2.3 Alkali Intercalation of C₆₀

The samples for all of the experiments desribed in this thesis were synthesized at The University of Pennsylvania by J.E. Fischer and coworkers. The process by which the alkali fulleride salts are made is high temperature solid state intercalation. Such processes are familiar in the synthesis of intercalated graphite compounds, such as KC₈ (a T_c = 140mK superconductor [88]). Stoichiometric quantities of high-purity oxygen-free C₆₀ and alkali metal are weighed and sealed in a vessel, e.g. quartz tube or oxygen-free copper cell. Then the cell is subjected to a baking and annealing process (at modest temperatures $\sim 150 ^\circ$C) over the course of weeks (e.g. [4]). For mixed compounds, this sometimes involves interrupting the baking process to mix or grind the material. The alkali vapour diffuses slowly into the lattice of C₆₀ molecules, and the result is a fine black powder. Other intercalation strategies have also been developed, such as reaction of (highly explosive) alkali azide compounds with C₆₀ or a previously intercalated C₆₀ compound[89], and dilution of higher stoichiometric alkali fullerides with pure C₆₀ (e.g. [46]). Samples were typically characterized by standard techniques such as x-ray diffraction and magnetization. The grain size of the powders was $\sim 10^4$ Å, but the x-ray linewidths give a crystalline coherence length, $\xi_{XTL}$of 500 - 1000 Å. From magnetization measurements in the superconducting state, the shielding fraction was typically 60 %, and the Meissner fraction 10 %. For the experiments described here, typically a few hundred milligrams of the powder was sealed under 1 atmosphere of 90% Ar / 10% He. Synthesis of large single crystals of C₆₀ intercalation compounds has proven quite difficult, but recently, a crystal with mass in the order of 100mg has been made.[90]

 

  

Figure 2.7: a) The time histogram of muon-positron decays detected in a single counter in Na₂CsC₆₀ in 10 mT transverse field. b) The same data combined with another histogram to remove the muon lifetime. A small non-relaxing signal, due to muons not stopping in the sample, is evident at late times. The relaxation is discussed in section ?.?.?.