Low-temperature specific heat of hydrogen-chemisorbed graphite–alkali-metal intercalation compounds

The results of low-temperature specific-heat measurements are presented for hydrogen-chemisorbed ${\mathrm{C}}_8$M (M=K,Rb), to clarify the effects of dissolved hydrogen on its electronic and lattice vibrational properties. The density of states at the Fermi level is suppressed through a charge transfer from ${\mathrm{C}}_8$M to hydrogen which is stabilized in interstitial sites in the intercalate layers. This finding is consistent with the results of ESR and positron-annihilation measurements. The introduction of hydrogen increases the acoustic and optic phonon energies for ${\mathrm{C}}_8$ ${\mathrm{KH}}_{\mathrm{x}}$ (x<0.1). The electronic effects of dissolved hydrogen are considered to be the cause of the increase. At the hydrogen-saturated concentration (${\mathrm{C}}_8$ ${\mathrm{KH}}_{2/3}$), the phonon energies are lowered by an increase in the effective mass of the intercalants due to the strong ionic coupling between K and H through bondings with hydrogen. In the case of the ${\mathrm{C}}_8$Rb system, the acoustic-phonon energies increase while the optic-phonon ones decrease through the introduction of small concentrations of hydrogen (x<0.1). The effects of dissolved hydrogen on the superconductivity of ${\mathrm{C}}_8$ ${\mathrm{KH}}_{\mathrm{x}}$ are discussed using the specific-heat results.