled by the population ratio of the new cation and the one to be replaced. The final material after ...

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confidence: 93%

Reversible Exchange of Tl and Hg Cations on the Superconducting “1212” Lattice

Xing

Zhao

2006

Advanced Materials

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“…A cation-exchange process developed recently by some of us has been proven to be a robust way to prepare highquality epitaxial thin films of Hg-based high temperature superconductors (Hg-HTSs) [1][2][3][4][5][6][7][8][9][10]. In the cation exchange process, an epitaxial HgBa 2 CaCu 2 O 6 (Hg-1212) film can be fabricated from an epitaxial TlBa 2 CaCu 2 O 7 (Tl-1212) or Tl 2 Ba 2 CaCu 2 O 8 (Tl-2212) precursor film by deflecting Tl cations out of the lattice and meanwhile diffusing Hg cations into it.…”

Section: Introductionmentioning

confidence: 99%

Converting Hg-1212 to Tl-2212 via Tl–Hg cation exchange in combination with Tl cation intercalation

Zhao¹,

Wu²

2007

Supercond. Sci. Technol.

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In a cation exchange process developed recently for epitaxy of HgBa 2 CaCu 2 O 6 (Hg-1212) thin films, TlBa 2 CaCu 2 O 7 (Tl-1212) or Tl 2 Ba 2 CaCu 2 O 9 (Tl-2212) precursor films were employed as the precursor matrices and Hg-1212 was obtained by replacing Tl cations on the precursor lattice with Hg cations. The reversibility of the cation exchange dictates directly the underlying mechanism. Following our recent success in demonstrating a complete reversibility within '1212' structure, we show the conversion from Hg-1212 to Tl-2212 can be achieved via two steps: conversion from Hg-1212 to Tl-1212 followed by Tl intercalation to form double Tl-O plans in each unit cell. The demonstrated reversibility of the cation exchange process has confirmed the process is a thermal perturbation of weakly bonded cations on the lattice and the direction of the process is determined by the population ratio between the replacing cations and that to be replaced.

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“…Several groups have recently reported the growth of Hgbased films with thicknesses between 1 and 80 lm [1][2][3]. Most of these films have showed single phase transition above 120 K with significantly high J c values.…”

Section: Introductionmentioning

confidence: 99%