Polyamorphism is the ability of a substance to exist in several different amorphous modifications. It is analogous to the polymorphism
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...

 of crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

line materials. Many amorphous substances can exist with different amorphous characteristics (e.g. polymers). However, polyamorphism requires two distinct amorphous states with a clear (first-order) phase transition between them. As amorphous solids are sometimes associated with liquid state, polyamorphism is also referred to as liquid–liquid phase transition.


Even though amorphous materials exhibit no long-range periodic atomic ordering, different amorphous phases of the same chemical substance can vary in other properties, such as density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

. In several cases sharp transitions have been observed between two different density amorphous states of the same material. Amorphous ice
Amorphous ice
Amorphous ice is an amorphous solid form of water, meaning it consists of water molecules that are randomly arranged like the atoms of common glass. Everyday ice is a crystalline material where the atoms are regularly arranged in a lattice whereas amorphous ice is distinguished by a lack of...

 is one important example (see also examples below). Several of these transitions (including water) are expected to end in a second critical point
Critical point (thermodynamics)
In physical chemistry, thermodynamics, chemistry and condensed matter physics, a critical point, also called a critical state, specifies the conditions at which a phase boundary ceases to exist...


Liquid–liquid transitions

Polyamorphism may apply to all amorphous states, i.e. glasses, other amorphous solids, supercooled liquids, ordinary liquids or fluids. A liquid–liquid transition however, is one that occurs only in the liquid state (red line in the phase diagram, top right). In this article liquid–liquid transitions are defined as transitions between two liquids of the same chemical substance. Elsewhere the term liquid–liquid transition may also refer to the more common transitions between liquid mixtures of different chemical composition.

The stable liquid state unlike most glasses and amorphous solids, is a thermodynamically stable equilibrium state. Thus new liquid–liquid or fluid-fluid transitions in the stable liquid (or fluid) states are more easily analysed than transitions in amorphous solids where arguments are complicated by the non-equilibrium, non-ergodic nature of the amorphous state.

Rapoport's theory

Liquid–liquid transitions were originally considered by Rapoport in 1967 in order to explain high pressure melting curve maxima of some liquid metals. Rapoport's theory requires the existence of a melting curve maximum in polyamorphic systems.

Double well potentials

One physical explanation for polyamorphism is the existence of a double well inter-atomic pair potential (see lower right diagram). It is well known that the ordinary liquid–gas critical point appears when the inter-atomic pair potential contains a minimum. At lower energies (temperatures) particles trapped in this minimum condense into the liquid state. At higher temperatures however, these particles can escape the well and the sharp definition between liquid and gas is lost. Molecular modelling
Molecular modelling
Molecular modelling encompasses all theoretical methods and computational techniques used to model or mimic the behaviour of molecules. The techniques are used in the fields of computational chemistry, computational biology and materials science for studying molecular systems ranging from small...

 has shown that addition of a second well produces an additional transition between two different liquids (or fluids) with a second critical point.

Examples of polyamorphism

The most famous case of polyamorphism is amorphous ice
Amorphous ice
Amorphous ice is an amorphous solid form of water, meaning it consists of water molecules that are randomly arranged like the atoms of common glass. Everyday ice is a crystalline material where the atoms are regularly arranged in a lattice whereas amorphous ice is distinguished by a lack of...

. Pressurizing conventional hexagonal ice crystals to about 1.6 GPa at liquid nitrogen
Liquid nitrogen
Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...

 temperature (77 K) converts them to the high-density amorphous ice. Upon releasing the pressure, this phase is stable and has density of 1.17 g/cm3 at 77 K and 1 bar. Consequent warming to 127 K at ambient pressure transforms this phase to a low-density amorphous ice (0.94 g/cm3 at 1 bar). Yet, if the high-density amorphous ice is warmed up to 165 K not at low pressures but keeping the 1.6 GPa compression, and then cooled back to 77 K, then another amorphous ice is produced, which has even higher density of 1.25 g/cm3 at 1 bar. All those amorphous forms have very different vibrational lattice spectra and intermolecular distances.

Yttria-alumina melts are another system reported to exhibit polyamorphism. Observation of a liquid–liquid phase transition in the supercooled liquid has been reported. Though this is disputed in the literature. Polyamorphism has also been reported in Yttria-Alumina glasses. Yttria-Alumina melts quenched from about 1900 °C at a rate ~400 °C/s, can form glasses containing a second co-existing phase. This happens for certain Y/Al ratios (about 20–40 mol% Y2O3). The two phases have the same average composition but different density, molecular structure and hardness. However whether the second phase is glassy or crystalline is also debated.
A first-order glass-liquid phase transition
Phase transition
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....

 has also been reported for silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

, and continuous changes in density were observed upon cooling silicon dioxide
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...

 or germanium dioxide
Germanium dioxide
Germanium dioxide, also called germanium oxide and germania, is an inorganic compound, an oxide of germanium. Its chemical formula is GeO2. Other names include germanic acid, G-15, and ACC10380...

. Although continuous density changes do not constitute a first order transition, they may be indicative of an underlying abrupt transition. Polyamorphism has also been observed in organic compounds, such as liquid triphenyl phosphite
Triphenyl phosphite
Triphenyl phosphite is the chemical compound with the formula P3. This colourless viscous liquid is the ester of phosphorous acid and phenol. It is used as a ligand in organometallic chemistry...

at temperatures about 200 K.

Polyamorphism is also a potentially important area in pharmaceutical science. The amorphous form of a drug typically has much better aqueous solubility (cf. the analagous crystalline form) but the actual local structure in an amorphous pharmaceutical can be different, depending on the method used to form the amorphous phase
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