Instabilities of intercalated graphite structures - EPR oscillations

Summary

EPR studies of HOPG-potassium intercalated (first stage) are described for three samples: C₈K, C₈K_ex (with excess of potassium) and C₆KO_y with y < 0.2 (C₄KO_x potassium double layer + C₈K, × < 0.2).

EPR of pristine HOPG and C₈K (stoichiometric) consists of one Dysonian line whereas C₈K_ex and C₆KO_y exhibit complex spectra. The analysis of these spectra allowed to decompose them into Dysonian and Lorentzian lines and to estimate their relative contributions. The temperature dependence of Dysonian component in C₈K_ex spectrum is the same as the EPR spectrum temperature dependence of stoichiometric C₈K. The second component, the Lorentzian one is due to localized electrons. This component follows Curie law on cooling. On lowering temperature, a very strong oscillations in the EPR spectrum have been observed in C₈K_ex. The temperature at which the oscillations start, the temperature range and the strength of these oscillations seem to occur at random. Such behavior can be related to local instability of stoichiometry x in C_xK samples. Redistribution of potassium ions between stiffened graphitic planes appears at low temperature also in two phase C₆KO_y system (C₄KO_x potassium double layer for x < 0.2, and C₈K). It confirms that alkali redistribution modulates conducting band and leads to magnetic non-resonant fluctuations