Ferroelectricity and Ferroelasticity in Organic Inorganic Hybrid

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Ferroelectricity and Ferroelasticity in Organic Inorganic Hybrid (Pyrrolidinium)3[Sb2Cl9] Martyna Wojciechowska, Anna Gagor, Anna Piecha-Bisiorek, Ryszard Jakubas, Agnieszka Ci#man, Jan K. Zar#ba, Marcin Nyk, Piotr Zielinski, Wojciech Medycki, and Andrzej Bil Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00962 • Publication Date (Web): 22 Jun 2018 Downloaded from http://pubs.acs.org on June 24, 2018

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Chemistry of Materials

Ferroelectricity and Ferroelasticity in Organic Inorganic Hybrid (Pyrrolidinium)3[Sb2Cl9] Martyna Wojciechowska,† Anna Gągor,†† Anna Piecha-Bisiorek,*† Ryszard Jakubas,† Agnieszka Ciżman,‡ Jan K. Zaręba,│ Marcin Nyk,│ Piotr Zieliński,║ Wojciech Medycki, ┴ Andrzej Bil† †

Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland

††

W. Trzebiatowski Institute of Low Temperature and Structure Research PAS, P.O. Box 1410, 50-950 Wrocław, Poland. ‡Division of Experimental Physics, Faculty of Fundamental Problems of Technology, University of Science and Technology, Wybrzeże Wyspiańskiego 27, 50–370 Wrocław, Poland.

│

Advanced Materials Engineering and Modelling Group, Faculty of Chemistry, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland.

║

H. Niewodniczański Institute of Nuclear Physics PAN, Radzikowskiego 152, 31-342 Kraków, Poland.

┴

Institute of Molecular Physics, PAS, M. Smoluchowskiego 17, 60-179 Poznań, Poland.

ABSTRACT: Perovskite-like materials exhibit desirable photophysical and electric properties that make them suitable for a remarkable breadth of applications in electronics and physics. In this contribution, we report on the multi-phase ferroelectric and ferroelastic phenomena in pyrrolidinium-based hybrid metal-organic material, (C4H8NH2)3[Sb2Cl9]. The title compound is the first pyrrolidinium derivative within the halobismuthates(III) and haloantimonates(III) families that is featured by the ferroelectric property. From a structural point of view, the crystal structure is built of [Sb2Cl9]3-∞ perovskite-like layers, interdigitated by layers of pyrrolidinium cations. The rich solid state dynamics of pyrrolidinium cations endowed (C4H8NH2)3[Sb2Cl9] with a complex sequence of temperature-dependent phase transitions. Remarkably, polar properties have been found to occur in all the six phases, including room-temperature phase I. Insights from variable temperature single-crystal X-ray diffraction, dielectric spectroscopy, and T1 spin-lattice relaxation measurements revealed the general mechanism of most phase transitions, as related to the progressive ordering of nonequivalent pyrrolidinium cations. Non-centrosymmetry is probed by room-temperature second harmonic generation (SHG), while the ferroelectric property was evidenced through P(E) and dielectric measurements. The experimental values of spontaneous polarization were justified and analyzed in the context of theoretical values derived from quantum-chemical calculations. Optical measurements show that the integrity of the sample survives all the phase transitions in spite of, sometimes significant, deformations of the unit cell. The changes of symmetry associated with structural phase transitions are accompanied by an intriguing evolution of the ferroelastic domain structure with temperature.

sharing MX6 octahedra enclosing 12-coordinate voids is INTRODUCTION responsible for their excellent physical properties and, The current global scientific trend of the search for thus, has received a tremendous interest from the viewnew, environmentally friendly smart materials for photoepoints of fundamental science and applications.10,11 lectronics and photovoltaics among hybrid, perovskiteIt is the rich diversity of the anionic substructures like materials is a vast discipline which have promoted a (from zero-, through one-, two-, or even threerapid progress over the last decade.1-5 The focus on perovdimensional architecture) and isoelectronicity with Sn(II) skites, described by the general formula AMX3 (where A = and Pb(II) that are mainly responsible for the huge interorganic cation, M = metal and X = Cl, Br, I) has evolved est of the Sb(III) or Bi(III) halides group.12,13 Haloantimoover time, to encompass a multitude of aspects of syn6-8 nates(III) and halobismuthates(III) of the general formula thetic and physical chemistry. Despite the presence of RaMbX3b+a, (where R denotes organic cations, M stands for toxic metals, the greatest commercial development in this Sb(III) or Bi(III) and X = Cl, Br, I) have garnered wide area is reserved mainly to inorganic ceramics such as interest of scientific community as they combine many barium titanate (BTO) or lead zirconate titanate (PZT).9 desirable features, e.g. facile synthesis and processing, Their unique three-dimensional structure of cornerACS Paragon Plus Environment

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biocompatibility, cost-effectiveness with the desired electrical and optical properties.14,15 There is a huge diversity of chemical compositions among halontimonates(III) and halobismuthates(III). Nevertheless, ferroelectricity is limited to few of them only, e.g. RMX4, R3M2X9, R2MX5 and R5M2X11.16 Many reports have been devoted to the crystals with the R3M2X9 composition, which are characterized by three terminal halogen atoms and three bridging ones. The [MX6] octahedra can form either infinite one-dimensional (1D) double chains17, two-dimensional (2D) layers18, discrete bi-octahedral (0D)19 or fouroctahedral (0D) units [M4X18]6-20. In spite of a wide variety of the anionic forms the ferroelectric properties were found to appear only in salts characterized by 0D and 2D anionic layers. Organic-inorganic hybrids based on five-membered non-aromatic heterocycles such as pyrrolidine and substituted analog – N-methylpyrrolidine, exhibiting ferroelectric properties are quite rare. Up till now only several of such examples are known; (pyrrolidinium)[MnCl3] (Tc = 295 K) and (pyrrolidinium)[MnBr3] (Tc = 219 K).21,22 As it was stated, both compounds exhibit intense red luminescence under a UV excitation as well as ferroelectric properties. Moreover, in the case of bromide analog a weak ferromagnetism is also observed. In the haloantimonates(III) and halobismuthates(III) family only two compounds can be treated as a representatives of lead-free perovskite halides (N-methylpyrrolidinium)3[Sb2Cl9] (Tc = 322 K) and (N-methylpyrrolidinium)3[Sb2Br9] (Tc = 323 K). Those compounds were found to show large ferroelectric polarizations (5.2 (Cl), 7.6 (Br) μC·cm-2, respectively) and pronounced semiconducting performances.23,24 Encouraged by these observations, we have embarked upon a study to exploit the self-assembly of pyrrolidinium cations with haloantimonate(III) complex anions. As a result, we demonstrate a new lead-free hybrid ferroelectric (pyrrolidinium)3[Sb2Cl9] (hereafter referred to as PCA) which is characterized by 2D corrugated layers, in contrast with the N-methylpyrrolidinium analogs containing 0D anionic substructure. The calorimetric and dielectric spectroscopy revealed a rich polymorphism in the solid state, while the observation under polarizing microscope disclosed unique evolution of the domain pattern in PTs. Crystal noncentrosymmetry was proved by the detection of the SHG effect at room temperature, whereas ferroelectric properties were confirmed by the observations of the ferroelectric hysteresis loops. These results are compared with those observed for similar ferroelectric from R3M2X9 – a subclass of haloantimonates(III) and halobismuthates(III) family.

