Molecular motion in solidH2at 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 ρ/${\rho }_0$ 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 ΔH yields an activation volume of 5.7±0.6 ${\mathrm{cm}}^3$/mol or 63% of the molar volume, a reasonable value for a vacancy diffusion mechanism. The smooth variation of ΔH/${\mathrm{kT}}_{\mathrm{melt}}$ with density also suggests that the diffusion mechanism remains the same for ρ/${\rho }_0$ between 1 and 3. The spin-lattice relaxation is controlled primarily by molecular reorientation. The observed density dependence $T_1$∝${\rho }^{5/3}$ indicates that molecular electric quadrupole-quadrupole interactions cause reorientation, even at the high temperatures and densities of this work.