Anomalously rotary polarization discovered in homochiral organic ferroelectrics

Molecular ferroelectrics are currently an active research topic in the field of ferroelectric materials. As complements or alternatives of conventional inorganic ferroelectrics, they have been designed to realize various novel properties, ranging from multiferroicity and semiconductive ferroelectricity to ferroelectric photovoltaics and ferroelectric luminescence. The stabilizing of ferroelectricity in various systems is owing to the flexible tailorability of the organic components. Here we describe the construction of optically active molecular ferroelectrics by introducing homochiral molecules as polar groups. We find that the ferroelectricity in (R)-(−)-3-hydroxlyquinuclidinium halides is due to the alignment of the homochiral molecules. We observe that both the specific optical rotation and rotatory direction change upon paraelectric-ferroelectric phase transitions, due to the existence of two origins from the molecular chirality and spatial arrangement, whose contributions vary upon the transitions. The optical rotation switching effect may find applications in electro-optical elements.

(1)circular dichroism spectroscopy (CD) is suggested to be perfomed to the compound even the optical rotation spectrum has been characterized.
(2)Piezoelectric effects is suggested to be characteized for the titled compound.
The manuscript was considerably improved. As I requested, the authors reanalyzed the X-ray diffraction data, and obtained the results that exactly I suggested (highly symmetric plastic phase for the high temperature phase). Now the manuscript is readable, and may be subjected to the reviewing. I think that the following points are still to be issued for the further revisions (sentences in the submitted manuscript is in italic).
The authors write the manuscript as if they are the first who noticed the plastic phase transition in the quinuclidinium derivatives that shows ferroelectric-paraelectric phase transition. However, very recently, there is an important precedent. The work is actually cited as reference 27 (J. Harada, et al. Nature Chemistry, 8, 946-952 (2016)), but the authors should mention that their work is the first report that shows the change of molecular reorientational motion in the crystalline phase induces ferroelectric-paraelectric transition. For example, the sentences in p.6, ″<i>The HT single crystal X-ray diffraction shows few peaks with relatively weak intensity especially for those in the relatively high-angle region. These diffraction characteristics remind us the plastic phase, which is characterized by high symmetry (often cubic), and by average structure Bragg reflections, both few in number and weak in intensity, accompanied by a large amount of diffuse scattering due to the severe orientational and/or displacive disorder. The high-temperature structure was……</i>″ should be re-written such as, ″The high temperature single crystal X-ray diffraction shows few peaks with relatively weak intensity especially for those in the relatively high-angle region. We noticed that this situation is very similar to that recently observed for the quinuclidinium salt that undergoes ferroelectric transition [Ref. 27]. In this case, the paraelectric phase (high temperature phase) becomes a cubic plastic phase. At the transition point, isotropic cubic phase changes to a polar rhombohedral phase. In our case, the hightemperature structure was……″ The authors used ″HTP″ and ″LTP″ without any definition.
If the high temperature phase is a cubic plastic crystal, one can align the polarization axes by applying an electric field, as demonstrated in ref. 27. The authors have tried such poling experiments? This could give a larger remnant polarization.
In p.7, the authors should add a reference for the sentence of ″<i>at the level for those of molecular plastic conductors</i>″.