EXPERIMENTAL SECTION Synthesis. The crystals of PCA were synthesized by the slow evaporation of stochiometric amounts of pyrrolidine(Sigma Aldrich, ≥99.0%) and antimony(III) oxide (Sigma Aldrich, 99.99%) in a concentrated 35% hydrochloric acid solution. The single transparent, platelet-like crystals were characterized by elemental analysis: C: 18.47

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(theor. 18.50), N: 5.35 (theor. 5.39), H 3.79 (theor. 3.88) while their phase purity was verified by powder X-ray diffraction, as shown in Figure S1 in the Supplementary Information (SI). Single-crystal XRD data were collected on XcaliburAtlas diffractometer, operating in κ-geometry, equipped with a two-dimensional CCD detector and Mo Kα radiation (0.71073 Å) source. Data were measured in a ω-scan mode with ∆ω=1.0o. The CrysAlis PRO software was used for data collection and processing.25 An open flow nitrogen cryosystem (Oxford Cryosystem) was used for experiments at low temperatures. The structures were solved by direct methods in three structural phases at 255, 225 and 208 K and refined using full-matrix least-squares methods with SHELXL2014/7 program.26 Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm was applied on all data. The restraints on C-C and C-N distances (DFIX) were used to fix disordered pyrrolidinium cations to be chemically reasonable in all the phases. In the heavily disordered Phase I the C and N atoms were refined with isotropic displacement parameters whereas SIMU and DELU restraints was used in the refinement of anisotropic displacement parameters of C and N in Phase II and IV. These phases were refined as inversion twins, additionally, the Phase IV was non-merohedrally twinned and refinement was performed for the dominating twin domain (98 percent of separated peaks). Hydrogen atoms were introduced in calculated positions and refined as riding atoms. The thermal parameters of hydrogen atoms were set to be equal to 1.2 times the thermal parameters of the corresponding parent atoms. Table 1 presents the results of the data collection and refinement together with the crystal description. Table S1 shows the selected interatomic distances. Powder X-ray diffraction measurements were performed with the use of X’Pert PRO Xray diffraction system equipped with PIXcel ultra-fast line detector, divergence slits and Soller slits for Cu Kα radiation. The measurements were done in a reflection mode in the Bragg-Brentano geometry. Thermal properties The differential Scanning Calorimetry (DSC) experiments were conducted on a PerkinElmer model 8500 differential scanning calorimeter on the polycrystalline material in the temperature range 100300 K under a nitrogen atmosphere in hermetically sealed Al pans. Calibration was performed with n-heptane and indium as standards. Dilatometric measurements were made with a thermomechanical analyzer Perkin Elmer TMA-7 in the temperature range 150-300 K. Simultaneous Thermogravimetric Analysis (TGA) and Differential Thermal Analysis (DTA) were performed on a Setaram SETSYS 16/18 instrument in the temperature range 300– 875 K with a ramp rate 5 K min−1. The scans were done in flowing nitrogen (flow rate: 1 dm3 h−1 ). Dielectric studies The complex dielectric permittivity, ε* = ε′ − iε″, measurements were conducted on PCA single crystal with the use of an Agilent E4980A Precision LCR Meter between 100 and 300 K in the frequency range between 135 Hz and 2 MHz. The overall errors were less

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than 5%. The dimension of the crystal was of the order of 5 x 3 x 0.75 mm3. Pyroelectric properties were tested with an electrometer/high resistance meter Keithley 6517B with temperature ramp 2 K min−1 in the same temperature

range. The ferroelectric hysteresis loop measurements were performed using a home-made high precision setup based on the Diamant–Drenck–Pepinsky (DDP) bridge.

Table 1. Experimental data for PCA Phase I

Phase II

Phase IV

Chemical formula

C12H30Cl9N3Sb2

C12H30Cl9N3Sb2

C12H30Cl9N3Sb2

Mr

778.94

778.94

778.94

Crystal system, space group

Trigonal, R3m

Trigonal, P31

Monoclinic, C2

Crystal data

Temperature (K)

255

225

208

a, b, c (Å)

8.8458(3), 8.8458(3), 30.2440(14)

8.8499(2), 8.8499(2), 30.1715(11)

14.8102(11), 9.1317(8), 30.110(4)

α, β, γ (°)

90, 90, 120

90, 90, 120

90, 90.189(8), 90

3

V (Å )

2049.48 (17)

2046.46 (12)

4072.1 (7)

V primitive cell

1/3 V

1V

½V

Z

3

3

6

Radiation type

Mo Kα

-1

2.86

µ (mm )

2.90

Crystal size (mm)

2.91 0.21×0.18×0.01

Data collection No. of measured, independent and observed [I > 2σ(I)] reflections

6692, 1332, 870

19971, 6443, 3962

17382, 6855, 4263

Rint

0.025

0.034

0.071

0.694

0.695

0.610

R[F > 2σ(F )], wR(F ), S

0.061, 0.194, 1.12

0.052, 0.119, 0.98

0.127, 0.360, 1.16

No. of reflections

1332

6443

6855

-1

(sin θ/λ)max (Å ) Refinement 2

2

2

No. of parameters

53

324

374

No. of restraints

16

216

215

H-atom treatment (∆/σ)max -3

ρ〉max, ρ〉min (e Å )

H-atom parameters constrained 0.916

0.033

0.358

0.67, -0.56

0.68, -0.56

4.60, -1.58

Absolute structure Absolute structure parameter

Refined as an inversion twin. 0.45 (13)

0.37 (8)

0.58 (17)

2

The R3m model has been chosen basing on the intensity statistics (E -1)=0.701; CFORM=3.39, the presence of weak SHG signal and a very small dielectric increment that appears during the phase transition to ferroelectric phase II, which is not expected for the phase transition from centro- to non-centrosymmetric phase. Computational methods For the total energy calculations we have employed a plane wave basis set defined by the energy cut-off of up to 400 eV, Troullier-Martinstype pseudopotentials27 and dense Monkhorst–Pack k-point meshes28 together with PBE (Perdew-Burke-Ernzerhof)29 and LDA30 density functionals, as implemented in Abinit

software suite31,32. The spontaneous polarization was calculated using a Berry phase approach.33,34 SHG measurements Prior to measurements, the crystalline powders of PCA and KDP (potassium dihydrogen phosphate, used herein as a reference material) were sieved through a mini-sieve set (Aldrich) to the 125-177



