Pressure-temperature phase diagrams and transitions mechanisms of

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Pressure-temperature phase diagrams and transitions mechanisms of hybrid organic-inorganic NH--N bonded ferroelectrics Anna Olejniczak, Marek Szafra#ski, and Andrzej Katrusiak Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00581 • Publication Date (Web): 24 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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Crystal Growth & Design

Pressure-temperature phase diagrams and transitions mechanisms of hybrid organic-inorganic NH--N bonded ferroelectrics Anna Olejniczak,1 Marek Szafrański2 and Andrzej Katrusiak*1 1

Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznań, Poland, 2

Faculty of Physics, Adam Mickiewicz University, Umultowska 89, 61-614 Poznań, Poland,

KEYWORDS: ferroelectrics, NH+···N hydrogen bonds, proton disorder, phase transition, high temperature, high pressure

The

ferroelectric-paraelectric

transitions

of

1,4-diazabicyclo[2.2.2]octane

(dabco)

tetrafluoroborate (dabcoHBF4) and perchlorate (dabcoHClO4), between their phases II (orthorhombic space group Pm21n) and I (tetragonal space group P4/nmm) proceed at TC=377 K and 378 K, respectively. On approaching phase I the anions are strongly shifted by about 0.8 Å and become disordered, but the protons remain ordered in the NH+···N bonds. In phase I, the disordered protons and anions approximate the tetragonal space group P4/mmm with a twicesmaller unit cell, but due to small displacements of the dabcoH+ cations the crystal acquires the symmetry of tetragonal space group P4/nmm, with the unit cell compatible with that of phase II. The structures have been determined by single-crystal X-ray and neutron diffraction.

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DabcoHBF4 has been also investigated at high pressure and its new phases have been revealed. The high pressure phases IV of dabcoHBF4 and III of dabcoHClO4 are isostructural. Highpressure in situ crystallizations in a diamond-anvil cell yield also hydrate dabcoHBF4·H2O of an unprecedented structure, where the crystallization water molecules are OH···O bonded into zigzag chains parallel to the NH+···N bonded cations.

Introduction 1,4-Diazabicyclo[2.2.2]octane monosalts can be described by a general formula dabcoHX, where HX stands for a monovalent acid. In the dabcoHX structures where X=Cl, Br, I, ClO4, BF4 and ReO4, the cations are NH+···N bonded into linear chains (Scheme 1) and these compounds exhibit exceptional dielectric properties and rich phase diagrams. At normal conditions the crystals of dabcoHClO41-6 and dabcoHBF41-4,7-10 are isostructural, with very similar positions of the tetrahedral anions and NH+···N bonded cations. It was found, that the dabcoHX complexes are prone to form polymorphic modifications of various types.1-14 DabcoHBF4 and dabcoHClO4 undergo a similar ferroelectric-paraelectric phase transition. Although the ferroelectric ambientconditions structures of dabcoHBF4 (phase II) and dabcoHClO4 (phase II) are highly isostructural, both of orthorhombic space group Pm21n, Z=2, their low-temperature phases as well as their phase diagrams are very different.5,7 The main aim of this present study is to confirm the structure of the crystals, when they transform to the paraelectric phases, and to complete the phase diagrams of these two compounds. DabcoHClO4 was studied as a function of pressure and temperature earlier,1-6 and dabcoHBF4 was investigated at various temperature conditions.1-4,7-10 Several dabcoHClO4 and dabcoHBF4 phases, as well as a dabcoHClO4 solvate with methanol, were determined. Those studies included the high-temperature phase I


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determination of dabcoHClO4 and dabcoHBF4 at 382 K and 405 K respectively.1,5,10 Then the symmetry of phase I was established as P4/mmm, Z=1, basing on the point-detector X-ray diffractometer measurements.3 Presently we have employed modern diffractometers equipped with 2-dimensional detectors as well as neutron-diffraction, in order to detect the subtle features of these strongly disordered structures. We have found that in the previous studies a set of weak reflections was overlooked and that the structure requires redetermination in a lower-symmetry space group P4/nmm, Z=2. Our present high-pressure studies on dabcoHBF4 reveal its similarities with dabcoHClO4 and the high-pressure preference to form hydrate dabcoHBF4·H2O.

