Titolo della tesi: Phase diagram, structure and spectroscopy of ordinary and high pressure ice: impact of quantum anharmonic nuclear motion
Water ice is a unique material, presenting the most complex phase diagram known in the literature, ranging from low to high temperature and from low to high pressures.
The low-pressure phases, like ordinary ice Ih and its proton-ordered counterpart ice XI, show intriguing physical properties, such as negative thermal expansion and
anomalous volume isotope effect (VIE). In the opposite regime, at high pressure, the features of the phase transition of dense ices VII/VIII to the symmetric ice X are still
open questions. The signatures of the phase transition are indirect (hydrogen atoms are invisible to X-ray diffraction and the limited data quality and uncertainties in the procedure of data correction in neutron scattering hampers an unambiguous interpretation) and come from vibrational spectroscopies that give contrasting results because of the strong anharmonic regime close to the phase transition. Experimental data need the support of theoretical simulations to understand the high-pressure phase diagram.
In this thesis, I explore the paramount importance of nuclear quantum fluctuations in the thermodynamic and vibrational properties of low and high pressure ice by
employing the stochastic self-consistent harmonic approximation. For what concerns the VIE in low-pressure ices, I prove that quantum effects on hydrogen are so strong
to be in a nonlinear regime: when progressively increasing the mass of hydrogen from protium to infinity (classical limit), the volume first expands and then contracts, with
a maximum slightly above the mass of tritium. I manage to accurately reproduce, for the first time, the low-energy phonon dispersion, possible thanks to the correct
treatment of nuclear quantum fluctuations, paving to way for the study of thermal transport in ice from first-principles.
I establish the second-order character of the high-pressure phase transition combining the results from classical and quantum simulations, where a continuous
transformation of one phase into the other, the presence of soft modes, and the absence of an hysteresis cycle is proven. I show the importance of including quantum
fluctuations that reduce the critical pressure of about 55 GPa at T=0 K, solving the problem of the strong underestimation of the critical pressure by the classical
approximations. I simulate the Infrared absorption spectra sampling a fine pressure grid close to the transition, revealing the sudden collapse of the stretching mode toward the low-energy regime in less than 10 GPa. Simultaneously, in the same range, I show that the low-energy translational mode (situated in a region where there are
no published data to date) increases its intensity by an order of magnitude. These two features can be regarded as unique signatures of the phase transition.