C-AXIS ELECTRICAL-CONDUCTIVITY AND THERMOELECTRIC-POWER OF SODIUM HYDRIDE GRAPHITE TERNARY INTERCALATION COMPOUNDS

The electronic structures of sodium hydride intercalated graphite (NaH-GIC's) C(an) NaH (a = 2.5-4.5, n = stage index), which have ionic triple atomic layers Na+-H--Na+ in their graphitic galleries, are investigated using c-axis resistivity and thermoelectric power measurements. The introduction of NaH in the graphitic galleries depresses the c-axis resistivity, in contrast to the case of the isostructural compounds KH-GIC's or KHg-GIC's. This suggests that the NaH intercalate does not contribute to the transport properties. The thermoelectric power, which is explained mainly in terms of phonon-drag effects, anomalously decreases as the stage index increases. This behavior is explained by the important role played by the domain-scattering mechanism of phonons in phonon-drag effects, and proves that the Fermi surface of graphitic pi electrons is small. The results of thermoelectric power measurements give the Fermi energy at E(F) = 0.45 eV for a stage-3 compound, which is considerably smaller than that for KHg-GIC's and KH-GIC's. From these experimental results, it is suggested that the ionic NaH intercalate with a stoichiometric composition of [Na]/[H] = 1 makes a complete charge transfer within the intercalate layer, resulting in the presence of a small concentration of graphitic pi-electron carriers and the absence of conduction carriers in the intercalate regions.