3. Phase transitions, critical properties

3.1 General

The Ehrenfest and Landau classifications recognize two types of change of state (or phase transitions).

• First-order (or discontinuous) transitions are accompanied by a sudden change in physical properties, with or without a change in symmetry (solid-liquid or liquid-vapor, for example). They involve phase transition enthalpy (or latent heat). They can end at an isolated critical point in the case of phases with the same symmetry (e.g. liquid-vapor).

• Second-order (or continuous) transitions are always accompanied by a change in symmetry described by an order parameter that is zero in the phase of highest symmetry (generally of highest temperature), non-zero in the least...