In a cation-exchange process, epitaxial Hg-based high-temperature superconducting (HTS) films are obtained from epitaxial Tl-based HTS precursor films by diffusing Tl cations out of, and Hg cations into the crystal lattice. In order to understand the cation diffusion mechanism, we have carried out a systematic study on conversion of Tl2Ba2CaCu2O8 precursor films to HgBa2CaCu2O6 films. Such a conversion was found to be composed of two steps. In the first step, Tl2Ba2CaCu2O8 collapses structurally into the (Tl,Hg)Ba2CaCu2O6 lattice, while in the second step, Tl cations diffuse out while Hg cations diffuse in, resulting in pure HgBa2CaCu2O6 phase. A time constant τ1 of about 45–60 min was found for the first stage, while the second stage τ2 is beyond 2τ1. This provides potential to optimize the cation exchange processing parameters for Hg-based HTS films and devices.

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