# Phase transition

### From Online Dictionary of Crystallography

Transition de phase (*Fr*); Phasenübergang (*Ge*); Cambio de estado ( *Sp*); Transizione di fase (*It*); Фазовый переход (*Ru*);相転移 (*Ja*).

A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another one by heat transfer. The term is most commonly used to describe transitions between solid, liquid and gaseous states of matter, and, in rare cases, plasma.

Phase transitions of crystallographic interest are those involving solid phases: with the change of the external conditions (temperature, pressure, applied field) the crystal structure undergoes a change that, when it is not accompanied by a change in the chemistry, relates polymorphs.

Phase transitions are classified on the basis of different criteria.

- Mechanistic classification (Buerger):
*reconstructive*(with changes in the pattern of chemical bonds),*displacive*(characterised by only small atomic shifts),*order-disorder*(of the atomic distribution on given Wyckoff positions). - Thermodynamic classification (Ehrenfest; Tisza):
*first-order*(exhibit a discontinuity in the first derivative of the free energy with respect to some thermodynamic variable),*second-order*(continuous in the first derivative but exhibit discontinuity in a second derivative of the free energy),*lambda*(the heat capacity shows a kink at the transition temperature)

While reconstructive phase transitions are always first order, displacive phase transitions can be both first and second order.