Electron spin resonance in neutron irradiated and in doped polycrystalline graphite—Part I

Abstract

The electron spin resonance of neutron irradiated and lamellar compound doped, incompletely graphitized, P33 carbon black was investigated. An improved temperature arrangement operating from room temperature down to liquid nitrogen was constructed which permits one to perform speedy measurements with a minimum of corrections involved. It is shown that the intensity vs. temperature curves for P33 can be split into a sum of contributions of localized spin centers and conduction carriers; the irradiation producing mainly localized spins and the doping changing only the carrier concentration. The two kinds of spin centers interact by exchange so that they produce a single line with a g-value intermediate between that for localized spin centers g free and for the conduction carriers g c . The g-value can be calculated using a simple mix formula, each type of spin contributing in proportion to its absorption intensity. The g c value for conduction carriers varies according to irradiation dose or amount of doping and is always lower than the directionally averaged graphite value due to a combined effect of displacement of the Fermi level and of the spreading apart of the layers, both causing a change in spin—orbit interaction. Extensive studies of variation in ESR parameters with irradiation dose, amount of doping (Li, Na, K, Rb, H 2SO 4 and ammonia), temperature of anneal and ambient temperature permitted one, among other things, to observe the transition of the Fermi level across the center of the band overlap (with Na and Li).