Unusual Long-Range Ordering Incommensurate Structural Modulations in an Organic Molecular Ferroelectric

The
incommensurate (IC) behaviors of ferroelectrics have been widely investigated
in inorganic oxides as an exciting branch for aperiodic materials,
whereas it still remains a great challenge to achieve such intriguing
effects in organic systems. Here, we present that successive ordering
of dynamic dipoles in an organic molecular ferroelectric, N-isopropylbenzylaminium trichloroacetate (1), enables unusual incommensurately modulated structures between
its paraelectric phase and ferroelectric phase. In particular, 1 exhibits three distinct IC states coupling with a long-range
ordering modulation. That is, the incommensurately modulated lattice
is ∼7 times as large as its periodic prototype, and the IC
structure is well solved using a (3 + 1)D superspace
group with the modulated wavevector q = (0, 0, 0.1589). To the best of our knowledge, 1 is
the first organic ferroelectric showing such a long-range ordering
IC structural modulation. In addition, structural analyses reveal
that slowing down dynamic motions of anionic moieties accounts for
its modulation behaviors, which also results in dramatic reorientation
of dipolar moments and concrete ferroelectric polarization of 1 (∼0.65 μC/cm2). The combination
of unique IC structural modulations and ferroelectricity makes 1 a potential candidate for the assembly of an artificially
modulated lattice, which will allow for a deep understanding of the
underlying chemistry and physics of aperiodic materials.