Electrical Conductivity and Superconductivity in Graphite Intercalation Compounds
This paper will provide a current review of the electrical conductivity of the donor and acceptor intercalation compounds of graphite based on the research of the group at Penn and their collaborators.
In the lithium graphitides, which have a different stoichiometry than the graphitides with larger alkali atoms, many interesting characteristics appear: the conductivity is higher for stage one than stage two, the conductivities for both stages is rather high, ∼2 × 105Ω-1, and the anisotropy of conductivity is only ten. For the potassium intercalated graphite at higher pressures, startling discontinuities in the c axis resistivity variation with pressure indicate transformations of the interplane ordering and concomitantly the intraplane packing. In other experiments, it was found that alteration of the planar density by alloying permits the synthesis of the unusual ternary series KxNa1-xCg where x is between 0.45 and 1. These ternaries were found to have lower in plane resistivities than the corresponding KCg compounds.
It has been known for some time that the acid acceptor compounds that form liquid-like intercalated layers (HNO3, UBBELOHDE) will, on cooling below 240 K, exhibit anomalies in the ρ (T) curve. This has been confirmed for compounds synthesized with SbF5, H2SO4 (electrolytically) and NO2SbF6. X-ray structural examination of the SbF5 compound reveals that ordering changes in the intercalated layer coincide with the abrupt changes of resistivity with temperature.
Low-temperature superconductivity measurements have been made on stage 1 and 2 K/Rb mercurographitides and thallographitides. Transitions have been observed in KHgC8 (II) at 1.90 K and RbHgC8 (II) at 1.44 K, but no transitions were observed in KHgC4 (I) or RbHgC4 (I) down to 1.3 K. On the other hand, transitions were observed in KTl1.5C8 (II) at 1.3 K. Suppression of the transition in a magnetic field was observed and anisotropy of the critical field as well.
KeywordsAnisotropy Graphite Lithium Hexagonal H2SO4
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