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.