Parent clamping approximation
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Parent clamping approximation (PCA), or high-symmetry approximation, is a term indicating the suppression of all distortions of the unit cell in a phase transition with group-subgroup relation.
Let G be the space group of the parent (higher-symmetry) phase, and H the space group of daughter (low-symmetry) phase. The group-subgroup relation is fulfilled if the translation subgroup T(H) of H is a (proper or trivial) subgroup of the translation subgroup T(G) of G, i.e. if the lengths of the basis vectors of H are commensurate with those of the basis vectors of G. A phase transition occurring under a change of external conditions (temperature, pressure, applied field etc.) is typically accompanied by a contraction or expansion of the unit cell, which makes the above assumption no longer valid. The parent clamping approximation is adopted precisely to assure the validity of translational symmetry descents.
- Section 18.104.22.168 in International Tables for Crystallography, Volume D.