topotactic transition
A transition in which the crystal lattice of the product phase shows one or more crystallographically equivalent, orientational relationships to the crystal lattice of the parent phase. Example: Transitions in which the anionic array is unchanged during the transition but cation reorganization occurs, as in:
PAC, 1994, 66, 577. 'Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)' on page 593 (