Enantiotropy

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Enantiotrope (Greek enantíos "opposite", trópos "turn") means convertible into one another and denotes substances with two or more different solid phases that can be reversibly converted into one another by changing the temperature . This means that at a temperature below the transition temperature, one phase is thermodynamically stable, the other is metastable ; above the transition temperature , the other phase is thermodynamically stable. In the case of liquid crystals , the transition between the liquid and the liquid-crystalline phase or between different liquid-crystalline phases is also referred to as the enantiotropic transition.

The phase transition between the modifications of enantiotropic substances does not generally take place immediately after the transition temperature is exceeded or not reached. The conversion takes place extremely slowly at a low temperature difference to the conversion temperature; the conversion can only be observed in practice at a higher temperature difference. The transition temperature can often be shifted by the application of pressure . Well-ordered crystals can be destroyed by converting them into another modification, with crystal twins often forming.

Enantiotropy can occur with polymorphic substances, as well as with allotropic elements. In contrast to enantiotropic conversions, conversions that can only proceed in one direction are called monotropic . Enantiotropy and monotropy of solids are important terms when describing polymorphism in compounds and allotropy in chemical elements.

Examples of enantiotropic substances:

  • Tin - The conversion of metallic β-Sn to α-Sn below 13.2 ° C is known as tin plague .
  • Iron - At room temperature the stable phase is α-iron ( body-centered cubic lattice ), at temperatures above 911 ° C it is γ-iron (face-centered cubic lattice).
  • Quartz - α-quartz is stable up to 573 ° C, above that β-quartz.
  • Caffeine - β-caffeine converts to α-caffeine at a temperature of 141 ° C, which then melts at 236 ° C.
  • Ammonium nitrate - The ammonium nitrate occurs in five modifications (transformation temperatures: −16, +35, +85 and +125 ° C).

Other meanings

In Heraclitus' constantly changing worldview, enantiotropy is the constant counteraction of things.