Abstract
Water freezes below 0 °C at ambient pressure ordinarily to ice I(h), with hexagonal stacking sequence. Under certain conditions, ice with a cubic stacking sequence can also be formed, but ideal ice I(c) without stacking-disorder has never been formed until recently. Here we demonstrate a route to obtain ice I(c) without stacking-disorder by degassing hydrogen from the high-pressure form of hydrogen hydrate, C(2), which has a host framework isostructural with ice I(c). The stacking-disorder free ice I(c) is formed from C(2) via an intermediate amorphous or nano-crystalline form under decompression, unlike the direct transformations occurring in ice XVI from neon hydrate, or ice XVII from hydrogen hydrate. The obtained ice I(c) shows remarkable thermal stability, until the phase transition to ice I(h) at 250 K, originating from the lack of dislocations. This discovery of ideal ice I(c) will promote understanding of the role of stacking-disorder on the physical properties of ice as a counter end-member of ice I(h).