Two-dimensional metallic hydrogen in the potassium-hydrogen-graphite ternary intercalation compound

A proton NMR study was carried out on the stage-1 potassium-hydrogen-graphite ternary intercalation compound ${\mathrm{C}}_4$ ${\mathrm{KH}}_{\mathrm{x}}$, prepared through direct intercalation of potassium hydride into graphite. Line-shape analysis revealed extremely dense two-dimensional packing of negatively charged hydride ions where hydrogen-hydrogen direct contact takes place. The metallic nature of hydrogen was observed through the Korringa term, (${\mathrm{T}}_1$T${)}^{\mathrm{-}1}$=6.4×${10}^{\mathrm{-}4}$ ${\mathrm{s}}^{\mathrm{-}1}$ ${\mathrm{K}}^{\mathrm{-}1}$, where ${\mathrm{T}}_1$ is the spin-lattice relaxation time. ${\mathrm{C}}_4$ ${\mathrm{KH}}_{\mathrm{x}}$ is thus proven to be an example of two-dimensional metallic hydrogen.