Phase transitions induced by proton tunneling in hydrogen-bonded crystals. Ground-state theory

A novel theory is presented for the ground-state equilibrium properties and phase behavior of hydrogen-bonded crystals in which quantum-mechanical tunneling is important. The approach is based on a variational correlated wave function and simultaneously treats short- and long-range proton correlations in addition to the quantum tunneling aspect. Application to an exactly solvable quantum spin model along with general statistic. ~l mechanical considerations suggest the theory is quantitatively reliable for many three-dimensional crystals of interest. Model calculations reveal a diverse set of possible phase behaviors as a function of applied pressure. The theory should be useful for predicting and interpreting a range of phenomena induced by high pressure (e.g., orderdisorder phase transitions, dielectric properties, hydrogen-bond symmetrization) for experimentally interesting systems such as the KDP-like (potassium dihydrogen phosphate) ferroelectrics and ice polymorphs.