μm size, while the sample of PCA after dielectric measurements was used without further sieving. The samples of those materials were placed between microscope glass slides and excited by ~130 fs length laser pulses from a Quantronix Integra-C regenerative amplifier operating at 800 nm at the repetition rate of 1 kHz. The focused beam (using aspheric lens of f = 150 mm, irradiation area of 0.4 cm2) was directed onto samples at about 45 degrees while the collecting optics, mounted to the optical fiber, was placed along the direction normal to the plane of the sample. The scattered fundamental radiation was optically removed with the use of 700 nm shortpass dielectric filter (FESH0700, Thorlabs). The spectra of the SHG emission were recorded by an Ocean Optics 2000 fibercoupled CCD spectrograph. To preserve a comparability of results, the geometry of the experimental setup and the intensity of the laser beam were the same for the whole experiment and all the samples. The laser beam intensity was attenuated by a Glan laser polarizer to the value of 700 mW, in order to limit the sample decomposition by femtosecond laser pulses. A different integrating time was used for the samples of PCA (10000 - 20000 ms) and for the KDP sample (50 ms, averaged 5 times). The intensity of the signal was normalized to the same time of collection. CAUTION: Work with the high-power laser brings danger to the eyes, especially in spectral range in which the beam is invisible. The adequate eye protection should be applied during measurements. Optical measurements The domain structure of PCA crystals was studied by means of an Olympus BX53 optical polarization microscope. The samples were mounted in a LINKAM THM-600 heating/cooling stage, where the temperature was stabilized to within 0.1 K. NMR measurements were performed on an ELLAB TEL-Atomic PS 15 spectrometer working at the frequency of 25 MHz. Spin–lattice relaxation times T1 were measured using a saturation sequence of π/2 pulses followed by a variable time interval τ and a reading π/2 pulse. The nonexponential recovery of magnetization was mostly observed in temperatures above 150 K. The changes of temperature of the sample were effectuated using the liquid nitrogen evaporation and were controlled by a UNIPAN 660 temperature controller operating on Pt 100 sensor providing a long-time temperature stability better than 1 K. Errors of the measured T1 values were estimated to be lower than 5%. The measurements of the proton NMR second moment were carried out with a wide-line ELLAB CW spectrometer operating at 26.8 MHz. The second moment values were calculated by the numeral integration of the first derivative of an absorption line and corrected for the finite modulation amplitude.

Figure 1. DSC data for heating and cooling runs (red and -1 blue line respectively, 10 K min , m = 16.51 mg)

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RESULTS AND DISCUSSION Thermal properties of PCA TGA/DTA results reveal that PCA is thermally stable up to about 460 K (Figure S2). DSC measurements show that PCA below room temperature undergoes complicated sequence of phase transitions. Clearly, the DSC curve (Figure 1) shows five PTs during cooling run and only four ones while heating. A well-separated phase behavior is seen only in the case of two extreme thermal anomalies which occur at: 241/242 K (I→II) and 189/197 K (V→VI and VI→IV’) (cooling/heating). The PT at 202 K (IV→V) is irreversible and is visible only on the cooling run while the heat anomaly occurring around 215-220 K manifests itself by a double shaped maximum which separates distinctly only at slow temperature changes (see grey inset in Figure 1). This suggests that the phase VI overheats and survives until the PT to the phase IV. The transition entropies (∆S) for all detected PTs are as follows: ∆S (I→II) =5 J/mol-1⋅ K-1, ∆S (II→III) =3 J/mol-1⋅ K-1 , ∆S (III→IV) =6 J/mol-1⋅K-1, ∆S (IV→V) =1.5 J/mol-1⋅ K-1. ∆S (V→VI) =12.5 J/mol-1⋅K-1. ∆S values clearly indicate that most of the PTs are of an orderdisorder type (except for PT (IV→V) which is close to the “displacive” type). Structural characterization PCA crystallizes in the room temperature phase (I) in the polar, trigonal R3m symmetry. The fundamental structural feature for this and other phases of PCA is that crystal structure is built up of [Sb2Cl9]3-∞ layers, composed of corner-sharing SbCl6 octahedra and pyrrolidinium counter-ions balancing the negative charge of the layers. The detailed projections of the crystal structure in Phase I are shown in the Figure 2.

Figure 2. Details of the PCA crystal structure in Phase I, T=255 K. (a) Packing, as seen along the a-direction, the inequivalent cations are marked by the Arabic numbers, the 3[Sb2Cl9] ∞ layers are drawn in the octahedral representation, the polar direction is perpendicular to the layers. (b) The single anionic layer in the ellipsoid representation. (c) Each inequivalent cation is disordered over the trigonal 3-fold axis giving nine different orientations for counterions; (d) The elements of the anionic layer, symmetry inequivalent sites are drawn with front ellipses.

The asymmetric unit comprises three halves of the pyrrolidinium ions, two antimony (Sb1 and Sb2), two terminal (Cl1 and Cl2) and one bridging (Cl3) atoms. Each pyrrolidinium cation has Cs symmetry with the nitrogen atom located on the mirror plane. This determines the direction of the dipoles associated to each pyrrolidinium ion. Additionally, each cation is dynamically disordered over the trigonal 3-fold axis, which gives rise to nine different spatial orientations for the pyrrolidinium ions in the structure of Phase I and in consequence, to a heavy disorder.


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The [Sb2Cl9]3-∞ layers have a C3v (3m) symmetry and extend in the (a,b)-plane. The neighboring layers are shifted by the (1/3, 2/3, -1/3) translation, the distance between the layers is equal to 1/3c (~10.08 Å). The pyrrolidinium cations 1 and 3 are located between the layers, whereas the cation 2 occupies the voids within them. The analysis of Sb-Cl bonds of PCA in Phase I reveals two different sets of Sb-Cl bonds in the structure, commonly referred to as bridging across terminal and terminal across bridging. Indeed, due to the trans-effect observed in haloantimonates(III) and -bismuthates(III)35 the terminal bonds are evidently shorter than the bridging ones, for example Sb1-Cl1 2.417(13) Å, Sb2-Cl2 2.378(10) Å and Sb1Cl3, equal to Sb2-Cl3 3.10(2) Å. The Cl1-Sb1-Cl3 and Cl3Sb2-Cl2 trans- angles are equal to 178°, whereas Cl-Sb-Cl cis-angles range from 88.6(4) to 92.9(3)°. At 241 K a structural PT leads to lowering of the crystal symmetry to the space group P31. The mechanism of the transformation is complex and involves both, a distortion of the [Sb2Cl9]3-∞ network and a partial ordering of the cations. In the new phase, the site symmetry of all atoms is reduced to C1. Figure 3 compares two single [Sb2Cl9]3-∞ layers and cations placements in both phases. In Phase II the symmetry of the layers is reduced from C3v (3m)to C1 and the number of spatial orientations of cations in each crystal void is reduced from three to two. In a similar fashion to Phase I the planes are symmetry related and the distance between them is equal to 1/3c (~10.06 Å). The asymmetric unit still contains two Sb centers but the number of inequivalent terminal atoms grows from two to six, and that of bridging atoms from one to three. Organic cations are still disordered; however, the disordered positions are not symmetry-related. Thus, the asymmetric unit contains six different cations with site occupation factors ranging from 0.4 to 0.6. The ordering process of the pyrrolidinium cations is shown in Figure 4. In Phase II the symmetry does not restrict the position of the nitrogen atom in the ring anymore, thus the possible placement of the NH2 group must be deduced from the electron density maps. The disorder present here renders the differentiation between the CH2 and NH2 groups almost impossible; accordingly, the NH2 group placement was assumed based on the shortest distance to the possible acceptor Cl (taking into account possible N-H…Cl hydrogen bonds). However, it must not be considered as a unique one and as such does not inform about the direction of the cations dipole moments. There are three different bridging and three different terminal Sb-Cl distances in each SbCl6 octahedron because of C1 symmetry. The terminal bonds range from 2.389(8) to 2.408(7) Å whereas the bridging bonds range from 3.033(9) to 3.096(9) Å. The Cl-Sb-Cl trans-angles change from 176.9(3) to 178.1(3)°, whereas the cis-angles

