Difference between revisions of "Parent clamping approximation"
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Parent clamping approximation (PCA), or highsymmetry approximation, is a term indicating the suppression of all distortions of the unit cell in a phase transition with groupsubgroup relation.
Let G be the space group of the parent (highersymmetry) phase, and H the space group of daughter (lowsymmetry) phase. The groupsubgroup 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 is typically accompanied by a contraction or expansion of the unit cell, which makes the above assumption no longer valid.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.
See also
 Section 3.4.2.5 in International Tables for Crystallography, Volume D.