Phase transitions in ferroelectric 4‐aminopyridinium tetrachloroantimonate(III) – revisited

New X‐ray diffraction studies on the crystal structure of ferroelectric [4‐NH 2 C 5 H 4 NH][SbCl 4 ] indicate that in the broad temperature range from 240 to 304 K covering the three intermediate phases, the crystal structure is modulated. Phase II is incommensurately modulated with modulation vector q = β b* , β varying from 0.60 to 0.66 and monoclinic C 2/ c (0β0) s 0 superspace group. Ferroelectric phase III is commensurate with q = b* and Cc (0β0)0 symmetry. Polar phase IV is incommensurately modulated with β varying from 0.66 to 0.70 and Cc (0β0)0 superspace group. In all phases only first‐order satellites are observed along the b* direction. Two types of periodic deformation are present in the structure of modulated phases. The 4‐aminopyridinium cations are subjected to occupation modulation whereas [SbCl 4 ] − n chains are displacively modulated. The paraelectric–ferroelectric phase transition is an example of the incommensurate–commensurate transition of the lock‐in type. A new mechanism for this transformation is proposed.