Scheme 1. One chain of NH+···N bonded 1,4-diazabicyclo[2.2.2]octane (dabco) cations in monosalts dabcoHX (X=Cl, Br, I, ClO4, BF4). The double wells represent the potential energy (Ep) as a function of the proton position in the homoconjugated H-bonds. Apart from the specific information about the transition and paraelectric phases of dabcoHBF4 and dabcoHClO4, the present study shows that quite general assumptions about the high-temperature paraelectric phases, that their symmetry is holoedric for the considered structure, may be too simplistic. The structural investigations of high-temperature phases are often hampered by strong non-harmonic vibrations and disorder lowers the intensity of reflections and resolution of data, which affects the accuracy of the results. High-temperature experiments also cause technical difficulties, like the softening of the glue used for mounting the crystal. Compared to four-circle diffractometers routinely used in the 1980s, presently much more efficient equipment for measuring the neutrons and X-rays diffraction data is available..


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Therefore we have undertaken this study of the high-temperature phases of the dabcoHBF4 and dabcoHClO4 ferroelectrics. Experimental Single crystals of dabcoHBF4 and dabcoHClO4 were selected of the precipitate obtained of the aqueous solutions evaporated slowly at 298 K. The phase transition between phases II and I in dabcoHBF4 and dabcoHClO4 proceeds at 377 K and 378 K, respectively, and the hightemperature measurements were carried out at 385 K for dabcoHBF4 and at 380 K and 400 K for dabcoHClO4 on the single crystals stuck with epoxy glue to a glass fiber. The crystals were covered with a thin layer of this glue to slow down the sublimation of the sample. High-pressure diffraction experiments were performed on the dabcoHBF4 samples crystallized in-situ in a modified high-pressure diamond anvil-cell (DAC).15 Single-crystals of neat dabcoHBF4 were obtained from saturated solutions: aqueous, methanol or a mixture of 1:1 (vol) water:methanol (Figure 1).

Figure 1. Single-crystals of dabcoHBF4 and its hydrate at 296 K (a-e): (a) dabcoHBF4 phase II at 0.41 GPa (from the H2O solution); (b) phase IV at 0.72 GPa (H2O); (c) the hydrate at 1.06 GPa (H2O); (d) dabcoHBF4 phase IV at 0.77 GPa (MeOH solution); (e) phase IV at 1.20 GPa (MeOH); and (f) the amorphous precipitate at 1.75 GPa (from MeOH).


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The precipitation of the crystalline sample in the form of fine powder was induced by isothermal compression of the saturated solution, and then the single crystal was grown at isochoric conditions from one seed left of the dissolved sample. It took on average four to five hours to grow a high-quality single crystal. Several crystallizations of dabcoHBF4 have been performed. In the first experiment the saturated aqueous solution of dabcoHBF4 was used and the single crystal was obtained at isothermal conditions. The X-ray diffraction measurement revealed that the crystal at 0.41 GPa is in phase II (Pm21n) (Figure 1a). At 0.72 GPa the isochoric crystallization yielded a new dabcoHBF4 phase IV (Figure 1b) of orthorhombic space-group symmetry Pc21n (Z=4), with unit-cell parameter c doubled compared to that in phase II. Recrystallizations at 1 GPa resulted in the solvated form: a monohydrate of dabcoHF4·H2O was obtained (Figure 1c). Isochoric recrystallizations of the aqueous solution starting at 296 K above 1.3 GPa were hampered by the crystallization of ice VI. In order to circumvent the formation of the hydrate and ice VI in the next crystallizations, methanol was used as a solvent and at 0.77 GPa phase IV was obtained at isochoric conditions (Figure 1d). The recrystallization at 1.2 GPa resulted also in phase IV (Figure 1e). Several recrystallizations above 1.5 GPa resulted in samples, which did not give the diffraction (Figure 1f). Pressure in the DAC chamber was calibrated by the ruby-fluorescence method16,17 with a Photon Control spectrometer affording an accuracy of 0.02 GPa; the calibration was performed before and after the diffraction measurement. The diffraction data were measured with a KUMA KM4-CCD diffractometer using MoKα radiation at 296 K. CrysAlisPro 171.37.3118 was used for recording reflections19 and preliminary data reduction. For the high-temperature measurements an Oxford Cryosystems 700 Series attachment was used. The intensities were corrected for the effects of DAC absorption, the sample shadowing by the gasket, and the sample