Since the low temperature phase is SHG active, I do not think that the most of the sentences in p.8 are not necessary. Please remove the following parts. ″<i>SHG is described by the third rank polar property tensor χ(2), analogous to the piezoelectric coefficient tensor, which vanishes in the 11 centrosymmetric point groups and the noncentrosymmetric point group 432. This is due to the restriction by point group symmetry, since all components of the χ(2) tensor are zero in these SHG-inactive point groups. For the 20 piezoelectric point groups, however, only 18 of them can allow the appearance of SHG response under the restriction of Kleinman's symmetry. The matrices of other two point groups 422 and 622 are zero. The two point groups require two nonvanishing components of χ(2) to follow the equation = −. Furthermore, to satisfy the requirement of all symmetry transformations, must be equal to . These lead to and equal to zero. This means that only the other 18 point groups are SHG-active. To derive symmetry information of the HTP, we measured the variable-temperature SHG response.</i>″, and ″<i>For point group 4, there are four nonvanishing second-order susceptibility tensors (, , and) under the restriction of Kleinman's symmetry,33 only two are independent (, ). The matrix is given as .</i>″(whole sentence with the matrix) Please move the following sentences in p. 19 to the section of ″methods″. ″<i>The first-principles calculations were performed within the framework of density functional theory (DFT) implemented in the Vienna ab initio Simulation Package (VASP). 45, 46 The energy cut-off for the expansion of the wave functions was fixed to 550 eV and the exchange−correlation interactions were treated within the generalized gradient approximation of the Perdew−Burke− Ernzerhof type. 47 For the integrations over the k-space we used a 4x4x1 k-point mesh. The experimental room temperature crystal structure was used as the ground state for evaluating the ferroelectric polarization. In order to evaluate the ferroelectric polarization, we consider the path connecting the non-polar to the polar structure by linearly interpolating the atomic positions</i>.″ The manuscript may be acceptable for publication, if the above points are properly revised.
Reviewer #2 (Remarks to the Author): The raised points have been well solved. The work is very important and the results are convincing. Thus, I'd like to recommend publication of the work in Nature Communications.

Response
(Our responses are marked in blue, those from the main text in purple) Reviewers' comments: Reviewer #1 (Remarks to the Author): This manuscript describes the optical rotation switching in homochiral organic ferroelectrics. The main point of this manuscript may be the finding of the optical activity change at the ferroelectric transition. The authors presented the structural, optical, and dielectric data, but I think that there are several serious faults and insufficiency in their interpretations.
1. The powder X-ray diffraction of HTP shows sharp peaks even in the relatively high-angle region.
Smaller number of peaks observed means that the symmetry of the unit cell becomes very high. What I could not really understand is why the authors did not attempt to index the powder pattern. For such a simple pattern, it is rather easy to index the peaks. I think that the authors should index the pattern and determine the lattice parameters (crystal system, cell volume and the Z value) based on the powder diffraction of HTP. This information is extremely important for further discussion.
Reply: We really appreciate the reviewer for question 1 and 2 which reminds us the plastic phase of the high temperature phase (HTP). In the first version, we just focus on whether the HTP is a paraelectric phase or not, and thus ignore more structural information which can be deduced from the X-ray diffraction. In this revision, we not only indexed the PXRD, but also collected the high-quality high-temperature single-crystal diffraction data and thus determined the high-temperature structure.
The results from the powder and single crystal are consistent. By combination the basic ferroelectric rule (Aizu rule), we find the determined space group is just the only permissible one (see the following text marked in purple from the main text). With the high temperature structure, all the properties and details can be well explained, including the ferroelectric mechanism, including the symmetry breaking, the SHG and pioezoelectric effect transition, optical rotation.
The added the related results and discussion are as following: To probe the high-temperature structural information, both the variable-temperature single crystal and powder X-ray diffractions were measured ( Figure S3, Supplementary Information). Patterns of the PXRD (powder X-ray diffractions) below T c match well with the simulated one from the single crystal structure, revealing the high crystallinity and purity of the phase ( Figure S4, Supplementary   Information). Many diffractions peaks observed at below T c disappear upon heating above T c . The smaller number of peaks observed means that the symmetry of the HTP becomes very high. The PXRD data at 363 K were refined with the Pawley method ( Figure S5, Supplementary Information), a cubic unit cell with a = 9.4666 Å was suggested, and the most possible space group is the F432.
