Ice VII
Encyclopedia
Ice VII is a cubic
crystalline form of ice
. It has a triple point
with liquid water and Ice VI at 355 K and 2.216 GPa, with the melt line extending to at least 715 K and 10 GPa. It can also be reached in the solid state by increasing the pressure on ice VI at ambient temperature. Like the majority of ice phases (including ice Ih
), the hydrogen atom
positions are disordered. In addition, the oxygen
atoms are disordered over multiple sites. The structure of ice VII comprises a hydrogen bond
framework in the form of two interpenetrating (but non-bonded) sub-lattices.
Ice VII is the only disordered phase of ice that can be ordered by simple cooling, and it forms (ordered) ice VIII
below 273 K up to ~ 8 GPa. Above this pressure, the VII-VIII transition temperature drops rapidly, reaching 0 K at ~60 GPa. Thus, ice VII has the largest stability field of all of the molecular phases of ice. The cubic
oxygen sub-lattices that form the backbone of the ice VII structure persist to pressures of at least 128 GPa; this pressure is substantially higher than that at which water loses its molecular character entirely, forming ice X.
Ordinary water ice is known as ice Ih
, (in the Bridgman
nomenclature). Different types of ice, from ice II
to ice XV
, have been created in the laboratory at different temperatures and pressures.
Scientists hypothesize that Ice VII may comprise the ocean floor of extrasolar planets (such as Gliese 436 b
and GJ 1214 b) that are largely made of water.
Cubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
crystalline form of ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
. It has a triple point
Triple point
In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases of that substance coexist in thermodynamic equilibrium...
with liquid water and Ice VI at 355 K and 2.216 GPa, with the melt line extending to at least 715 K and 10 GPa. It can also be reached in the solid state by increasing the pressure on ice VI at ambient temperature. Like the majority of ice phases (including ice Ih
Ice Ih
thumb|Photograph showing details of an ice cube under magnification. Ice Ih is the form of ice commonly seen on earth.Ice Ih is the hexagonal crystal form of ordinary ice, or frozen water. Virtually all ice in the biosphere is ice Ih, with the exception only of a small amount of ice Ic which is...
), the hydrogen atom
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
positions are disordered. In addition, the oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
atoms are disordered over multiple sites. The structure of ice VII comprises a hydrogen bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
framework in the form of two interpenetrating (but non-bonded) sub-lattices.
Ice VII is the only disordered phase of ice that can be ordered by simple cooling, and it forms (ordered) ice VIII
Ice VIII
Ice VIII is a tetragonal crystalline form of ice formed from ice VII by cooling it below 5 °C. It is more ordered than ice VII, since the hydrogen atoms assume fixed positions....
below 273 K up to ~ 8 GPa. Above this pressure, the VII-VIII transition temperature drops rapidly, reaching 0 K at ~60 GPa. Thus, ice VII has the largest stability field of all of the molecular phases of ice. The cubic
Cubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
oxygen sub-lattices that form the backbone of the ice VII structure persist to pressures of at least 128 GPa; this pressure is substantially higher than that at which water loses its molecular character entirely, forming ice X.
Ordinary water ice is known as ice Ih
Ice Ih
thumb|Photograph showing details of an ice cube under magnification. Ice Ih is the form of ice commonly seen on earth.Ice Ih is the hexagonal crystal form of ordinary ice, or frozen water. Virtually all ice in the biosphere is ice Ih, with the exception only of a small amount of ice Ic which is...
, (in the Bridgman
Percy Williams Bridgman
Percy Williams Bridgman was an American physicist who won the 1946 Nobel Prize in Physics for his work on the physics of high pressures. He also wrote extensively on the scientific method and on other aspects of the philosophy of science.- Biography :Bridgman entered Harvard University in 1900,...
nomenclature). Different types of ice, from ice II
Ice II
Ice II is a rhombohedral crystalline form of ice with highly ordered structure. It is formed from ice Ih by compressing it at temperature of 198 K at 300 MPa or by decompressing ice V. When heated it undergoes transformation to ice III....
to ice XV
Ice XV
Ice XV is a crystalline form of ice, the proton-ordered form of ice VI. It is created by cooling water to around 130 K at 1 GPa .Ordinary water ice is known as ice Ih,...
, have been created in the laboratory at different temperatures and pressures.
Scientists hypothesize that Ice VII may comprise the ocean floor of extrasolar planets (such as Gliese 436 b
Gliese 436 b
Gliese 436 b is a Neptune-sized extrasolar planet orbiting the red dwarf star Gliese 436. It was among the smallest known transiting planets in mass and radius until the much smaller Kepler discoveries started coming in 2010.-Discovery:...
and GJ 1214 b) that are largely made of water.