Editing Phase transition (section)

==Types of phase transition==

===States of matter===
[[File:Phase diagram of water simplified.svg|thumb|250px|A simplified phase diagram for [[water]], showing whether solid ice, liquid water, or gaseous water vapor is the most stable at different combinations of temperature and pressure.]]
{{See also|vapor pressure|phase diagram}}

Phase transitions commonly refer to when a substance transforms between one of the four [[state of matter|states of matter]] to another. At the phase transition point for a substance, for instance the [[boiling point]], the two phases involved - liquid and [[vapor]], have identical free energies and therefore are equally likely to exist. Below the boiling point, the liquid is the more stable state of the two, whereas above the boiling point the gaseous form is the more stable. 

Common transitions between the solid, liquid, and gaseous phases of a single component, due to the effects of temperature and/or [[pressure]] are identified in the following table:
{{Table of Phase Transitions}}

For a single component, the most stable phase at different temperatures and pressures can be shown on a [[phase diagram]]. Such a diagram usually depicts states in equilibrium. A phase transition usually occurs when the pressure or temperature changes and the system crosses from one region to another, like water turning from liquid to solid as soon as the temperature drops below the [[freezing point]]. In exception to the usual case, it is sometimes possible to change the state of a system [[Adiabatic|diabatic]]ally (as opposed to [[adiabatic invariant|adiabatic]]ally) in such a way that it can be brought past a phase transition point without undergoing a phase transition. The resulting state is [[metastable]], i.e., less stable than the phase to which the transition would have occurred, but not unstable either. This occurs in [[superheating]] and [[supercooling]], for example. Metastable states do not appear on usual phase diagrams.

===Structural===
{{See also|Polymorphism (materials science)}}
[[File:Pure iron phase diagram (EN).svg|thumb|250px|A phase diagram showing the [[allotropes of iron]], distinguishing between several different [[crystal structure]]s including [[Allotropes of iron|ferrite]] (α-iron) and [[austenite]] (γ-iron).]]

Phase transitions can also occur when a solid changes to a different structure without changing its chemical makeup. In elements, this is known as [[allotropy]], whereas in compounds it is known as [[polymorphism (materials science)|polymorphism]]. The change from one [[crystal structure]] to another, from a crystalline solid to an [[amorphous solid]], or from one amorphous structure to another ([[polyamorphism|{{not a typo|polyamorphs}}]]) are all examples of solid to solid phase transitions.

The [[martensitic transformation]] occurs as one of the many phase transformations in carbon steel and stands as a model for [[displacive phase transformations]]. Order-disorder transitions such as in alpha-[[titanium aluminide]]s. As with states of matter, there is also a [[metastable]] to equilibrium phase transformation for structural phase transitions. A metastable polymorph which forms rapidly due to lower surface energy will transform to an equilibrium phase given sufficient thermal input to overcome an energetic barrier.

===Magnetic===
{{See also|Magnetic structure}}
[[File:MnSi magnetic phase diagram.png|class=skin-invert-image|thumb|250px|A phase diagram showing different [[magnetic structure]]s in the same crystal structure of [[Manganese monosilicide]].]]

Phase transitions can also describe the change between different kinds of [[magnetic ordering]]. The most well-known is the transition between the [[ferromagnetism|ferromagnetic]] and [[paramagnetism|paramagnetic]] phases of [[magnet]]ic materials, which occurs at what is called the [[Curie point]]. Another example is the transition between differently ordered, [[Commensurability (mathematics)|commensurate]] or incommensurate, magnetic structures, such as in cerium [[antimonide]]. A simplified but highly useful model of magnetic phase transitions is provided by the [[Ising Model|Ising model]].

===Mixtures===
[[File:Binary phase diagram of NiTI (phase and temperature).JPG|class=skin-invert-image|thumb|250px|A binary phase diagram showing the most stable chemical compounds of [[titanium]] and [[nickel]] at different [[mixing ratio]]s and [[temperature]]s.]]

Phase transitions involving [[Solution (chemistry)|solution]]s and [[mixture]]s are more complicated than transitions involving a single compound. While chemically pure compounds exhibit a single temperature [[melting point]] between solid and liquid phases, mixtures can either have a single melting point, known as [[congruent melting]], or they have different [[liquidus and solidus|liquidus and solidus temperatures]] resulting in a temperature span where solid and liquid coexist in equilibrium. This is often the case in [[solid solution]]s, where the two components are isostructural.

