Ice IV

Samples of ice IV have been made and recovered in a metastable state at liquid‐nitrogen temperature by using the nucleators discovered by Evans and a modification of his technique. No appreciable thermal effect that can be associated with a phase transition was observed on cooling. During the course of the preparation, measurements were made of the melting lines of ice III, IV, V, and VI. The intersections of these lines allows the coordinates of the hitherto unreported triple points III‐IV‐L and III‐VI‐L for both H₂O and D₂O and IV‐VI‐L for H₂O to be obtained as well as new values of the coordinates of known triple points. The equilibrium pressure p_e of a first‐order phase transformation is given by $p_e\mathrm{=-\; \Delta \; A\; /\; \Delta \; V}$ where ΔA and ΔV are the changes of the Helmholtz energy and the volume at the transition. By using this equation, the effect of isotopic substitution on solid—solid transitions is shown to be caused by the change of vibration frequencies at a transition and for the phases of ice is largely caused by the change of the zero‐point energy of the vibrations.