Discussion and Conclusions

Besides the characterization measurements that were discussed in this chapter, several additional experiments were attempted that were not as successful. Among those attempted are optical transmission and resistivity.

Optical experiments are an obvious possibility considering that different T_c's correspond to different sample colors. The apparatus necessary to perform reflectivity measurements is not readily available at MIT, so it was decided to try transmission measurements instead. Transmission experiments on C₈K and C₂₄K were previously performed by Zhang and Eklund at the University of Kentucky.[275] The chief difficulty in optical transmission work with metals is getting a thin enough sheet that a measurable amount of light can actually be passed through. In fact, preparing even a transparent sheet of HOPG proved to be tough, let alone intercalating it in a intact condition. The lesson from these efforts is that reflectivity measurements would be much practical to perform if a suitable apparatus could be located.

Another experiment for which a need is felt is low-temperature resistivity. Surprisingly, in-plane resistivity measurements have not been reported below 100 K on C₄KHg,[70] even though c-axis resistivity data has been published down to liquid-helium temperature.[85] The possibility of a charge-density wave in low- T_c C₄KHg samples[55] provides plenty of motivation for performing this experiment. In addition, more information about normal-state transport would be helpful in order to compare the critical field data to existing theories.[186]

One attempt was made to measure the in-plane resistivity of C₄KHg from room temperature to liquid-helium temperature. Four silver-paint leads were placed on the sample inside a glovebag, and the sample was sealed inside a glass-tube with epoxy, a technique developed by previous students.[79] Despite the fact that the leads did not pull off the a-face of the sample, and despite the fact that the sample showed no signs of discoloration or deintercalation, a resistivity about 10⁴ times the previously reported value of 20 mu Omega-cm[72] was measured at room temperature. The leads all appeared to have the same resistivity, so no obvious cause for the failure of this experiment could be identified. Perhaps silver paint is simply too reactive to use with C₄KHg. The best idea would be to repeat this experiment using another contact medium, such as the gold paste that has been successfully used with C₈K.[141]

Both the reflectivity and resistivity experiments still need to be performed on C₄KHg. However, a lot has been learned about the samples used in the superconductivity experiments from characterization efforts that were successful. From the x-ray experiments, it was discovered that the low- T_c and higher- T_c specimens have approximately the same structure. Good agreement was found with the assertion of Timp[248] and Kim et al.[130] that a temperature difference during intercalation produces less-ordered GIC's with a greater fraction of beta phase. It is worth noting that every specimen that showed any C₄KHg peaks in x-rays was found to be superconducting with T_c >= 0.7 K.

The neutron scattering experiments showed that the x-ray diffraction experiments were somewhat misleading, in that x-ray data sometimes indicated the presence of a single phase where neutrons indicated two phases. This finding may have important implications for understanding the hydrogenation experiments. Analysis of neutron diffraction data showed slight differences between gold and pink samples whose significance is not immediately clear. Good general agreement was found with the previous structural analysis of C₄KHg.[272]

Raman scattering experiments on C₄KHg showed no difference between the pink and gold phases with experimental error. This is despite the observation of zone-folded phases indicative of in-plane ordering by Timp.[247] The lack of zone-folded peaks in the recent Raman studies could be due to surface disorder stemming from laser-induced heating or slight accidental exposure to air.

Various measurements were also performed to find out the chemical composition of the specimens used for the low-temperature and diffraction experiments. The proposal that the low T_c of gold C₄KHg samples is caused by Hg deficiency[207] does not seem to hold up. In fact, support is found for the idea that the low- T_c phase has a higher mercury/potassium ratio,[272] although other factors are probably also important.

The CsBi-GIC's used in the low-temperature studies appear to be very similar to those made in other laboratories. The x-ray and electron[222] diffraction patterns, color, and chemical composition from RBS all agree with the results reported by Lagrange and coworkers,[145] the original discoverers of these new materials. T_c measurements on CsBi-GIC's are described in Section .

In general the current study of the synthesis of KHg- and CsBi-GIC's has confirmed the findings of previous investigators.[145,248,272] Extensive characterization efforts have allowed the identification of some correlations between the structural and chemical properties of a GIC and its superconductivity. The superconductivity measurements are described in detail in the following chapters.