Enthalpy Change from Pure Cubic Ice I c to Hexagonal Ice I h .

The preparation of pure cubic ice without hexagonal stacking faults has been realized only recently by del Rosso et al. ( Nat. Mater. 2020, 19, 663-668) and Komatsu et al. ( Nat. Commun. 2020, 11, 464). With our present calorimetric study on the transition from pure cubic ice to hexagonal ice we are able to clarify the value of the enthalpy change Δ H c→h to be -37.7 ± 2.3 J mol -1 . The transition temperature is identified as 226 K, much higher than in previous work on ice I sd . This is due to a catalytic effect of hexagonal faults on the transition, but even more importantly due to a relaxation exotherm that was not properly identified in the past.