two. The darkened octahedra are located below, whereas the lightened above the (a, b) plane. (b) The anionic substructure along the a direction. Neighboring octahedra rotates clockwise and anti-clockwise. Similar to Phase I all planes are symmetry-related.

Figure 4. The ordering of pyrrolidinium cations with temperature lowering. In R3m phase, cations occupy three different crystal voids 1, 2, 3. In each position, pyrrolidinium cation is dynamically disordered over the 3-fold axis. This gives 9 different spatial orientations. In Phase P31, partial ordering of pyrrolidinium cations takes place. In each cavity, pyrrolidinium ions are at least disordered over two inequivalent placements. Further cooling blocks rotations. In C2 phase, there are six symmetry independent states with one cation disordered over the 2-fold axis.

change from 86.1(3) to 93.1(2)°. The transition has a minor influence on the Sb-Sb distances which vary from 6.1158(1) Å in phase I to 6.1153(1), 6.0776(1) and 6.1103(1) Å in Phase II. Further cooling leads to successive structural transformations: to Phase III, IV, V, and VI. Due to a very narrow temperature range of existence of the phase III (~2 K) we were not able to detect the symmetry change using the xray diffraction. The next phase which was solved was the Phase IV. Structural changes in Phases V and VI led to a drastic reduction of the diffracted intensities which were insufficient for structure calculations. Around 215 K the symmetry of the PCA crystal changes from trigonal P31 to monoclinic and the crystal becomes non-merohedrally twinned. The solution at 208 K from intensities of the dominating domain gave the best result for C2 symmetry. Due to the C centering, the volume of the monoclinic cell is doubled compared to the trigonal phases; however, the metric of the primitive monoclinic structure corresponds approximately to the trigonal unit cell. The c-lattice parameter is almost preserved, although the polar direction is completely changed and is perpendicular to the c-axis, along monoclinic b-direction (which is equivalent to the a and b directions from both trigonal phases) as implied by symmetry. Noteworthy is that the space groups P31 and C2 are not group-subgroup related. The PT, thus, cannot be continuous. The fact that the sample preserves its integrity indicates a special twinning law involving a shuffling of atomic planes. The transformation is related to further ordering of the cations and strong deformation of [Sb2Cl9]3-∞ layers which is illustrated in Figures 4 and 5. The asymmetric unit is built of three Sb atoms and 14 Cl ligands, additionally, three of them are disordered over split positions, with site

3-

Figure 3. (a) Comparison of two single [Sb2Cl9] ∞ layers and pyrrolidinium cations placements in Phase I and II, the view along the c direction. The symmetry of the layers is reduced from 3m to 1, the number of spatial orientations of pyrrolidinium cations in each crystal void is reduced from three to

Figure 5. (a) The anionic network in Phase IV with six independent cation positions. (b) Deformation of anionic framework results in disordered Cl sites (transparent section) and 3two inequivalent [Sb2Cl9] ∞ layers. One of them has C2



symmetry. The polar direction changes by 90° compared to Phases I and II.

occupation factors ~0.5; the number of pyrrolidinium cations located in different crystal cavities grows to six. One of the pyrrolidinium cation is disordered over 2-fold axis among two equivalent sites with 1/2 occupancy. It seems that rotations of the whole molecules are blocked in Phase IV; however huge displacement parameters indicate that small rotations and librations are still present. The movements are expected especially for C4H8NH2+ cations located in the voids of [Sb2Cl9]3-∞ layers. In phase IV two neighboring anionic layers are symmetry independent. The spacing between them changes from 1/3 c (10.08, 10.06 Å) in Phase I and II to two different distances in Phase IV, 9.8534(1), 10.0436(1) Å; thus the lattice evidently contracts in the c-direction. There are three inequivalent SbCl6 octahedra; two of C1 symmetry build one of the layers whereas the second one, of C2 symmetry, exclusively builds the second layer. Noteworthy, despite the fact that the translational symmetry is reduced in Phase IV the site symmetry of one layer actually heightens with temperature lowering. In Phase IV the terminal bonds range from 2.25(4) to 2.66(3) Å whereas the bridging bonds range from 2.91(4) to 3.151(19) Å. The Cl-Sb-Cl trans-angles change form 169.4(7) to 179.3(5) Å, whereas the cis-angles change from 79.4(7) to 99.7(10)°.

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The analyses of the configurational arrangement/disorder of molecules based on the structural and calorimetric methods is helpful in understanding of molecular mechanism of PTs (see works by OnodaYamamuro36 and Butler37). The calorimetric results (the entropy effect) confirmed to a large degree the molecular mechanism of phase transitions in PCA. It should be emphasized that the thermal effects obtained from the differential scanning calorimetry are barred with errors even +/- 20%. The the total value of ∆S from DSC is close to 28 J/mol K (five PTs) whereas ∆S value estimated from available X-ray results is comparable. Ideed, in the Phase I we have three nonequivalent disordered cations distributed between three sites. Assuming that in the ordered Phase (VI) all cations are completely frozen (one possible position) the entropy change (∆Stheor = 3Rln3, 27.4 J/K⋅mol) is quite close i.e. to the experimental one. Some differences are observed when comparing particular PTs. Taking into account the structural results, PTs (IV→V) and (V→VI) should be characterized by small value of ∆S whereas the experimental value for (V→VI) is the largest (12.5 J/K⋅mol). It may suggest that another subtle type of motions of cations is present, which was not accounted for the structural considerations, e.g. a pucking motion of the pyrrolidinium cation between twisted and envelope conformations. The presence of this type of dynamics is a probable scenario, as can be inferred from temperatureresolved proton NMR studies (vide infra).

Figure 6. (a) Temperature dependence of the real part of complex electric permittivity measured along the c-axis during cooling; P-E hysteresis loops at: (b) 187.5 (Phase VI), 205 (Phase IV), 215 (Phase III) and 220 K (Phase II).

