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Figure 19
(a) The asymmetric unit of thiourea at 123 K, with mirror planes in the ac plane of the unit cell. Note that individual molecules keep the same mirror symmetry in both structures, but the loss of inversion symmetry from room to low temperature makes two half-molecules inequivalent in the latter. (b) The packing of the P21ma setting of the low-temperature form of thiourea (as given by Mercury). The absence of parallelism between pairs of molecules leaves a net dielectric moment, causing ferroelectricity along the a axis. (c) The packing of the room-temperature form of thiourea along the b axis (as given by Mercury), showing the perfectly antiparallel orientation of pairs of molecules preventing ferroelectricity.

Journal logoJOURNAL OF
APPLIED
CRYSTALLOGRAPHY
ISSN: 1600-5767
Volume 57| Part 5| October 2024| Pages 1618-1639
https://doi.org/10.1107/S1600576724007659
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