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absorption; the reflections overlapping with diamond reflections were eliminated. OLEX2-1.2,20 SHELX-L, and SHELX-S21 were used to solving the structures by direct methods and refining the models by full-matrix least-squares. Anisotropic temperature factors were generally applied for non-hydrogen atoms, but the isotropic thermal parameters were occasionally retained for the atoms with unreasonable thermal ellipsoids. The H-atoms in the structures were calculated from molecular geometry, with the distances N-H equal 0.86 Å and C-H 0.97 Å, and the Uiso factors of H atoms equal to 1.2 times Ueq of the carrier atoms. Neutron-diffraction measurements were carried out on a single crystal of dabcoHClO4, 1.0x1.85x4.0 mm in size, on a D9 diffractometer, equipped with a small 2D detector, installed at the high-flux reactor of Institut Laue-Langevin in Grenoble, with the monochromatic beam λ=0.84040 Å. The neutron data have been corrected for the extinction and absorption and the structural model was refined with SHELX-L. The H atoms were located from the difference Fourier map and refined with anisotropic temperature factors. The crystal data and refinement details are summarized in Tables 1 and 2; the experimental and structural details have been deposited in the CIF format in the Cambridge Structural Database Nos. 1823145-1823154. Structural drawings have been prepared using the X-Seed interface of POV-Ray.22,23 Calorimetric studies were performed in the temperature range 95–420 K using a differential scanning calorimeter (DSC) Q2000 (TA Instruments). The indium standard was used to calibrate temperature and enthalpy as well as synthetic sapphire for specific heat calibration. The samples were heated/cooled with a rate of 10 K/min. The entropy change ∆S was calculated from the Cp(T) dependences according to the formula:

∆S =

T2

∫

T1

C p (T ) − C b (T ) T

dT ,


6

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where Cb(T) represents the baseline and T1 and T2 are the limits of the thermal anomaly integration. Table 1. Selected crystal data for high-temperature neat structures of dabcoHClO4 at 0.1 MPa. dabcoHClO4

phase I

phase I

phase II

phase II

Temperature (K)

400.0(2)

380.0(2)

368.0(2)1

370.0(2)

Crystal system

tetragonal

Space group

1

P4/nmm

orthorhombic

P4/nmm

Pm21n

Pm21n

Unit cell a (Å)

9.4541(3)

9.4341(3)

9.0570(16)

9.0475(12)

b (Å)

9.4541(3)

9.4341(3)

9.7261(11)

9.7061(11)

c (Å)

5.3702(3)

5.3611(3)

5.3688(6)

5.3626(10)

Z/Z'

2/1

2/1

2/1

2/1

Volume (Å3)

479.98(4)

477.15(4)

472.93(11)

470.92(12)

Dcalc (g·cm-3)

1.471

1.480

1.138

1.500

Final R1/wR2 (I>2σ1)

0.1052/0.3129

0.1072/0.3365

0.0803/0.1397

0.501/0.1238

Neutron diffraction data

Table 2. Selected crystal data for high-temperature and pressure structures of dabcoHBF4 and its hydrate. dabcoHBF4

phase I

Temperature (K)

385.0(2)

Pressure (GPa)

0.0001

Crystal system

tetragonal

Space group

phase II

phase IV

phase IV

phase IV

monohydrate

1.20(2)

