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Figure 3
Top-down view of two distorted variants of the K2NiF4 structure showing the differences between [{\rm X}_3^+] and [{\rm X}_4^+] irreps that correspond to the order parameter (0, a). For clarity, the octahedra in the upper layer are depicted as orange octahedra, and those in the lower layer in blue. The [{\rm X}_3^+] irrep leads to ϕ tilts around an axis parallel to [110] that have the same sense in all layers (left). The [{\rm X}_4^+] irrep produces the same ϕ tilts in the upper layer, but the direction of those tilts is reversed in adjacent layers (right).

IUCrJ
Volume 10| Part 4| July 2023| Pages 385-396
ISSN: 2052-2525
https://doi.org/10.1107/S2052252523003743