Amorphous ice
Amorphous ice is a form of solid water, which is distinguished by the fact that the water molecules are arranged irregularly, as in a glass, so no long-range order exists. This results in amorphous ice by the 15 known crystalline forms of ice is different.
The predominant form is fixed on the ground ice (Ih), which has a regular hexagonal crystal structure. In interstellar space, however, the amorphous forms are considered to be dominant.
As with crystalline ice, there are various forms of amorphous ice, this circumstance is called polyamorphism. The various amorphous forms are differentiated by their density:
- Low-density amorphous ice (LDA )
- High-density amorphous ice (HDA ) and
- Very - high-density amorphous ice ( VHDA ).
Molding
Low Density amorphous ice (LDA )
Low Density amorphous ice, or low- density amorphous ice (LDA ) is the longest known form of amorphous ice.
An ability to produce amorphous material is to cool it so fast that the material can not form a crystalline structure (see vitrification ). By condensation of water vapor on a cooled copper rod could be shown with the aid of the Debye -Scherrer method already in 1935 that forms a solid without crystalline structure below the glass transition temperature of water (about 130 K at 1 bar). This form was initially amorphous solid water called (ASW ).
Other manufacturing capabilities were developed in 1980, in which an n- heptane -water emulsion is sprayed into a cryogenic liquid, or a water aerosol is sprayed onto a cryogenic copper plate with supersonic flow. This cooling rate 106-107 K / s can be achieved. This form is called due to their production as hyperquenched glassy water ( HGW ).
A third possibility consists in HDA (see below) to warm at ambient pressure. This form converts at about 120 K in low-density amorphous ice.
These three types of production, all of which result in a density of about 0.94 g/cm3 were initially considered to be different shapes. Johari et al published in 1996, that ASW and HGW have a glass transition temperature of 135 K at ambient pressure, while it lies at 129 K for LDA. However, recent evidence suggests that all three types of generation are likely to lead to the same form of amorphous ice is referred to as LDA.
High-density amorphous ice (HDA )
1984 physicists discovered by Osamu Mishima another form of amorphous ice, which can be produced rather than by a change in temperature by compression. It showed that at a temperature of 77 K and a pressure of 10 kbar hexagonal ice to a certain extent " melts " and goes into a glassy amorphous state. This form of amorphous ice has a higher density of 1.17 g/cm3 and is therefore also high- density amorphous ice (HDA ) called. HDA and LDA can be converted by changing the pressure and temperature inside each other. Here, a sharp transition was observed.
Very - high-density amorphous ice ( VHDA )
This form was found in 1996 by Mishima, at pressures of 1-2 GPa, as it heated to 160 K HDA. The resulting form has a density of 1.26 g/cm3 - very high - density amorphous ice ( VHDA ).
Initially VHDA was not considered as a separate form, to 2001 Loerting among others This suggested.