Temperature dependence of the zero point kinetic energy in ice and water above room temperature

•The proton's kinetic energy shows a different temperature dependence between the solid and liquid phases.•Harmonic theories reproduce experiments if librations are taken into account for the solid and are neglected for the liquid.•The role of H-bonding and quantum fluctuations on the proton dynamics is highlighted.

By means of Deep Inelastic Neutron Scattering we determined the temperature dependence of the proton kinetic energy in polycrystalline ice Ih between 5 K and 271 K. We compare our results with predictions form Path Integral quantum simulations and semiclassical quasi-harmonic models with phase-dependent frequencies. The latter show the best agreement with the experiment if the librational contribution is properly taken into account. The kinetic energy increase with temperature in ice is also found to be approximately a factor ∼ 5 smaller than in the case of liquid water above room temperature, highlighting the role played by anharmonic quantum fluctuations in the two phases.

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