1.06(2)

296(2) 0.41(2)

0.72(2)

0.77(2)

orthorhombic

P4/nmm

Pm21n

Pc21n

orthorhombic

Pc21n

Pc21n

P212121

Unit cell a (Å)

9.3000(8)

8.559(4)

8.457(3)

8.44392(16)

8.3393(19)

5.2510(10)

b (Å)

9.3000(8)

9.512(5)

9.448(3)

9.431(2)

9.3503(14)

9.423(4)

c (Å)

5.3550(4)

5.3260(16)

10.655(3)

10.664(7)

10.596(5)

18.852(16)

Z/Z'

2/1

2/1

4/1

4/1

4/1

4/1

Volume (Å3)

463.16(6)

433.6(3)

851.3(5)

848.7(6)

826.3(5)

932.8(9)


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Dcalc (g·cm-3)

1.434

1.532

1.560

1.565

1.608

1.559

Final R1/wR2 (I>2σ1)

0.0937/0.3233

0.0514/0.0952

0.0906/0.1540

0.0626/0.1508

0.0548/0.0924

0.0774/0.1805

Results and Discussion Despite significantly different low-temperature and high-pressure behavior of dabcoHBF4 and dabcoHClO4, their ambient-conditions phases II and high-temperature phases I are very similar. The phase diagrams of these crystals, compared in Figure 2, are very similar in the upper-left part (the pressure up to 0.5 GPa and temperature above 350 K). We have now established that their phases next to phase II on high-pressure side, i.e. phase III of dabcoHClO4 and phase IV of dabcoHBF4, are isostructural too.

Figure 2. Phase diagrams p-T of (a) dabcoHClO4 and (b) dabcoHBF4. Solid lines have been drawn through the transition points determined by calorimetric and dielectric measurements (full circles according to Refs. 2, 5 and 7) and the dotted line according to the in situ crystallizations of dabcoHClO4. Triangles mark the structures determined by X-ray and neutron diffraction.


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The high-temperature phases I of dabcoHClO4 and dabcoHBF4 are isostructural (Figure 3). We found that although they both approximate the structural model previously described for dabcoHClO4, of space group P4/mmm and Z=1, in fact due to small displacements of ions along [z] the unit cell is larger (Z=2) and the space group is P4/nmm. The subtle nature of this superstructure can be appreciated from the dramatic drop of intensity of the reflections with the sum of Bragg indices h+k odd. The intensities of these reflections measured by X-ray diffraction are very low. Threfore, we have employed the neutron diffraction, which are much more suitable for investigating the crystal built of light elements at high temperature. Structural transformations between phases I and II are illustrated by one of the strongest such reflections, 034, measured as a function of temperature by the neutron-diffraction (Figure 4). This plot also illustrates the extent of gradual changes in the dabcoHClO4 crystal structure on approaching the transition temperature at 378 K, the discontinuous character of this transition, as well as the subtle distortion of the symmetry of phase I from the small-cell (Z=1) pseudo-symmetric space group P4/mmm (requiring that the reflection 034 intensity be equal to 0).


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Figure 3. High-temperature structures of dabcoHBF4 phase I at 385 K, dabcoHClO4 phase I at 380 K and dabcoHClO4 phase II at 368 K, all viewed along directions [001] and [010]. All these drawings include partly occupied atomic sites; for their phase I it concerns all atoms, while in dabcoHClO4 phase II at 368 K half-occupied are two sites of the Cl and of one O atoms of the ClO4 anion. Displacements δcz, δaz and δacy of cations and anions, related to the crystal symmetry, are indicated (to be explained).


