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Figure 6
M1+ mode (present in the P4/mbm model but not in the Cmce model). The arrows describe a displacive degree of freedom of the Bi sites within the perovskite blocks: note that the Bi displacement alternates `towards' or `away from' the central octahedral site in alternate layers (see text).

IUCrJ
Volume 6| Part 3| May 2019| Pages 438-446
ISSN: 2052-2525
https://doi.org/10.1107/S2052252519003804