Configurational Energetics in Ice $\mathrm{I}h$ Probed by Compton Scattering

Temperature-induced changes in the ground-state electron momentum density of polycrystalline ice $\mathrm{I}h$ are studied with high accuracy by Compton scattering utilizing synchrotron radiation. A unique feasibility of the technique to provide direct experimental information on configurational enthalpies and heat capacities is demonstrated. The configurational enthalpy, obtained with an accuracy of 1.5 meV, evolves linearly with temperature above $T=100\mathrm{K}$. Consequently the configurational heat capacity is found to be constant, $c_p^{\mathrm{config}}=(0.44\pm 0.11)\mathrm{J}{\mathrm{g}}^{-1}{\mathrm{K}}^{-1}$, in this temperature regime. Obtaining these quantities experimentally is fundamentally important for evaluating the accuracy of molecular-dynamics simulations schemes.

Figure 1

Compton profile differences of polycrystalline ice $\mathrm{I}h$ acquired at various temperatures, the reference profile being acquired at $T=100\mathrm{K}$. The spectra are vertically offset for clarity. The solid line is the experimental data of Ref. 23, i.e., the difference between Compton profiles of water and polycrystalline ice $\mathrm{I}h$, scaled to fit to each temperature difference.

Figure 2

Temperature-induced changes in the configurational enthalpy $\Delta H^{\mathrm{config}}$ (per molecule) of ice $\mathrm{I}h$ as determined from the present experimental data. The reference temperature is $T=100\mathrm{K}$. The slope of the linear least-squares fit to the data above $T=100\mathrm{K}$ provides the corresponding configurational heat capacity $c_p^{\mathrm{config}}$.