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Figure 4. Integrated intensity of reflection 034 as a function of temperature measured for a single crystal of dabcoHClO4 by neutron diffraction. In all determined phases of dabcoHClO4 and dabcoHBF4 the dabcoH+ cations are linked by NH+···N hydrogen bonds into chains. The NH+···N bonds are ideally linear in phases I and II and in the other phases they are slightly bent (Figure S1). The close cations of the neighboring chains are shifted along [z], i.e. along the chains direction (Figure S2), by 0.51 Å and 0.43 Å in phases I of dabcoHBF4 and dabcoHClO4, respectively, compared to about 0.45 and 0.40 Å in phases II (Figure 5). The anions in phases I of dabcoHBF4 and dabcoHClO4 become disordered at Wyckoff site e (¼ ¼ ½ etc, of site symmetry C2h) and therefore their displacements along [z] are reduced to 0 Å, compared to about 0.8 Å in phase II. The cations and anions are similarly displaced in phases II and I, but systematically larger in dabcoHBF4 than in dabcoHClO4, which suggests the correlation of the z-displacements of anions and cations. It is also characteristic that in phase II the z-displacements of cations are about 2 times smaller than those of anions, about 0.4 Å compared to 0.8 Å, respectively. In phases II and III of dabcoHClO4 and in phase II of


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dabcoHBF4 the z-displacement of the cation remains constant at about 0.4 Å up to TC. Above TC in phases I, the z-displacements of cations increase to ca. 0.43 and 0.51 Å in dabcoHClO4 and dabcoHBF4, respectively. The largest z-displacements of BF4 and ClO4 suggest that it is the dynamics of the anions that drive the structures to the phase transitions: starting above 250 K the z-displacements of the anions start gradually decreasing from ∼0.8 Å to the zero value around TC. On transition to phase I also the relative displacements of δacy of cations and anions along [y] become equal to 0, as required by symmetry (Figures 3 and 5). The temperature dependence of δacy is similar for dabcoHBF4 and dabcoHClO4. At the transition the δacy displacement changes sharply, but a small pre-transition effect appears within phase II gradually reducing δacy to ca. 0.40.5 Å. Our measurements and the previous dielectric studies show that the first-order phase character of the phase transitions and the abrupt changes of properties are considerably reduced by the pre-transitional gradual effects on approaching TC. They indicate that the disorder of anions strongly destabilizes phase II of both compounds.

Figure 5. Displacements of neighboring cations and anions: (a) the z-displacements between neighboring anions (triangles) and the z-displacements between neighboring cations dabcoH+


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(circles, calculated for the centroids of all C6N2 atoms, cf. Figures 3 and S2); (b) as well as the ydisplacements between the cation and anion. The z-, x- and y-displacements values are listed in Table S1. Plots 5a, 5b and Table S1 compile the presently measured data and those reported previously.5,10 Both the dabcoHClO4 and dabcoHBF4 structures in phases I are strongly disordered, as illustrated in Figures 3 and S4. The own symmetry of the dabcoH+ cation is incompatible with the 4-fold axis passing through the nitrogen atoms, hence the cation is orientationally disordered in several partially occupied sites of ethylene bridges. According to our structural refinements, the disordered sites of the carbon atoms are located at most 1 Å apart, which is comparable to the resolution of the recorded X-ray diffraction data. According to this structural information the cations can dynamically rotate about the direction of NH+···N hydrogen bonds, parallel to the [z] axis, which is consistent with the results of 1H-NMR studies.24 In phase I, both the anions ClO4 and BF4 similarly are located at the special position in this way that one of the bonds, Cl–O and B–F is collinear with the 4-fold axis, while the three remaining atoms are disordered around. Moreover, these three disordered atoms lie approximately in one plane and the anion is disordered with respect to this plane to its both sites, as illustrated in Figure 3. So the Cl and B atoms are disordered too, and located apart 0.530.56 Å in dabcoHClO4 and 0.32 Å in dabcoHBF4. Finally, the presently revealed space-group symmetry P4/nmm does not imply the disordering of protons in hydrogen bonds NH+···N linking the cations into chains. The considerable disorder of the structures and large thermal parameters does not allow reliable location of this proton from the X-ray diffraction data. However, due to this strong disorder of cations and anions, it is likely