Dielectric and ferroelectric characterization Armed with an understanding of structural peculiarities of solid-state processes that occur in PCA at various temperatures, we have moved our attention to the characterization of electric properties of observed crystal phases. The plot of dielectric permittivity measured as a function of temperature is depicted in Figure 6(a). The sequence of the PTs present in dielectric permittivity plots is in agreement with those revealed by the DSC scans (vide supra). All the (five) structural PTs below RT involve polar phases. A prototypic paraelectric phase is to be inferred above the decomposition/melting point. Between 275 and 216 K is present a significant increase in the ε’ value (from 80 to 115) with a weak anomaly at ca. 241 K. The PT at 216 K is accompanied by an abrupt decrease of ε’ down to 30 (excluding the anomaly for the II→III PT). The remaining lower temperature PT is accompanied by a visible dielectric increment (∆εc ≈ 5). The strongest dielectric response around II→III→IV PTs (∆εc ≈ 85) resembles those encountered for antiferroelectric materials however phase IV is polar (C2), noncentrosymmetric (typical for ferroelectrics). A drastic reduction of ε’ at 216 K is due to the rapid freezing of polar cations motion

which is confirmed by the X-ray diffraction results. In Phase IV five of the nonequivalent cations are well ordered whereas the remaining one is distributed between two sites. The dynamics of the latter cation seems to be responsible for the lowest temperature PT. It should be added that within Phases II and III only the traces of the low frequency relaxation processes are visible, whereas below 214 K the dynamics of the organic cations is distinctly diminished (see Figure S4). It should be emphasized that the dielectric response over the Phase I is dominated to a large degree by the conductivity phenomena. We carried out (see Figure S5) the measurements of the AC conductivity (σt = σDC +σAC, where σt is total conductivity) at different temperatures and frequencies (ω). In the low frequency region, the conductivity is found to be almost frequency independent which corresponds to the DC conductivity, while in the kilohertz frequency region, the conductivity increases with frequency. The frequency dependence of conductivity can be explained by an expression given by Jonscher’s power law38 (in details see SI): σAC=σ0+Aσn , where σ0 is the limiting zero frequency conductivity, A is preexponential constant and n express the temperature dependent degree of an interaction between mobile ions and ω is angular frequency. The estimated n parameter


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increases with temperature (0.39 and 0.50), which suggests that the conduction mechanism in PCA crystals may be attributed to small polaron quantum mechanical tunneling.39 The activation energy of the AC conductivity is found to be c.a. 0.45 eV which is a typical activation energy associated with the conductivity process of organic cations.40 Taking into account the conductivity value PCA may be classified as a weak semiconductor material over the Phase I. The results of the hysteresis loops measured at different temperatures are illustrated in Figure 6(b). The presence of the reversible spontaneous polarization (Ps) below 241 K confirms the ferroelectric properties of all the lower temperature phases. Due to the enhanced conductivity phenomena over the phase I it was impossible to detect the hysteresis loop (more information in SI). It should be emphasized that the value of Ps (4⋅⋅10-3 µC cm-2) at 220 K corroborated by the hysteresis loop (Figure 6b) is distinctly reduced in the following phases III, IV and V by one order. There are two possible mechanisms of such an effect; i) a strong increase in the coercive field which does not enable us to obtain the saturated values of Ps below 216 K; ii) the change in the direction of the polar axis during the phase P31→C2. Thus, we can observe the tracing of the Ps with respect to the proper polar direction b. Pyroelectric measurements (presented in Figure S6) were carried out in the temperature range covering all PTs of PCA. Significant changes in the pyroelectric current (Ipyr) were observed in the vicinity of I→II PT. Ipyr appears to be reversible under a DC electric field (2 kVcm1 ), however it is visible only when close to Tc(I→II). The remaining PTs are hardly visible in the pyroelectric measurements because the changes in the spontaneous polarization values detected along the c-axis are rather small. The pyroelectric current and, consequently, the measurement of the spontaneous polarization was possible only in the direction perpendicular to the crystal plates. This direction is parallel to the polar axis over the Phases I and II. The measured value of Ps in Phase II is significantly lower than that PsIIcalc = 0.7 µC cm-2 deduced from the projection of the permanent dipole moment of the pyrrolidinium cation disordered as in Figure 2. The reason for this difference may lie in the polarizability of the determination of the molecular dipoles in the disordered cations and enhanced polarizability of two dimesional anionic layers. More precise assessment was possible in more ordered Phase IV. The positions of atoms in the unit cell determined for Phase IV, which is a disordered one, are not directly suitable for ab initio calculations therefore an ordered model has been constructed in order to calculate polarization of the phase. The focal disordered structural motif is a pyrrolidinium cation whose nitrogen atom may point along c axis towards or against the direction of the axis, equiprobably. To account for this effect, the symmetry has been reduced to P1 and a model with an alternate arrangement of each setting in consecutive (010) layers has been created. The disorder originating from

other structural elements has been removed by choosing the most probable atomic positions according to their site occupations, see PhaseIV_ordered.cif appended to the SI. The calculated spontaneous polarization being approximately PsIVb = -2.0± n x 3.6μC cm-2, is directed along the shortest crystal axis, with exactly zero projections onto a and c directions. The addend, n x 3.6 μC cm-2, where n = 0, 1, 2.., is so-called polarization indetermination quantum, whose immanent appearance is due to the nonperiodic character of a position operator r. Further details of the calculations can be found in the SI. The measurement confirms the theoretical prediction in that the spontaneous polarization along the c axis in phases III, IV, V and VI is rather weak. The highest plausible value resulting from the monoclinic angle and the possible corresponding surface corrugation (see SI for details) is PsIVmesured ≈0.003 x PsIVb that is in accordance with the data obtained.

Figure 7. A comparison of averaged SHG traces for PCA (red) and that of KDP (black). Note that collection times of those signals were equal to 10000 ms and 50 ms, respectively; since the intensities are normalized to the same time of collection, the SHG is multiplied by 4000 to ensure visibility of the trace; (b) SHG traces recorded after five consecutive 20 s long collection windows, which indicate fast decomposition of the sample.

