Phase transitions in ferroelectric 4-aminopyridinium tetrachloroantimonate(III) - revisited.

New X-ray diffraction studies on the crystal structure of ferroelectric [4-NHCHNH][SbCl] 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 = βb*, β varying from 0.60 to 0.66 and monoclinic C2/c(0β0)s0 superspace group. Ferroelectric phase III is commensurate with q = 2\over 3b* 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] 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.