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 Delta 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.