Nuclear Magnetic Resonance in the Cesium-Graphite Intercalation Compounds

Abstract

The ${\mathrm{C}}^{13}$ and ${\mathrm{Cs}}^{133}$ 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 ${\mathrm{C}}^{13}$ 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 $T_1$ for ${\mathrm{C}}^{13}$ 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 ${\mathrm{C}}_8$Cs, ${\mathrm{C}}_{24}$Cs, ${\mathrm{C}}_{36}$Cs, ${\mathrm{C}}_{48}$Cs, ${\mathrm{C}}_{60}$Cs, 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 ${\mathrm{C}}^{13}$ $T_1$'s. None of the usual relaxation mechanisms is conclusively identified with the relatively short ${\mathrm{C}}^{13}$ $T_1$'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 ${\mathrm{C}}_8$Cs at 1.3 °K. The measured quadrupolar splitting is 16.8 ± 1 kHz. The Knight shift for cesium in ${\mathrm{C}}_8$Cs is measured to be (0.29 ± 0.01%), independent of temperature from 300 to 1.3 °K. In ${\mathrm{C}}_{24}$Cs and ${\mathrm{C}}_{36}$Cs, the Knight shift was zero within ± 0.02%. The experimentally measured $T_1$ for ${\mathrm{Cs}}^{133}$ in ${\mathrm{C}}_8$Cs was 7.5 ± 0.5 sec, while the $T_1$'s in ${\mathrm{C}}_{24}$Cs and ${\mathrm{C}}_{36}$Cs were 27 ± 6 and 48 ± 10 min, respectively. The ${\mathrm{Cs}}^{133}$ Knight-shift and relaxation-time results support the view that the cesium is partially ionized in ${\mathrm{C}}_8$Cs and completely ionized in the cesium-poorer stages.