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

Abstract

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 ...

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

“…The conversion between Tl-2212 and Hg-1212, however, differs from the conversion within the '1212' structure [10] because of the c-axis lattice variation. From figure 1, the c-axis lattice constant was 1.48 nm for Tl-2212 and 1.27 nm for Hg-1212.…”

“…This means that lattice decomposition and recrystallization at 760 • C is unlikely to occur. on the reversibility of cation exchange between Tl-1212 and Hg-1212 [10], suggests that the cation exchange is a simple perturbation over the volatile cations on the existing lattice and the process is bidirectional, with its direction determined by the population ratio between the two cations involved.…”

“…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.…”

“…Motivated by this, we have recently carried out a systematic study on the cation-exchange mechanism. Within the '1212' structure, the cation exchange process has been found completely reversible and the direction of the process is determined by the relative population ratio of Tl and Hg cations [10]. This suggests that the cation exchange within the '1212' structure is simply a perturbation of the volatile cations, such as Tl (or Hg), on the '1212' lattice which is followed by the replacement of them with the other type of volatile cations such as Hg (or Tl).…”

An analytical approach to transient homovalent cation exchange problems

“…The result is a surface reaction. Cation binding and exchange plays a fundamental role in a number of environmental and industrial applications, including bacterial growth (Maurya and Subramaniam, 2007;Shephard et al, 2008), refinement of fuel products (Bhattacharyya et al, 2006), development of superconducting materials (Xing et al, 2006), and elaboration of food products (protein separation), among others. The cation-exchange capacity of different natural and artificial materials can be used to remove undesired molecules from water, such as dyes (Wu et al, 2008), enzymes (Turmanova et al, 2005), etc.…”