Ice IX is a form of solid water stable at temperatures below 140 K and pressures between 200 and 400 MPa. It has a tetragonal crystal lattice and a density of 1.16 g/cm³, 26% higher than ordinary ice. It is formed by cooling ice III from 208 K to 165 K (rapidly—to avoid forming ice II). Its structure is identical to ice III other than being pro...