Instabilities of intercalated graphite structures - EPR oscillations

EPR studies of HOPG-potassium intercalated (first stage) are described for three samples: C8K, C8Kex (with excess of potassium) and C6KOy with y < 0.2 (C4KOx potassium double layer + C8K, x < 0.2). EPR of pristine HOPG and C8K (stoichiometric) consists of one Dysonian line whereas C8Kex and C6KOy exhibit complex spectra. The analysis of these spe ctra allowed to decompose them into Dysonian and Lorentzian lines and to es timate their relative contributions. The temperature dependence of Dysonian component in C8Kex spectrum is the same as the EPR spectrum temperature de pendence of stoichiometric C8K. The second component, the Lorentzian one is due to localized electrons. This component follows Curie law on cooling. O n lowering temperature, a very strong oscillations in the EPR spectrum have been observed in C8Kex. The temperature at which the oscillations start, t he temperature range and the strength of these oscillations seem to occur a t random. Such behavior can be related to local instability of stoichiometr y x in CxK samples. Redistribution of potassium ions between stiffened grap hitic planes appears at low temperature also in two phase C6KOy system (C4K Ox potassium double layer for x < 0.2, and C8K). It confirms that alkali re distribution modulates conducting band and leads to magnetic non-resonant f luctuations.