The most obvious conclusion to be extracted from the numbers in
Table is that the bismuth in the CsBi-GIC's is
acting as an acceptor of electrons, similar to behavior of the heavy metal
in other trilayer ternary GIC's. This conclusion is drawn due to the lower
EF and fC of the bismuth GIC's compared to the binary
Cs-GIC's, and also due to the fairly high E2g2 Raman frequencies
displayed by the CsBi-GIC's.[270] According to Refs. [37]
and
[194], a higher Raman frequency for donor compounds is indicative of a smaller
in-plane carbon-carbon distance and thereby a smaller fC.
Therefore, all evidence points to less cesium charge per carbon atom being
resident in the graphite layers than in C8Cs and possibly even C24Cs.
The picture then is of positively charged cesium layers and negatively charged bismuth and carbon layers. Since it is generally accepted that the superconducting MHg-GIC's and MTl-GIC's also have layers of alternating charge[110], this insight would appear to be of little help in understanding the apparent lack of superconductivity in the CsBi-GIC's. It is hard to see why back transfer of charge from the carbon layers would reduce Tc in the MBi compounds when it seems to raise Tc in the MTl- and MHg-GIC's.
A closer look at Table shows that, roughly
speaking, GIC's with | fC | less than about 0.042 (= 1/24) are
not found to be superconducting. An accumulation of experimental
evidence[89,62,66,199] supports the conclusion that
in the stage 2 alkali metal GIC's that the alkali metal is fully ionized,
and in fact it seems reasonable to suppose that in any ternary compound
where | fC | ;SPMlt; 0.04 the alkali metal atom will be fully
ionized. The thinking behind this is that the graphitic pi bands have
such a high affinity for the alkali-metal s-electron that the only reason
that fC can fall very low is for the heavy metal acceptor to
outcompete the pi-band for the s-charge. Therefore a low fC in a
ternary compound implies that fM = +1. The data accumulated in
Table
therefore suggests that superconductivity in
ternary GIC's is suppressed by an empty s band, a conclusion in keeping
with the Al Jishi model[4] of superconductivity in binary graphite
intercalation compounds. This model suggests that s-band occupancy is
necessary for superconductivity in GIC's.
The next natural question to arise is that of why physically the
s-band occupation should be imperative for superconductivity. More
intuition on this question can be developed by examining quantities closely
linked to s-band occupancy, namely the c-axis resistivity and the
resistivity anisotropy. Table , taken directly from the
work of McRae and Marêché[166], shows that the compounds
identified above as having an empty s-band also have a high rhoc and a
high resistivity anisotropy == rhoc/rhoa. High values for the
anisotropy and rhoc indicate an almost two-dimensional band structure,
often with hopping conduction along ^c.[227,229]
Table: Two tables prepared by McRae and Marêché[166] which
list the c-axis resistivity and resistivity anisotropy of many GIC's, both
donors and acceptors. A== rhoc/rhoa.
Correct sources for these numbers are given in Ref. [166].
The chain of reasoning developed in this section seems to imply
that two-dimensionality of band structure tends to suppress
superconductivity in GIC's. This is a conclusion which has been previously
reached by leading superconductivity experts[163,102] when
they turned their attention to the problem of superconductivity in GIC's.
The reasons why two-dimensional superconductivity might be expected in the
closely related transition metal dichalcogenides intercalation compounds
and not in the GIC's is discussed in Section .
The apparent lack of superconductivity in the MBi-GIC's is a great disappointment. Many experimentalists wanted to take advantage of the new materials' increased air stability and reported higher Tc's to extend the scope of experiments that could reasonably be performed on superconducting GIC's. Despite the letdown, attempts to answer the question of why the MBi-GIC's are not superconducting have led to some insight about the properties of the MHg-GIC's, which are the topic of the rest of this thesis. Finally, it should be mentioned that the irreproducibility of transition temperatures in other intercalation compounds leaves one with the hope that superconductivity may yet be confirmed in the MBi-GIC's.