Heat capacity, adsorption isotherms, and quasielastic neutron scattering measurements of bilayer deuterium hydride adsorbed on graphite

Abstract

Bilayer films of deuterium hydride (HD) adsorbed on exfoliated graphites have been studied using heat capacity, adsorption isotherms, and quasielastic neutron scattering (QENS) measurements. The second layer of HD shows solid ($S$), liquid ($L$), and gaseous ($G$) [vapor ($V$)] phases, with two-dimensional (2D) characteristics. We determined the $S\ensuremath{-}L\ensuremath{-}V$ triple line at ${T}_{t}(2\mathrm{D})=8.44$ K, and the $L\ensuremath{-}V$ critical temperature at ${T}_{c}(2\mathrm{D})=11.45$ K. QENS identified the $L$ phase and measured its translation diffusion coefficient ${D}_{T}(\ensuremath{\approx}{10}^{\ensuremath{-}5} {\mathrm{cm}}^{2}{\mathrm{s}}^{\ensuremath{-}1} \mathrm{at} 9.25 \mathrm{K})$. Isosteric heats, latent heat, and entropy change at melting, and solid Debye temperatures are calculated. The results are used to construct a general graph of critical and triple points of adsorbed gases as a function of their quantum parameter.