Structural and electronic properties of hydrogen in the potassium-hydrogen-graphite ternary intercalation compoundC8KH0.55: A nuclear-magnetic-resonance study

A proton NMR study was carried out on a second-stage potassium-hydrogen-graphite intercalation compound, ${\mathrm{C}}_8$ ${\mathrm{KH}}_{\mathrm{x}}$ (x=0.55), in a powder form within the temperature range 1.5–77 K. An inhomogeneously broadened NMR signal (around 40 kHz in full width at half maximum) was observed, but the intrinsic linewidth of the spin isochromat, about 3.6 kHz, was determined from the spin-echo envelope. The shape of the echo envelope was explained by assuming a regular two-dimensional structure of dissociated hydride ions. The inhomogeneous broadening may possibly be ascribed to localized electron spins of the order of 100 ppm. The spin-lattice relaxation mechanisms were divided into two contributions, the metallic process and the process due to localized electron spins. Analysis of the metallic relaxation component revealed the existence of a small local density of states at the Fermi level at hydrogen sites, N($E_F$ ${)}_{\mathrm{H}}$=0.014 states per eV per spin per hydrogen atom. The hydrogen atomic plane was revealed to be weakly metallic. A possible electronic structure is discussed.