Configurational Energetics in Ice Ih Probed by Compton Scattering

Temperature-induced changes in the ground-state electron momentum density of polycrystalline ice Ih 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  K. Consequently the configurational heat capacity is found to be constant, c_p^config=(0.44±0.11)  J g^-1 K^-1, in this temperature regime. Obtaining these quantities experimentally is fundamentally important for evaluating the accuracy of molecular-dynamics simulations schemes.