O Ice to Megabar Pressures*

Hydrogen bonds and proton mobility play a fundamental role for understanding the evolution from distinct molecular phases of ice to non-molecular forms under pressure, but the nature of the quantum disorder in the material across the transition has not been directly probed. We present the first synchron based broadband far-IR spectroscopy to megabar (100 GPa) pressures that reveal a significant increase in IR response and a multiplicity of excitations as a function of pressure between through the quantum disordered phase associated with hydrogen-bond symmetrization at 60-90 GPa. Features are found that were not observed or predicted in previous experimental and theoretical work, including recent simulations. The associated large static dielectric response accompanying the transition, as suggested by early simulations, is reproduced by first-principles calculations. The measurements provide crucial constraints on the many-body properties of ice in this quantum-disordered regime associated with the transition.