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

Abstract

New X-ray diffraction studies on the crystal structure of ferroelectric [4-NH2C5H4NH][SbCl4] 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 vectorq= βb*, β varying from 0.60 to 0.66 and monoclinicC2/c(0β0)s0 superspace group. Ferroelectric phase III is commensurate withq= 2\over 3b*andCc(0β0)0 symmetry. Polar phase IV is incommensurately modulated with β varying from 0.66 to 0.70 andCc(0β0)0 superspace group. In all phases only first-order satellites are observed along theb*direction. Two types of periodic deformation are present in the structure of modulated phases. The 4-aminopyridinium cations are subjected to occupation modulation whereas [SbCl4]−nchains 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.