The HT single crystal X-ray diffraction shows few peaks with relatively weak intensity especially for those in the relatively high-angle region. These diffraction characteristics remind us the plastic phase, which is characterized by high symmetry (often cubic), and by average structure Bragg reflections, both few in number and weak in intensity, accompanied by a large amount of diffuse scattering due to the severe orientational and/or displacive disorder. 31, 32 The high-temperature structure was refined in the space group F432, a = 9.5084 (18)   In general, it's not easy to determine the space group of a plastic phase reliably because the above-mentioned X-ray diffraction characteristics. In this case, the space group determined from the X-ray diffraction is just the one and the only one satisfying symmetry requirement for the ferroelectric phase transition. The ferroelectric phase has the point group 4. According to Aizu rule (see Table I  contains the 4 2 fourfold screw axes. The 4 2 fourfold screw axis of the space group F432 becomes the 4 1 fourfold screw axis of the space group 4 1 upon the transition from the paraelectric to ferroelectric phase, which determines not only the c LTP , but also the a LTP and b LTP as well as the origin which is located on the 4 1 fourfold screw axis. That's why the relationship of the two temperature cells is a HTP ≈ a LTP + b LTP , b HTP ≈ -a LTP +b LTP , c HTP ≈ 0.5c LTP (Figure 1 a). Therefore, the crystal of 1 is of the 432F4 ferroelectric species among the 88 ferroelectric species.
2. If HTP has a high-symmetry lattice and contains "severe disorder", the molecules are considered to undergo a high degree of dynamic reorientation (rotation). This suggests that the HTP is a plastic phase in which the molecules are nearly freely rotating. For such a case, electron density becomes featureless (spherically distributed electrons at the lattice sites), and the molecular framework and chirality may be vanished. Enantiomorphic point groups may be applicable only when the chirality in the lattice is warranted (This is the case when the site symmetry derived from the point 1 is consistent with the molecular chirality. In this case, electron density can be reproduced by superimposing multiple orientations of the molecular framework (restricted reorientation)). Because the authors should discuss the structure of HTP taking the results obtained from the point 1 into account, the present point group assignment for HTP being persistent in non-centrosymmetric may be fundamentally wrong. Reply: We agree the reviewer with the viewpoint on the relationship between crystal symmetry and the molecular chirality. Therefore, we avoid to deduce the high-temperature symmetry from the molecular chirality in the manuscript, but from the X-ray diffraction as suggested by the reviewer. Accordingly, we delete Table 1 and related discussion. As replied in question 1, the high-temperature space group can be exclusively determined from the X-ray diffraction.
3. Though the authors mention "the SHG signal undergoes a clear transition from non-zero to zero intensity" in p. 7, the SHG intensity in HTP shown in Figure 2b is not zero. This made the reviewer in confusion. If the SHG intensity has a finite value (higher than the noise (background) level), completely different discussion has to be given. Reply: In this revision, we calibrated our instrument, and re-measured the variable-temperature SHG. The result is shown in Figure R1. Since our structure analysis definitely reveals the 432F4 phase transiTIon, we think the result is reasonable. The result is included in the revised manuscript. Figure R1. The result of variable-temperature SHG. 4. Optical activity change at the structural transition with substantial symmetry change is rather natural. The authors' explanation is not a fault, but I do not think that this observation is significantly unique. Reply: We agree that the symmetry requirements for optical activity transitions have been well known, and the phenomena have been observed in many cases including ferroelectric crystals. However, as far as we known, the past investigations on optical activity change were focused mainly on the optical activity crystals without chiral molecules. Currently, the design of molecular ferroelectrics to realize novel functions have attracted great interest. Optical activity of the recent emerging molecular ferroelectrics hasn't been reported before, but it is really important for understanding the ferroelectricity. For example, we observed in this case that the optical axis can be changed because the ferroelectric crystal is multiaxial. The integration of optical activity into molecular ferroelectrics would add the degree of freedom for device design. In addition, as reminded by the reviewer that both the ferroelectricity and the optical activity change are related to the plastic phase transition, we are aware that it is interesting that in the optically active point group (432), the optically active molecules act as in the solution.
