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 statistical 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., order-disorder phase transitions, dielectric properties, hydrogen-bond symmetrization) for experimentally interesting systems such as the KDP-like (potassium dihydrogen phosphate) ferroelectrics and ice polymorphs.