# Transition temperature

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In [crystallography](/source/crystallography), the '''transition temperature''' is the [temperature](/source/temperature) at which a material changes from one [crystal](/source/crystal) state ([allotrope](/source/Allotropy)) to another.<ref>{{Cite book |last=Daintith |first=John |title=A Dictionary of Chemistry |publisher=[Oxford University Press](/source/Oxford_University_Press) |year=2008 |isbn=9780199204632 |edition=6th |language=en |chapter=allotropy |url=https://www.oxfordreference.com/display/10.1093/oi/authority.20110803095404490 }}</ref> More formally, it is the temperature at which two [crystalline forms](/source/Crystal_system) of a substance can co-exist in [equilibrium](/source/Chemical_equilibrium). For example, when [rhombic](/source/Orthorhombic_crystal_system) sulfur is heated above 95.6&nbsp;°C, it changes form into [monoclinic](/source/Monoclinic_crystal_system) sulfur; when cooled 
below 95.6&nbsp;°C, it reverts to rhombic sulfur. At 95.6&nbsp;°C the two forms can co-exist. Another example is [tin](/source/tin), which transitions from a [cubic](/source/Cubic_crystal_system) crystal below 13.2&nbsp;°C to a [tetragonal](/source/Tetragonal_crystal_system) crystal above that temperature.<ref>{{Citation |title=Chemical and Physical Information |date=August 2005 |work=Toxicological Profile for Tin and Tin Compounds |url=https://www.ncbi.nlm.nih.gov/books/NBK599937/ |access-date=2026-03-23 |publisher=Agency for Toxic Substances and Disease Registry (US) |language=en}}</ref>

In the case of [ferroelectric](/source/Ferroelectricity) or [ferromagnetic](/source/Ferromagnetism) crystals, a transition temperature may be known as the [Curie temperature](/source/Curie_temperature).

<!--Commented out information that seems inconsistent with the primary definition:

Transition temperature is the temperature at which a [solid](/source/solid) changes [state](/source/State_of_matter), either becoming softer when [heated](/source/Heat) or more brittle when cooled.

Transition temperature is the temperature above and below which [solubility](/source/solubility) changes in a noticeably different way. On a graph the transition temperature is deduced by the intersection of two lines each of which have a different gradient.
-->

== See also ==
* [Crystal system](/source/Crystal_system)

Category:Crystallography
Category:Threshold temperatures

== References ==
{{Reflist}}{{crystallography-stub}}
{{thermodynamics-stub}}

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