SHG measurements In the preceding section we have signaled that we were not successful in detecting the dielectric hysteresis loop for Phase I, due to the sample’s large conductivity at room temperature. Since a non-centrosymmetric space group is a prerequisite for the presence of macroscopic polarization, the detection of SHG property provides an unequivocal proof of the crystal noncentrosymmetry. Accordingly, the detection of SHG for phase I of PCA can be considered as i) a tool for verification if the assignment of noncentrosymmetric space group is correct, and ii) an indirect proof for phase I capability for room-temperature ferroelectric response. Our Kurtz-Perry-type40,41 SHG measurements have shown that PCA in Phase I indeed generates second harmonic upon irradiation with 800 nm, 130 fs long laser pulses, attesting to its noncentrosymmetry and ferroelectricity. To obtain SHG intensity values that could be used for semi-quantitative estimation of SHG efficiency of PCA, we applied a ‘minimum dose technique’ in which the time of collection window was reduced to 10 seconds, and the sample was continuously swept across the laser beam. In this manner, the photochemical damage of the material was limited, with an additional advantage of averaging the signal intensity due to collection of SHG signal over various crystal orientations. Integration of averaged spectra with minimized decomposition effects and comparison with integral intensity of SHG generated by KDP reveals that PCA generates second harmonic



approximately 14400 times weaker than KDP, as shown in Figure 7. We suppose that such a low relative SHG efficiency stems from destructive interference of the produced second harmonic response, which is a result of a

Page 8 of 21

scattering between ferroelectric domains of the opposite polarity. In the SI are provided details concerning the stability of the PCA towards femtosecond laser radiation.

Figure 8. Domain structures evolution at different temperatures and different phases. * Photo lightened by 10%

Domain structure in PCA The PT I→II (R3m→P31) belongs to a ferroelastoelectric species.42 The lowest order macroscopic order parameter is the piezoelectric tensor (e.g. in Phase R3m the directions of a purely longitudinal in-plane component of the piezoelectric polarization in response to a tensile or compressive stress in the plane (a,b) are determined by the mirror reflection planes, which restriction is lifted in the P31 symmetry). The two low-symmetry ferroic states (enantiomorphs) are related by a mirror plane m parallel to the trigonal axis. The domains are not visible in the polarization microscopy without external stress (see Figure 8, Phase II). Although Phase III appears in a narrow temperature range, it exhibits a clear domain pattern with preferentially flat domain walls. The domain walls form angles close to 60o (120o). This indicates that the symmetry reduction involves a loss of the three-fold axis. A more precise insight into this domain structure reveals an angle close to 64o between the domain boundaries. Examples of these angles are shown in Figure 8, Phase III. The angle is very close to that 63.310 deduced for the (W’,W’) domain walls, from the spontaneous in-plane strain in Phase IV at 210 K (see below). This follows from the fact that the loss of the trifold axis with conserved perpendicularity of the c-axis to the (a, b) plane results in domains analogous to those described by Szklarz et al.43 The crystal structure is known for Phase IV (space group C2). Looking at the lattice constants obtained in the SC-XRD measurements we see that the extension of the unit cell (multiplication of Z by a factor of 2) is related to the centering in the plane (a,b) in the monoclinic phase. This centering transforms a hexagonal unit cell into a rectangular one. The elementary rectangle in the Phase II has the size 15.3285 Å by 8.8499 Å (directions (1,1,0) and (1,-1,0) in the hexagonal system). The corresponding dimensions in Phase IV are 14.8102 Å and 9.1317 Å. The small 2D strain tensor between Phase II and IV then is: −0.03381 0 ߝ=ቀ ቁ. (1) 0 0.0318 o When rotated by 45 the strain becomes: −ߝ௣ −ߝ௦ −0.00098 −0.0328 ߝ=ቀ ቁ = ቀ −ߝ −ߝ ቁ. (2) ௦ ௣ 0 −0.00098 This demonstrates that the contraction in the plane (a,b) (expressed by the diagonal elements) is weak and that the shear is remarkable ε12 = -0.0328. It is this plane strain that plausibly determines the ferroelastic domain

structure in Phase III. The structure survives in Phase IV, but the angles between the domain walls grow (see Figure 8, Phase IV). In addition to that, a system of fine stripes appears in Phase IV. They may result from the distortion introduced by the deviation of the monoclinic angle from 90o. Such being the case a relief should appear on the surface. It can be detected by surface probe microscopy. The boundaries of the fine domains should be parallel to the monoclinic b axis (unique axis). The corresponding measured angles are given in Figure S9 (see also movie F1 included as SI). One can see that the angles of the fine domain walls and the previous ferroelastic domain walls are about 52.69o and 72.31o on the side and on the other of a ferroelastic wall. The origin of the difference is clear because the ferroelastic domain wall is not invertible; the domains result from a loss of trifold axis so there is no symmetry that would transform mutually the domains. The angle of the lattice edge b with the (W’,W’) domain wall should amount to 58.343o. It lies between the measured values. In summary, a comparison of the XRD and optical data in the Phase sequence II→III→IV suggests a loss of the trifold axis in the phase transition II→III without a variation of the right angle between the c axis and the (a,b) plane, whereas the latter deviation seems to be involved in the phase transition III→IV. The evolution is clearly seen in the movies F2 and F3.

Proton NMR studies in PCA The NMR measurements were carried out from 95 K to 290 K (on heating). The temperature dependence of the second moment of the proton resonance lines, M2, of PCA is shown in Figure 9(a). It is seen that between 95 and 150 K the second moment has a plateau of about 22 G2. Above 150 K the M2 value continually decreases to about 8 G2 passing without any abrupt changes over PTs and only the third anomaly at 242 K reveals little changes. The full analysis of the temperature dependence of M2 is presented in the SI. The obtained results indicate that below 150 K the cations are ordered and at room temperature the isotropic reorientational motion is not still reached. It is this anisotropy that plausibly produces the spontaneous polarization in Phase I.

Figure 9. (a) Temperature dependence of the second moment (M2) of the proton NMR lines for PCA; (b) The temperature dependence of spin–lattice relaxation times (T1) of


Page 9 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials PCA. The vertical lines show the transition temperatures determined by DSC.

The spin-lattice relaxation time (T1) measurements disclosed long and short components (Figure 9(b) and SI). We are able to fit the T1 data with the BPP-type equation44 only for the long component of relaxation times measured just below 197 K: ଵ

்భ

= ‫(ܥ‬

ఛ

ଵାఠబమ ఛమ

+

ସఛ

ଵାସఠబమ ఛమ

,

(3)

where C denotes the motional constant, τC the correlation time and ωo the Larmor frequency. The temperature dependence of the correlation times in the temperature range of this fit is described by the Arrhenius law τc = τc0 exp(Ea/RT). The obtained parameters are given in Table S2. The visible nearly constant values of relaxation times below 150 K may be attributed to the possible influence of an additional relaxation process of protons via quadrupolar nuclei of 35Cl.45-50 For the other measured relaxation times only the activation energies have been estimated (see Table S2) and it is interesting that three different ranges of values are found: : 2.1, 8.4 and 21 kJ⋅mol-1, respectively. The activation energies values point out to different motional states of the pyrrolidinium cations in PCA.48-50 The following kinds of motion of the cations in the wide temperature range may be considered: (i) a pucking motion of the pyrrolidine ring between twisted and envelope conformations, (ii) a small-angle libration of the pyrrolidinium cation, (iii) the rotations about the pseudo C5 axis perpendicular to the pyrrolidinium ring, (iv) isotropic rotation and cationic diffusion/rotational diffusion.

self-

It is known that the cationic self-diffusion requires very high activation energies (more than 46 kJ⋅mol-1.46 The two smaller activation energy ranges between 2.1 and 8.4 kJ⋅mol-1 should be assigned to a small-angle librations and to conformational changes between twisted and envelope forms of the pyrrolidine ring, respectively. The present 1H NMR studies are in an agreement with single crystal XRay diffraction (the model of reorientational motions of cations), thermal (DSC) and dielectric spectroscopy results. The main conclusions are as follows; I.

