polymorphic transition

A reversible transition of a solid crystalline phase at a certain temperature and pressure (the inversion point ) to another phase of the same chemical composition with a different crystal structure.
Note:
In a liquid-crystal transition, this term refers to phase changes in the smectic state or columnar discotic state.
Examples:
  1. The transitions of SiO2 (quartz-type) at 1143 K to SiO2 (tridymite-type), and at 1743 K to SiO2 (cristobalite-type).
  2. The transition of β-AgI (wurtzite-type structure) to α-AgI (body-centred-cubic structure) at 418 K. Synonymous with enantiotropic transition.
Source:
PAC, 1994, 66, 577 (Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)) on page 588