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Figure 3
The four basis symmetry modes present in a polar distortion of the perovskite structure along z. The 12-dimensional subspace of all possible polar distortions is spanned by this set of basis modes, plus analogous sets along x and y. Note that, while the first three modes involve a single atom per unit cell, the mode [{\varepsilon}_{4}^{z}] involves two atoms per cell, and therefore the normalization condition implies that the atomic displacements in this fourth basis mode are smaller by a factor of 1/21/2.

Journal logoJOURNAL OF
APPLIED
CRYSTALLOGRAPHY
ISSN: 1600-5767
Volume 47| Part 4| August 2014| Pages 1165-1179
https://doi.org/10.1107/S1600576714010693
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