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that protons can be disordered, too. It is also possible that the NH+···N bonded chains remain polarized in the opposite directions, like in ferroelectric phases II. The main difference between the presently described models of phase I in dabcoHClO4 and dabcoHBF4 compared to the previous model is that there are no mirror planes perpendicular to the chains (Figure 3), so the cations and anions are not restricted by symmetry to be located at precisely the same level along [z]. Hence, the z-displacements of the cations in phase I (Figure 5). There is no symmetry restriction for disordering the protons in the NH+···N bonds, either. In order to understand the structural changes on approaching the paraelectric transition within phase II, we have performed the neutron-diffraction experiment at (TC-10) K=368 K (Table 2). This structure is shown in Figures 3 and 6. Most strikingly, this structural determination shows that the proton remains ordered even at (TC-10) K. The thermal ellipsoids clearly show the main direction of vibrations of the cation, the strongest about the [z] axis, but also very strong about the mass centre of the cation. Surprisingly, the thermal vibrations of atoms in the anion are smaller, however the anion is disordered in two sites, like in phase I. It shows that the tumbling motion of the anion is easiest activated and that the strong rotations of the cations precede the phase transition. On approaching TC the rotations about axis [z] of the dabcoH+ cations, the largest ions in the structure, contribute strongly for increasing the symmetry of orthorhombic phase II toward tetragonal phase I. The occupation factor of two sites of disordered perchlorate anion are 0.80:0.20 at (TC-10) K. According to X-ray diffraction measurement at 370 K the site occupation of the perchlorate group is 0.73:0.27.


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Figure 6. The crystal structure of dabcoHClO4 in phase II at 368 K (TC-10 K), with thermal ellipsoids plotted at the 50% probability level. The entropy change in dabcoHBF4 and dabcoHClO4 was determined previously by DSC and DTA methods,1,2,24 but the data were analyzed only in the vicinity of the ferroelectricparaelectric phase transition. However, our recent study5 indicates that a large pre-transitional thermal effect occurs in these materials in a wide temperature range below the transition temperature, as can be clearly seen in Fig. 7. The total entropy changes associated with the transition could be obtained after carefully approximating the baseline of Cp(T) and then by integrating its anomalous parts. It occurs that about 50% of the entropy gain comes from the pretransitional phenomena. The DSC runs show that the anomalies are strongly extended toward low temperature and that the structures become progressively disordered starting at about 230250 K. Moreover, the entropy gain testifies that the disorder should involve, apart from the C3 reorientations of dabcoH+ cations observed by 1H NMR technique,24 also other modes of motions, including jumps of the ions between different sites in the crystal lattice. The interpretation of calorimetric data is supported by the results of electric permittivity


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measurements in dabcoHBF48 and in dabcoHClO4.5 In both crystals the electric permittivity anomalously increase much stronger along the non-polar direction [z] than the polar direction [y]. This large increase of electric permittivity can be ascribed to dipolar fluctuations of ionic hopping along the [z] direction. The calorimetric and dielectric characteristics of the crystal are consistent with the progressing structural disorder observed by X-ray and neutron diffraction in phase II on approaching TC.

Figure 7. The entropy change associated with the ferroelectric-paraelectric phase transitions and pre-transitional effects in dabcoHBF4 (a) and dabcoHClO4 (b). In the insets showing the DSC cooling and heating runs the transition peaks have been truncated in order to better emphasize the contributions from the pre-transitional phenomena. Phase II of dabcoHClO4 is stable up to 0.22 GPa and above this pressure it transforms to phase III. Phase II of dabcoHBF4 at 0.45 GPa transforms to its phase IV. DabcoHClO4 and dabcoHBF4 are isostructural in their room-temperature phases II (space group Pm21n), as well as in their high-pressure phases III and IV (space group Pc21n), respectively. The pressure-induced phase IV retains the ferroelectric properties. The transition between the ferroelectric phases III and IV


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is associated only with a subtle dielectric anomaly, as evidenced in our previous study [7]. It is consistent with the symmetry of phase IV. The discontinuous changes of the unit-cell parameters and of molecular volume in dabcoHBF4 as a function of temperature (Figure 8) testifies that the transitions between phases I, II and III are of the first-order type. The dielectric and calorimetric data indicate that the transition between phases II and IV is of the second-order type.7 It is confirmed by the presently measured diffraction data showing that the superstructure reflections are hardly detectable behind the boundary between phases II and IV. The intensity of these reflections doubling the unit-cell dimension along [z] gradually increase with pressure.