Clear anomalies in T1 (spin-lattice relaxation time) around the PTs temperatures confirme the orderdisorder mechanism of transitions with the key role of the pyrrolidinium cations.

II.

Librations motions and conformational changes (twisted↔envelope) are reflected in the traces of the dielectric relaxation processes of dipolar cations over phases II, III and IV. These motions contribute also to the entropy transition through the PT (IV→ V) and (V→VI).

III.

Three-site disorder models of three nonequivalent cations in phase I correspond well with M2 values which exclude the overall reorientational motion of cations up to RT in the Phase I.

CONCLUSIONS A Cambridge Structural Database survey (version 5.32, January 2018) shows that there is a substantial relationship between polar symmetry of the compound and a motif of the anionic sublayers. A total of 109 entries involving the [M2X9]3- anionic units were found: 80 for isolated bioctahedra (0D), 6 for [M2X9]∞3− double chains (1D), 13 for layer structure (2D), and 10 for [M4X18]6- tetramers. Among the 20 total polar materials the ferroelectric order was found int0 crystals; 6 with 2D anionic units and 4 for OD (isolated bioctahedra) (see Table S3). Taking into account Table S3 one can see the trend that 2D ferroelectric structure is characteristic for the chlorine and bromine analogs whereas OD-type ferroelectric are encountered in various halogen analogs. It should be emphasized that ferroelectricity within R3M2X9-type structure is found only for small-size cations. It is interesting that 2D ferroelectrics are characterized by the presence of highly disordered organic cation (one of three) placed inside the cavities of the anionic layers. The paraelectricferroelectric PT is usually due to the freezing out of these dipolar cations. The Curie temperature Tc for 2D-type subgroups ferroelectrics increases markedly with the size of cations, from methylammonium up to trimethylammonium. For the PCA (the largest cation – pyrrolidinium) Tc is shifted up to the decomposition of crystal. PCA appears to be the first pyrrolidinium ferroelectric within the halobismuthates(III) and haloantimonates(III) family and third known example among all the organic-inorganic hybrid. PCA reveals a complex sequence of solid-solid PTs (six phases), furthermore polar phases cover, at least I, II, III and IV ones. Mechanism of most PTs governed mainly by order of nonequivalent three types of pyrrolidinium cations was established by single-crystal x-ray diffraction, dielectric spectroscopy and T1 spin-lattice relaxation measurements. PCA is characterized by enhanced dielectric constants covering phases I – IV. Especially, worthy of attention is the PT II→III/IV accompanied by strong dielectric anomaly (∆ε ≈ 90) characteristic of paraelectricferroelectric transition, however in this case we deal with the ferroelectric-ferroelectric transformation. In analogy to many iono-molecular crystals the integrity of the sample survives all the solid-solid PTs in spite of the deformations of the unit cell. The changes of symmetry manifest themselves in coherent domain boundaries. A part of the domains have been observed under the microscopy in polarized light. In fact, the existence of domains is expected already in the RT phase I because it shows a spontaneous polarization (see Figure S6). The trigonal symmetry of Phase I implies a uniaxial nature of its ferroelectricity with domains polarized perpendicular to the observation plane. The domains are not distinguishable in the polarized light. Neither possible flux closure domains could have been discerned. The loss of the mirror planes in the PT I→II is also optically invisible. A clear ferroelastic planar domain walls in Phase III corre-



spond to the loss of the three-fold axis. A comparison with Phase IV suggest a symmetry change in which the c crystallographic axis remains perpendicular to the plane (a, b) whereas a quasi pure spontaneous shear strain in the plane (a, b) seems to be similar to that confirmed crystallographically in Phase IV. Interestingly enough the monoclinic symmetry of Phase IV implies a spontaneous polarization in the observation plane (a, b), namely parallel to the unique axis b. A strong reduction in the real part of the electric susceptibility in the c direction in Phase III (Figure 6), along with high losses (Figure S4) may suggest the existence of this polarization component already in this phase. The presence of a c-component of polarization in Phase III remains an open question (see Figure S6). The fine domain structure in Phase IV seems to be related with the deviation of the monoclinic angle from 90o. The hypothesis can be verified with a surface probe method. The same should be said on the domain structure in Phase V of unknown crystallographic structure. The pattern seems a coarse grained variant of the fine domain structure in Phase IV. In summary, the material combines ferroelectricity and ferroelasticity. In principle the multiferroic character of the compound is even richer as the symmetry P31 of Phase II allows for a spontaneous pseudovector along the c axis. Therefore, a magnetization of whatever nature can be also present. A piezoelectric effect is also allowed in this symmetry. An interesting property of PCA is that its the polar axis rotates by about 90o from the c direction in Phases I and II to the monoclinic b direction in the remaining phases. This makes the material interesting for electrostriction/piezoelectricity measurements and for the corresponding applications.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by Wroclaw Centre of Biotechnology, programme: The Leading National Research Centre (KNOW) for years 2014–2018 (contest no. 12). A grant of computer time from the Wrocław Center for Networking and Supercomputing (WCSS) is gratefully acknowledged. Jan K. Zaręba is supported by the Foundation for Polish Science (FNP).

REFERENCES (1)

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ASSOCIATED CONTENT Supporting Information The supporting Information available: Details of PXRD, thermal, dielectric, 1H NMR, SHG and optical properties. CCDC: 1825918-1825920.

Corresponding Author *E-mail: [email protected] Bisiorek)

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AUTHOR INFORMATION (Anna

ORCID

Piecha-

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A. Piecha-Bisiorek: 0000-0002-0314-4478 R. Jakubas: 0000-0002-2464-8309 A. Ciżman: 0000-0002-8906-4080

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W. Medycki: 0000-0001-5494-5729 A. Gągor: 0000-0002-8651-2925

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A. Bil: 0000-0003-0702-6409

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J. K. Zaręba: 0000-0001-6117-6876 M. Nyk: 0000-0002-0329-4038

