Molecular motion in solid H2 at high pressures.

Abstract

Solid molecular hydrogen has been studied with proton nuclear magnetic resonance in a diamond anvil cell. Pressures from 18 to 68 kbar were used, resulting in melting temperatures from 160 to 350 K and relative densities \ensuremath{\rho}/${\ensuremath{\rho}}_{0}$ as high as 3. At temperatures above 0.7${T}_{\mathrm{melt}}$, translational self-diffusion narrows the resonance line. The pressure variation of the activation enthalpy \ensuremath{\Delta}H yields an activation volume of 5.7\ifmmode\pm\else\textpm\fi{}0.6 ${\mathrm{cm}}^{3}$/mol or 63% of the molar volume, a reasonable value for a vacancy diffusion mechanism. The smooth variation of \ensuremath{\Delta}H/${\mathrm{kT}}_{\mathrm{melt}}$ with density also suggests that the diffusion mechanism remains the same for \ensuremath{\rho}/${\ensuremath{\rho}}_{0}$ between 1 and 3. The spin-lattice relaxation is controlled primarily by molecular reorientation. The observed density dependence ${T}_{1}$\ensuremath{\propto}${\ensuremath{\rho}}^{5/3}$ indicates that molecular electric quadrupole-quadrupole interactions cause reorientation, even at the high temperatures and densities of this work.