The preparation of
pure cubic ice without hexagonal stacking
faults
has been realized only recently by del Rosso et al. (Nat. Mater.2020,19,663−66832015533) and Komatsu et al. (Nat. Commun.2020,11,46432015342). 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 ΔHc→hto be −37.7 ± 2.3 J mol–1. The transition temperature is identified as 226 K, much higher
than in previous work on ice Isd. 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.