Ice Ic without stacking disorder by evacuating hydrogen from hydrogen hydrate

Water freezes below 0 °C at ambient pressure, ordinarily to ice Ih with an ABAB... hexagonal stacking sequence. However, it is also known to produce "ice Ic" nominally with an ABCABC... cubic stacking sequence under certain conditions1, and its existence in Earth's atmosphere, or in comets is debated. "Ice Ic", or called as cubic ice, was first identified in 1943 by König, who used electron microscopy to study the condensation of ice from water vapor to a cold substrate. Subsequently, many different routes to "ice Ic" have been established, such as the dissociation of gas hydrates, warming amorphous ices or annealing high-pressure ices recovered at ambient pressure, freezing of $μ$- or nano-confined water. Despite the numerous studies on "ice Ic", its structure has not been fully verified, because the diffraction patterns of "ice Ic" show signatures of stacking-disorder, and ideal ice Ic without stacking-disorder had not been formed until very recently. Here we demonstrate a route to obtain ice Ic without stacking-disorder by degassing hydrogen from the high-pressure form of hydrogen hydrate, C$_2$, which has a host framework that is isostructural with ice Ic. Surprisingly, the stacking-disorder free ice Ic is formed from C$_2$ via an intermediate amorphous or nano-crystalline form under decompression, unlike the direct transformations that occur in the cases of recently discovered ice XVI from neon hydrate, or ice XVII from hydrogen hydrate. The obtained ice Ic shows remarkable thermal stability until the phase transition to ice Ih at 250 K; this thermal stability originates from the lack of dislocations, which promote changes in the stacking sequence. This discovery of ideal ice Ic will promote understanding of the role of stacking-disorder on the physical properties of ice as a counter end-member of ice Ih.