Structural Mechanisms of Transitions between Ices with Different Densities—Features of Water Phase Diagrams

Structural mechanisms are developed for transitions between ices with either high or low density. Fractions of hydrogen bonds retained in the course of the transitions are calculated. Relative degrees of space filling by molecules of the considered ices are estimated. An assumption is formulated and partially substantiated that boundary exists between ice phases in the low-temperature region of the PvT phase diagram of ices. A new method for construction of PvTb diagrams of water is justified using the results of the structural analysis. The hydrogen bond concentration b serves as the second order parameter of phase transitions.