In conclusion, though the data of optical activity and dielectric properties seem to be acceptable, I think that the total material characterization is insufficient and unacceptable. In addition, the main point of the subject of this manuscript may not have a sufficient scientific impact. Therefore, I do not recommend publication of this manuscript in Nature Communications. Reply: As answered in question 1, the high-temperature structure was determined, all the observed phenomena/properties can be well accounted for. All ferroelectric properties are well done and as we are aware, this is very unique homochiral ferroelectric example never found before. Extraordinarily this phase transition can be attributed to plastic phase transition with the help of the excellent reviewer and is unprecedented. So we do believe that this work can be published in the esteemed journal Nature Communication.

Reviewer #2 (Remarks to the Author):
The authors in this work report a very interesting homochiral organic ferroelectrics with anomalously rotary polarization. The compound exhibits both ferroelectric properties and notable optical rotation during the paraelectric-ferroelectric phase transitions. Thus, present work will be great helpful to rationally design new ferroelectric with interesting optical activities. I'd like to recommend publication of the work in Nature Communications after the following minor revisions.
(1) circular dichroism spectroscopy (CD) is suggested to be perfomed to the compound even the optical rotation spectrum has been characterized. Reply: As suggested, we first measure the Uv-vis in the wavelength range of 200 to 800 nm. No absorption peak was observed. This is reasonable since the molecule contains saturate single bonds (sp 3 , no conjugation system or double bond). Therefore, CD spectrum will show no absorption peaks. Therefore we can not measure the CD spectrum in commercial CD instrument.
(2) Piezoelectric effects is suggested to be characteized for the titled compound. Reply: As suggested, we measured the piezoelectric effects. The results and discussion are added as following: The piezoelectric effect is the similar physical property which depend on the crystal symmetry. We thus also measured piezoelectric effect in the temperature range from room temperature to 373 K. As expected, the d 33 undergoes a transition from the non-zero to zero at around T c ( Figure S4   The manuscript was considerably improved. As I requested, the authors reanalyzed the X-ray diffraction data, and obtained the results that exactly I suggested (highly symmetric plastic phase for the high temperature phase). Now the manuscript is readable, and may be subjected to the reviewing. I think that the following points are still to be issued for the further revisions (sentences in the submitted manuscript is in italic).
Reply: We thank the reviewer for the positive assessment of our revision. We also appreciate the reviewer for the valuable suggestions on the technological writing.
These suggestions, ranging from the data analysis to the technological writing, can help us improve the quality of the paper indeed. We think we benefit greatly by these sugestions.
The authors write the manuscript as if they are the first who noticed the plastic phase transition in the quinuclidinium derivatives that shows ferroelectricparaelectric phase transition. However, very recently, there is an important precedent. The work is actually cited as reference 27 (J. Harada, et al. Nature Chemistry, 8, 946-952 (2016)), but the authors should mention that their work is the first report that shows the change of molecular reorientational motion in the crystalline phase induces ferroelectric-paraelectric transition. For example, the sentences in p.6, ″The HT single crystal X-ray diffraction shows few peaks with relatively weak intensity especially for those in the relatively high-angle region. These diffraction characteristics remind us the plastic phase, which is characterized by high symmetry (often cubic), and by average structure Bragg reflections, both few in number and weak in intensity, accompanied by a large amount of diffuse scattering due to the severe orientational and/or displacive disorder. The high-temperature structure was……″ should be re-written such as, ″ The high temperature single crystal X-ray diffraction shows few peaks with relatively weak intensity especially for those in the relatively high-angle region. We noticed that this situation is very similar to that recently observed for the quinuclidinium salt that undergoes ferroelectric transition [Ref. 27]. In this case, the paraelectric phase (high temperature phase) becomes a cubic plastic phase. At the transition point, isotropic cubic phase changes to a polar rhombohedral phase. In our case, the high-temperature structure was……″