Nuclear Magnetic Resonance in the Cesium-Graphite Intercalation Compounds

The C¹³ and Cs¹³³ nuclear magnetic resonances have been studied in powdered samples of the cesium-graphite intercalation compounds at temperatures between 1.3 and 4.2 ^∘{}K. The spin-lattice relaxation times and the line shapes of both nuclear species in the compounds and of the C¹³ nucleus in pure graphite were measured in an effort to determine the nature of the conduction-electron states in these substances. At 4.2 ^∘{}K, using pulse techniques, the measured values of T₁ for C¹³ are 1.6 ±{} 0.3, 2.6 ±{} 0.4, 3.9 ±{} 0.5, 5.6 ±{} 0.5, 7.6 ±{} 0.8, and 30 ±{} 5 min in C₈Cs, C₂₄Cs, C₃₆Cs, C₄₈Cs, C₆₀Cs, and pure graphite, respectively. Both the Salzano-Aronson binding model for the compounds and a tight-binding extension to the Slonczewski-Weiss band model for graphite are shown to account qualitatively for the cesium concentration dependence of the C¹³ T₁'s. None of the usual relaxation mechanisms is conclusively identified with the relatively short C¹³ T₁'s. The temperature dependence also remains unaccounted for. Echo techniques at helium temperatures establish the shape of the 500-G-wide cesium quadrupolar spectrum in C₈Cs at 1.3 ^∘{}K. The measured quadrupolar splitting is 16.8 ±{} 1 kHz. The Knight shift for cesium in C₈Cs is measured to be (0.29 ±{} 0.01%), independent of temperature from 300 to 1.3 ^∘{}K. In C₂₄Cs and C₃₆Cs, the Knight shift was zero within ±{} 0.02%. The experimentally measured T₁ for Cs¹³³ in C₈Cs was 7.5 ±{} 0.5 sec, while the T₁'s in C₂₄Cs and C₃₆Cs were 27 ±{} 6 and 48 ±{} 10 min, respectively. The Cs¹³³ Knight-shift and relaxation-time results support the view that the cesium is partially ionized in C₈Cs and completely ionized in the cesium-poorer stages.