On paramagnetism in ¯uorinated graphite: EPR and solid state NMR study M. Panich a, * , A.I. Shames a , T. Nakajima b a Department of Physics, Ben-Gurion University of the Negev, Be'er Sheva 84105, Israel b Division of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606, Japan Received 25 October 1999; accepted 25 September 2000 Abstract The study of the origin of paramagnetism in ¯uorinated graphics by means of EPR and 19 F NMR spin±lattice relaxation measurements is presented. We show that paramagnetism in ¯uorinated graphite is caused by local magnetic moments of dangling bonds. Localized spins in the high ¯uorine concentration region behave like isolated spins. In the partially ¯uorinated samples, strong exchange interaction between localized moments is observed. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: D. Electron paramagnetic resonance (EPR); D. Nuclear magnetic resonance (NMR) 1. Introduction Intercalation of graphite with various guest species yields charge-transfer compounds. Such compounds show an order of magnitude increase in an in-plane conductivity upon intercalation due to injection of carriers from the intercalate layer to the conduction band of graphite. The magnetic susceptibility of graphite is large and diamagnetic, in graphite intercalation compounds, the value of x k is reduced in comparison to graphite and tends to be positive (para- magnetic) for donors and negative for acceptors [1]. The observed susceptibility results from diamagnetic contribu- tion from the core electrons, paramagnetic spin (or Pauli) contribution from the free carriers and an orbital contribu- tion from the valence electrons. For nonmagnetic acceptor or donor compounds, it is temperature independent [1]. However, ¯uorine-graphite intercalation compounds (GICs) and graphite ¯uorides exhibit a behavior that is different from other GICs. For a dilute regime ¯uorine- GICs are conductors and, like all other GICs, also exhibit temperature-independent (Pauli) spin susceptibility charac- teristic for conduction electrons. Nevertheless, the suscept- ibility of the insulating C x F compound readily exhibits a Curie-law temperature dependence coming from localized moments [2]. Recently, the same dependence of susceptibil- ity was obtained in activated carbon ®bers which are micro- porous systems comprised of a random network of nano- sized graphites [3]. Such magnetic behavior was attributed to the presence of dangling bonds having a localized spin. However, Saito et al. suggested that in C x F the doped elec- tron is strongly localized around the F site [4]. To obtain atomic level information about such an effect, we carried out EPR and spin±lattice 19 F NMR relaxation measurements of ¯uorinated graphites, which are presented in this paper. From EPR spectra, we expect to receive information about the structure of paramagnetic centers and spin exchange. Nuclear spin±lattice relaxation rate strongly depends on concentration and arrangement of paramagnetic centers and is also an effective tool to study them. 2. Sample preparation and characterization Polycrystalline graphite ¯uoride (C 2 F) n was prepared by reaction of natural graphite with ¯uorine gas at temperatures in the range 350±4008C. The sample is black colored ®ne powder. Powdered graphite ¯uorides (CF) n have been prepared by ¯uorination of natural graphite and petroleum coke with F 2 at approximately 5008C. Basically, petroleum coke takes the same layered structure as graphite; however, Journal of Physics and Chemistry of Solids 62 (2001) 959±964 0022-3697/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0022-3697(00)00264-X www.elsevier.nl/locate/jpcs * Corresponding author. Tel.: 1972-7-647-2458; fax: 1972-7- 647-2903. E-mail address: pan@bgumail.bgu.ac.il (M. Panich).