Ice Ic (pronounced "ice one c" or "ice I c") is a metastable cubic crystalline variant of ice. Hans König was the first to identify and deduce the structure of ice Ic.[1] The oxygen atoms in ice Ic are arranged in a diamond structure and is extremely similar to ice Ih having nearly identical densities and the same lattice constant along the hexagonal puckered-planes.[2] It forms at temperatures between 130 and 220 kelvins (−143 and −53 degrees Celsius) upon cooling, and can exist up to 240 K (−33 °C) upon warming,[3][4] when it transforms into ice Ih.
Apart from forming from supercooled water,[5] ice Ic has also been reported to form from amorphous ice[2] as well as from the high-pressure ices II, III and V.[6] It can form in and is occasionally present in the upper atmosphere[7] and is believed to be responsible for the observation of Scheiner's halo, a rare ring that occurs near 28 degrees from the Sun or the Moon.[8]
Ordinary water ice is known as ice Ih (in the Bridgman nomenclature). Different types of ice, from ice II to ice XIX,[9] have been created in the laboratory at different temperatures and pressures.
Some authors have expressed doubts whether ice Ic really has a cubic crystal system, claiming that it is merely stacking-disordered ice I (“ice Isd”),[10][11][12] and it has been dubbed the ″most faceted ice phase in a literal and a more general sense.″[13]
However, in 2020, two research groups individually prepared ice Ic without stacking disorder. Komatsu et al. prepared C2 hydrate at high pressure and decompressed it at 100 K to make hydrogen molecules extracted from the structure, resulting in ice Ic without stacking disorder.[14] Del Rosso et al. prepared ice XVII from C0 hydrate and heated it at 0 GPa to obtain pure ice Ic without stacking disorder.[15] Pure ice Ic prepared in the latter method transforms into ice Ih at 226 K with an enthalpy change of -37.7 J/mol.[16]