c-axis resistivity of ${\mathrm{MoCl}}_5$ graphite intercalation compounds

The c-axis resistivity ${\mathrm{\rho }}_{\mathit{c}}$ of stage-2, -3, -4, -5, and -6 ${\mathrm{MoCl}}_5$ graphite intercalation compounds (GIC’s) has been measured in the temperature range between 4.2 and 300 K with and without an external magnetic field along the c axis. The resistivity for stage-3, -4, and -5 ${\mathrm{MoCl}}_5$ GIC’s shows a logarithmic increase (lnT) with decreasing temperature at low temperatures. The stage-4, -5, and -6 ${\mathrm{MoCl}}_5$ GIC’s show a negative longitudinal magnetoresistance (MR), while the stage-2 and -3 ${\mathrm{MoCl}}_5$ GIC’s show a positive longitudinal MR. Because of high anisotropy in resistivity, the c-axis conduction is dominated by the in-plane conduction. The logarithmic behavior and negative MR in ${\mathrm{\rho }}_{\mathit{c}}$ are discussed in terms of the two-dimensional weak localization effect occurring in the graphite basal plane. The possibility of the Kondo effect is not ruled out because of possible exchange interactions between conduction electrons and ${\mathrm{Mo}}^{5+}$ spins having an effective magnetic moment of 1.83–1.95${\mathrm{\mu }}_{\mathit{B}}$. The temperature dependence of ${\mathrm{\rho }}_{\mathit{c}}$ varies with the stage number: metallic behavior for stage-2 and semiconductorlike behavior for stage-5 ${\mathrm{MoCl}}_5$ GIC. The resistivity ${\mathrm{\rho }}_{\mathit{c}}$ of stage-3 ${\mathrm{MoCl}}_5$ GIC has a local maximum around 267 K. These behaviors of ${\mathrm{\rho }}_{\mathit{c}}$ are also discussed in terms of conduction mechanisms including conduction paths, phonon- and impurity-assisted hopping, and small polaron effect. © 1996 The American Physical Society.