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In crystallography, the '''transition temperature''' is the temperature at which a material changes from one crystal state (allotrope) to another.<ref>{{Cite book |last=Daintith |first=John |title=A Dictionary of Chemistry |publisher=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 of a substance can co-exist in equilibrium. For example, when rhombic sulfur is heated above 95.6 °C, it changes form into monoclinic sulfur; when cooled below 95.6 °C, it reverts to rhombic sulfur. At 95.6 °C the two forms can co-exist. Another example is tin, which transitions from a cubic crystal below 13.2 °C to a tetragonal 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 or ferromagnetic crystals, a transition temperature may be known as the Curie temperature.
<!--Commented out information that seems inconsistent with the primary definition:
Transition temperature is the temperature at which a solid changes state, either becoming softer when heated or more brittle when cooled.
Transition temperature is the temperature above and below which 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
Category:Crystallography Category:Threshold temperatures
== References == {{Reflist}}{{crystallography-stub}} {{thermodynamics-stub}}