Several unresolved questions pertinent to the superconductivity of
the KHg-GIC's have been discussed at length in previous chapters. Among
the most intriguing of these is why C8KHg has a higher Tc
than C4KHg, how the superconducting properties of the various phases
of C4KHg differ, and whether two-dimensional superconductivity can
be observed in GIC's. These questions are difficult to approach on a
first-principles basis because of the large unit cell and complex band
structure of the ternary compounds. As a result, much of the current
understanding has been derived from qualitative comparisons of ternary
GIC data with that from different materials, principally the binary GIC's
and TMDC's discussed in Chapter .
When Lagrange and his colleagues at the University of Nancy
announced the synthesis of the MBi-BIC's (M = K, Rb, or Cs) in
1985,[146], they opened up a whole new arena for the
exploration of these issues. The new ternary GIC's were not only reported
to be superconducting, but they also possessed many of the more unusual
features of the KHg-GIC's. Specifically, Lagrange et al. said that
the CsBi-GIC's show a higher Tc for stage II than stage I, the same
surprising stage dependence that is exhibited by the
MHg-GIC's.[167] Secondly, for each alkali metal and each stage
they reported two different phases of the MBi-GIC's, with the phases
distinguished by different sandwich thicknesses.[146,167]
These phases were said to have different superconducting transition
temperatures,[167] perhaps similar to the situation in the
KHg-GIC's [see Chapter ]. The announcement of the MBi-GIC's
also gave new life to the search for two-dimensional superconductivity in
GIC's since the rubidium compounds could be prepared in stages up to
seven.[146]
The discovery of the MBi-GIC's spurred a great deal of experimental activity, in large part because of the high Tc of about 4 K[146] reported for the CsBi alpha-phase material. The results of further studies on the superconductivity of these materials are reported below.