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.