Sample preparation and characterization is quite an important topic for the ternary graphite-based superconductors. A thorough discussion of specimen synthesis and handling is obligatory not only because it is customary, but because the details of these techniques appear to impact the results of the experiments discussed in the following chapters in non-trivial ways. Reports in the literature on superconducting GIC's have often been confusing or even contradictory for reasons possibly related to differences between the samples prepared by the various research groups. Because of the inadequate information about sample preparation in many papers (as in, ``The samples were prepared using the technique of Ref. X ...''), one is often left wondering whether in-house experiments and those by other groups were performed on comparable materials. It is the aim of this chapter to give enough information about the samples used in the current experiments so that anyone who reproduces these efforts will not have to ask a similar question.
The leaders in the field of ternary GIC discovery and synthesis are the group at the University of Nancy. Under the leadership of Profs. Hérold and Lagrange, a team of students and staff has synthesized an impressive array of ternary GIC superconductors. The first reported were the MHg-GIC's, where M stands for one the heavy alkali metals.[150] Soon after the same authors described the preparation of the MTl-GIC's,[150,73] and recently they announced the discovery of the large class of MBi-GIC's.[146] All of these materials have been reported to be superconducting. Lagrange[145] has recently reviewed the lessons learned about the intercalation chemistry of this entire class of compounds.
It should be noted that the Nancy group has also synthesized ternary GIC's whose superconductivity seems probable, but has not been experimentally verified. Besides the alkali-metal/heavy-metal ternaries mentioned above, whose intercalant sandwich consists of multiple metal layers, there are also ternaries with monolayer intercalant sandwiches. Among these candidates are solid solutions of known GIC superconductors, such C8K1-xRbx,[24] and others such as sodium-barium GIC's[25,26] whose superconductivity seems quite likely in terms of existing models.[4,86] These materials have received a lot of attention from a structural characterization standpoint,[39,40] but few cryogenic transport studies have been performed on them. The reader will find more information on these compounds in a recent review by Solin and Zabel.[218] There is also a whole new class of alkali-metal/antimony compounds whose low-temperature[83,71] properties are unknown. Despite the fundamental interest in these materials, they will not be discussed further here, since determination of their superconductivity is left for future investigators.
The superconductivity of the alkali-metal/heavy-metal ternary
GIC's is the focus of the remainder of this work. Sample
preparation issues are particularly important for
superconductivity experiments because they presumably are the
key to understanding why such a wide range of transition
temperatures has been reported for some superconducting
GIC's. For example, Tc for C4KHg has
been variously reported in the range from 0.7[120] to 1.6 K,[247] while Tc for
C4CsBi0.5 has been quoted from 4.05
K[146] to <=0.5 K.[36] In the next few sections an
attempt will be made to relate sample preparation conditions
to reproducibility problems with superconductivity
experiments in GIC's.