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Bieder, J.; Bokhanchuk, A.; et al. Recent Developments in the ABINIT Software Package. Comput. Phys. Commun. 2016, 205, 106–131. Vanderbilt, D.; King-Smith, R. D. Electric Polarization as a Bulk Quantity and Its Relation to Surface Charge. Phys. Rev. B 1993, 48 (7), 4442–4455. King-Smith, R. D.; Vanderbilt, D. Theory of Polarization of Crystalline Solids. Phys. Rev. B 1993, 47 (3), 1651–1654. Laane, J.; Jagodzinsky, P. W. Low-frequency vibrational spectra of bromo- and iodobismuthates and the observation of a trans effect. Inorg. Chem. 1980, 19, 44–49. Onoda-Yamamuro N.; Matsuo T.; Suga H. J. Phys. Chem Solids 1990, 51, 1383-1395. Butler K. T.; Walsh A.; Cheetham A. K.; Kieslich G. Chem. Sci. 2016, 6316. Jonscher A. K. Dielectric Relaxation in Solids; Chelsea Dielectrics: London, UK, 1983. Elliot S. R. A.C. conduction in amorphous chalcogenide and pnictide semiconductors.Adv. Phys. 1987, 36, 1352187. Kurtz, S. K.; Perry, T. T. A Powder Technique for the Evaluation of Nonlinear Optical Materials. J. Appl. Phys. 1968, 39, 3798-3813. Graja, A. Production of the Second Harmonic of Light in Ammonium Pentaborate and other Powdered Piezoelectric Crystals. Phys. Status Solidi B 1968, 27, K93-K97. Newnham, R. E. Properties of Materials: Anisotropy, Symmetry, Structure; OUP Oxford, 2004. Szklarz, P.; Pietraszko, A.; Jakubas, R.; Bator, G.; Zieliński, P.; Gałązka, M. Structure, phase transitions and molecular dynamics of [C(NH2)3]3[M2I9], M=Sb, Bi. J. Phys.: Condens. Matter, 2008, 20, 255221(1-12). Bloembergen, N.; Purcell, E. M.; Pound; V. Relaxation Effects in Nuclear Magnetic Resonance Absorption Phys. Rev. 1948, 73, 679-712. Bloembergen, N.; Purcell, E. M.; Pound; V. Relaxation Effects in Nuclear Magnetic Resonance Absorption Phys. Rev. 1948, 73, 679-712. Ono, H.; Ishimaru, S.; Ikeda, R.; Ishida, H. New mesophases in ionic crystals: piperidinium perchlorate and nitrate studied by 1H, 2H, 14N and 35Cl NMR. Chem. Phys. Lett. 1997, 275, 485-490. Ono, H.; Ishimaru, S.; Ikeda, R.; Ishida, H. Ionic plastic phase in piperidinium hexafluorophosphate studied by solid NMR, X-Ray diffraction, and thermal measurements. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 650-655. Ishida, H.; Ikeda, R.; Nakamura, D. Proton NMR studies on the motion of cations in the three solid phases of methylammonium perchlorate including cationic selfdiffusion in its highest-temperature solid phase. Bull. Chem. Soc. Japan 1987, 60, 467-474. Bednarska-Bolek, B.; Jakubas, R.; Medycki, W.; Nowak, D.; Zaleski, J. Structure, phase transitions and molecular motions in ferroelastic (C4H8NH2)SbCl6 (C4H8NH2)Cl. J. Phys.: Condens. Matter 2002, 14, 3129–3142. Medycki, W.; Świergiel, J.; Hołderna-Natkaniec, K.; Jurga, K.; Jakubas, R. Solid State Nucl. Mag. 2004, 25, 129–132. Jakubas R., Bednarska-Bolek B., Zaleski J., Medycki W., Hołderna-Natkaniec K., Zieliński P., Gałązka M. Structure, phase transitions and molecular dynamics in ferroelastic crystal pyrrolidinium hexachloroantimonate(V), [C4H8NH2][SbCl6]. Solid State Sci. 2005, 7, 381–390.



Figure 1. DSC data for heating and cooling runs (red and blue line respectively, 10 K min-1, m = 16.51 mg) 287x201mm (300 x 300 DPI)


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Figure 2. Details of the PCA crystal structure in Phase I, T=255 K. (a) Packing, as seen along the a-direction, the in-equivalent cations are marked by the Arabic numbers, the [Sb2Cl9]3-∞ layers are drawn in the octahedral representa-tion, the polar direction is perpendicular to the layers. (b) The single anionic layer in the ellipsoid representation. (c) Each inequivalent cation is disordered over the trigonal 3-fold axis giving nine different orientations for counterions; (d) The elements of the anionic layer, symmetry inequivalent sites are drawn with front ellipses. 205x141mm (300 x 300 DPI)



Figure 3. (a) Comparison of two single [Sb2Cl9]3-∞ layers and pyrrolidinium cations placements in Phase I and II, the view along the c direction. The symmetry of the layers is reduced from 3m to 1, the number of spatial orientations of pyrroli-dinium cations in each crystal void is reduced from three to two. The darkened octahedra are located below, whereas the lightened above the (a, b) plane. (b) The anionic sub-structure along the a direction. Neighboring octahedra rotates clockwise and anti-clockwise. Similar to Phase I all planes are symmetry-related. 197x175mm (300 x 300 DPI)


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Figure 4. The ordering of pyrrolidinium cations with tem-perature lowering. In R3m phase, cations occupy three different crystal voids 1, 2, 3. In each position, pyrrolidinium cations is dynamically disordered over the 3-fold axis. This gives 9 different spatial orientations for pyrrolidinium cati-ons. In Phase P31, partial ordering of pyrrolidinium cations takes place. In each cavity, pyrrolidinium ions are at least disordered over two inequivalent placements. Farther cool-ing blocks rotations of pyrrolidinium cations. In C2 phase, there are six symmetry independent states with one cation disordered over the 2-fold axis. 207x122mm (300 x 300 DPI)



Figure 5. (a) The anionic network in Phase IV with six inde-pendent cation positions. (b) Deformation of anionic framework results in disordered Cl sites (transparent sec-tion) and two inequivalent [Sb2Cl9]3-∞ layers. One of them has C2 symmetry. The polar direction changes by 90° com-pared to Phases I and II. 177x176mm (300 x 300 DPI)


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Figure 6. (a) Temperature dependence of the real part of complex electric permittivity measured along the c-axis during cooling; P-E hysteresis loops at: (b) 187.5 (Phase VI), 205 (Phase IV), 215 (Phase III) and 220 K (Phase II). 387x198mm (300 x 300 DPI)



Figure 7. (a) A comparison of averaged SHG traces for PCA (red) and that of KDP (black). Note that collection times of those signals were equal to 10000 ms and 50 ms, respectively; since the intensities are normalized to the same time of collection, the SHG is multiplied by 4000 to ensure visibility of the trace; (b) SHG traces of PCA recorded after five consecutive 20 s. long collection windows, which indicate fast decomposition of the sample. 288x201mm (300 x 300 DPI)


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Figure 8. Domain structures evolution at different temperatures and different phases. * Photo lightened by 10% 222x159mm (300 x 300 DPI)



Figure 9. (a) Temperature dependence of the second mo-ment (M2) of the proton NMR lines for PCA; (b) The tem-perature dependence of spin–lattice relaxation times (T1) of PCA. The vertical lines show the transition temperatures determined by DSC. 90x132mm (300 x 300 DPI)


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84x47mm (300 x 300 DPI)


Ferroelectricity and Ferroelasticity in Organic Inorganic Hybrid

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