Anisotropy and two-dimensional effects in the ESR properties ofOsF6-graphite intercalation compounds

Systematic ESR and susceptibility studies were carried out on highly oriented pyrolytic graphite intercalated with ${\mathrm{OsF}}_6$. For all stages and ${\mathrm{OsF}}_6$ concentrations the ESR results show anisotropic g values, thermal broadening, and residual widths. The anisotropic g value is explained assuming a spin Hamiltonian with an effective spin, S=(3/2), appropriate to the quartet ground state of the 5$d^3$ configuration of ${\mathrm{Os}}^{5+}$ in a slightly distorted octahedral symmetry. The susceptibility measurements support this identification and yield zero-field splitting parameters D=12±5 K for stage I and D=30±10 K for higher stages. The differences may be partially correlated with variation of the ‘‘sandwich’’ thickness of these compounds. The charge transfer per intercalant species is one electron—independent of stage. The anisotropic thermal broadening is attributed to a Korringa-like mechanism due to anisotropic exchange interaction between the localized 5d and the π-like mobile electrons. Using the theory of Redfield, we have developed equations for the ESR thermal broadening under conditions of anisotropic exchange and g values. Comparison with experimental data suggests almost isotropic 5d-π exchange interaction for stage I, but some anisotropy for higher stages. The data suggest that the density of carriers at the Fermi level is almost stage independent. By using second-moment calculations the angular variation of the residual width can be explained by two-dimensional dipolar interaction as well as small anisotropy in the ion-ion spin exchange interaction.