Supplementary Information

Figure S1. The powder X-rays diffraction (PXRD) pattern of 1, verifying the purity of the bulk

phase.

Figure S2. The result of the thermal gravity analysis of 1, showing good thermal stability up to 500

K.

Figure S3. DSC (differential scan calorimetry) curves for 1 in the heating and cooling runs,

showing thermal anomalies corresponding to two sequential phase transitions. The heights of

each couple of peaks in the heating and cooling runs is somewhat different. Several heating and

cooling cycles were monitored to exclude the irreversibility (results not shown).

Figure S4. Perspective views of the crystal structures of the HTP (space group, Pbca) at 403 K (a)

and the ITP (space group, Pbca) at 373 K (b) of 1, showing the similarities of the packings and the

orientational changes of the cations due to due to the rotations of the ammonium heads and/or the

pyridinium rings along the C–C single bonds upon the phase transition. Hydrogen atoms were

omitted for clarity. The organic cations are numbered for convenience of comparison (see Figure S5).

(b)

(a)

Figure S5. Comparison of the orientational states of the organic cations in two cells of the HTP and

one cell of the ITP, showing the orientational changes of the cations due to the rotations of the

ammonium heads and/or the pyridinium rings along the C–C single bonds upon the phase transition.

(a, b, c and d) Projections of the selected organic cations (see Figure S4) in the HTP (space group,

Pbca) at 403 K. (e, f, g, and h) Projections of the selected organic cations (see Figure S4) in the ITP

(space group, Pbca) at 373 K. Hydrogen atoms were omitted for clarity.

(h)(g)(f)(e)

(d)(c)(b)(a)

Figure S6. Comparison of the crystal structures between the paraelectric and ferroelectric phases of

compound 1. (a) Project of 1 along the c-axis at 403 K. The read dotted lines indicated the c-glides.

(b) Project of compound 1 along the c-axis at 293 K. Hydrogen atoms were omitted for clarity.

Figure S7. Equatorial plane projection of point group of D2h in the paraelectric phase and C2v in the

ferroelectric phase. Symmetry breaking occurs with an Aizu notation of mmmFmm2.

(b)

(a)

Figure S8. J–V (current density versus external voltage) curve and P–E hysteresis loop of 1

measured by using the double-wave method.

Figure S9. PFM characterization of the ferroelectric thin film of 1. PFM phase (a) and amplitude

images (b) and topographic images (c) for the thin film. (d) Phase and amplitude signals as functions

of the tip voltage for a selected point, showing local PFM hysteresis loops. The film consists of

polycrystalline (2-(ammoniomethyl)pyridinium)SbI5, with microcrystal in the size of several m.

The size and shape of each microcrystal can be seen in the morphology image as shown in (b). Due

to the polycrystalline nature, the polarization direction is different on each microcrystal, which can

be observed on the amplitude of the out-of-plane component in (c). For the PFM spectroscopic study,

we picked several spots on different microcrystal with maximum out-of-plane amplitude to

demonstrate the polarization switching capability.

Figure S10. Demonstration of polarization reversal on thin-film surface of 1. (a) Initial state. (b)

After the switching with positive tip bias of 40 V for 125 ms. Bias was applied at the center of the

selected area (dashed line in the phase and amplitude image). The yellow and purple regions in phase

image indicate the regions with polarization oriented upward and downward respectively.

Figure S11. The photoluminescence emission spectrum of 1. The absorption spectrum is shown for

comparison.

Figure S12. The UV-Vis transmission spectrum for the thin film deposited on a quartz substrate. The

Solid state diffuse reflection spectrum for powdered crystals was depicted together for comparison.

Figure S13. A zoom-in view of the energy band structure of 1 for a clear view of the VB maximum and CB minimum.