There are also a number of phase transitions involving three phases: a [[eutectic]] transformation, in which a two-component single-phase liquid is cooled and transforms into two solid phases. The same process, but beginning with a solid instead of a liquid is called a [[eutectoid]] transformation. A [[peritectic]] transformation, in which a two-component single-phase solid is heated and transforms into a solid phase and a liquid phase. A [[peritectoid]] reaction is a peritectoid reaction, except involving only solid phases. A [[monotectic]] reaction consists of change from a liquid and to a combination of a solid and a second liquid, where the two liquids display a [[miscibility gap]].<ref>{{cite book | last1=Askeland | first1=Donald R. | last2=Haddleton | first2=Frank | last3=Green | first3=Phil | last4=Robertson | first4=Howard | title=The Science and Engineering of Materials | date=1996 | isbn=978-0-412-53910-7 | page=286| publisher=Chapman & Hall }}</ref>

Separation into multiple phases can occur via [[spinodal decomposition]], in which a single phase is cooled and separates into two different compositions.

Non-equilibrium mixtures can occur, such as in [[supersaturation]].

===Other examples===
[[File:Argon ice 1.jpg|thumb|A small piece of rapidly melting solid [[argon]] shows two concurrent phase changes. The transition from solid to liquid, and gas to liquid (shown by the white condensed water vapour).]]
Other phase changes include:

* Transition to a [[mesophase]] between solid and liquid, such as one of the "[[liquid crystal]]" phases.
* The dependence of the [[adsorption]] geometry on coverage and temperature, such as for [[hydrogen]] on iron (110).
* The emergence of [[superconductivity]] in certain metals and ceramics when cooled below a critical temperature.
* The emergence of [[metamaterial]] properties in artificial photonic media as their parameters are varied.<ref>{{cite journal|author=Rybin, M.V.|title= Phase diagram for the transition from photonic crystals to dielectric metamaterials|journal=Nature Communications|page=10102 |year=2015|doi=10.1038/ncomms10102|pmid= 26626302 |display-authors=etal|volume=6|pmc=4686770|arxiv= 1507.08901|bibcode= 2015NatCo...610102R}}</ref><ref>Eds. Zhou, W., and Fan. S., [https://www.sciencedirect.com/bookseries/semiconductors-and-semimetals/vol/100/suppl/C Semiconductors and Semimetals. Vol 100. Photonic Crystal Metasurface Optoelectronics''], Elsevier, 2019</ref>
* Quantum condensation of [[boson]]ic fluids ([[Bose–Einstein condensate|Bose–Einstein condensation]]). The [[superfluidity|superfluid]] transition in liquid [[helium]] is an example of this.
* The [[Symmetry breaking|breaking of symmetries]] in the laws of physics during the early history of the universe as its temperature cooled.
* [[Isotope fractionation]] occurs during a phase transition, the ratio of light to heavy isotopes in the involved molecules changes. When [[water vapor]] condenses (an [[equilibrium fractionation]]), the heavier water isotopes (<sup>18</sup>O and <sup>2</sup>H) become enriched in the liquid phase while the lighter isotopes (<sup>16</sup>O and <sup>1</sup>H) tend toward the vapor phase.<ref>{{cite web
 | year=2004
 | author= Carol Kendall|author-link=Carol Kendall (scientist)
 | title= Fundamentals of Stable Isotope Geochemistry
 | url= http://wwwrcamnl.wr.usgs.gov/isoig/res/funda.html
 | publisher= USGS
 | access-date= 10 April 2014
}}</ref>

Phase transitions occur when the [[thermodynamic free energy]] of a system is [[analytic function|non-analytic]] for some choice of thermodynamic variables (cf. [[phase (matter)|phases]]). This condition generally stems from the interactions of a large number of particles in a system, and does not appear in systems that are small. Phase transitions can occur for non-thermodynamic systems, where temperature is not a parameter. Examples include: [[quantum phase transition]]s, dynamic phase transitions, and topological (structural) phase transitions. In these types of systems other parameters take the place of temperature. For instance, connection probability replaces temperature for percolating networks.
{{Condensed matter physics|expanded=States of matter}}