subgroup-supergroup transition
A transition in which the space-group symmetry of the lower symmetry phase is a subgroup of that of the higher symmetry phase. Example: The transition of the low-temperature polymorph of quartz characterized by space-group symmetry \(P3_{1}2\) (trigonal), to the high temperature polymorph of quartz with space-group symmetry, \(P6_{2}22\) (hexagonal). Synonymous with @[email protected] (note: this term, strictly speaking, is no longer in use).
PAC, 1994, 66, 577. (Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)) on page 591 [Terms] [Paper]