Figure 8. Lattice dimensions of dabcoHBF4 as a function of temperature (red lines and symbols) and pressure (black symbols and lines): (a) unit-cell parameters; and (b) molecular volume (V/Z). The lines joining the points are for guiding the eye only. The vertical dashed lines mark phase boundaries. Plots 8a and 8b compile the presently measured data and those reported previously.10 Monohydrate dabcoHBF4·H2O


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High pressure can influence the course of crystallization of materials and the weak hydrogen bonds are relevant for the formation of new polymorphs or solvates.25-31 For all dabco salts studied at high-pressure conditions, either new phases or solvates were formed.32-38 Likewise, we have found that dabcoHBF4 forms a hydrate at 1.06 GPa, when crystallized from water solution. No other solvates have been obtained, although recrystallizations of dabcoHBF4 from methanol or ethanol or their 1:1 or 3:1 mixtures with water in isochoric conditions were tried. It appears that water is most prone to crystallize with dabco salts. In those hydrates, the water molecule usually is involved in bonds NH+···O or OH···N and the NH+···N bonds are not formed (Figure S6). In this respect the new hydrate dabcoHBF4·H2O is exceptional, because in its structure cations dabcoH+ are NH+···N hydrogen bonded into linear chains, similar to those in dabcoHX salts, while water molecules are OH···O bonded into zigzag chains with BF4 anions attached on both sides by OH···F bonds. The disordered NH+···N bonds in polycationic chains are analogous to those in dabcoHX relaxors, and therefore it is possible that monohydrate dabcoHBF4·H2O is a relaxor ferroelectric, too. This feature resembles another hydrated NH+···N bonded relaxor ferroelectric, dabco hydrochloride trihydrate (dabcoHCl·3H2O).39 In the dabcoHCl·3H2O hydrate the Cl − anions and water molecules are disordered, occupying the same sites in corrugated sheets. The structure of dabcoHBF4·H2O is ordered except for the protons in the NH+···N bonds, the latter being characteristic of NH+···N bonded ferroelectrics. This crystal can be considered as a host-guest structure, where the (H2O···BF4)n ribbons are contained between NH+···N bonded polycationic chains (Figure 9).


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Figure 9. Crystal structure of hydrate dabcoHBF4·H2O (a) projection along the chains parallel to axis [x]; and (b) one zigzag OH···O and OH···F bonded chain and two neighboring NH+···N bonded polycations. Shortest contacts are marked by dashed lines. Conclusions We have shown that ferroelectric crystals dabcoHClO4 and dabcoHBF4 are isostructural in their paraelectric (I) and ferroelectric (II) phases. Their phases III and IV, respectively, are isostructural, too. The mechanism of pretransitional transformations and dynamics in phase II indicate that the largest role in destabilizing phase II is played by the tumbling of anions and strong rotations of the cations, while the protons remain ordered to (TC-10) K at least. This observation is consistent with the marginal negative isotope effect of the H/D substitution of the acidic protons with deuterons on the TC value, of about 2 K lower for the deuterated forms,1 compared to the positive isotope effect on TC of over 100 K for KD2PO4 than that in KH2PO4.40 The high-pressure recrystallization of dabcoHBF4 of aqueous solution yields the monohydrate dabcoHBF4·H2O with NH+···N bonded polycationic chains, with the disordered protons, analogous to those in NH+···N bonded stoichiometric anisotropic relaxors dabcoHI,13 dabcoHBr12 and dabcoHCl·3H2O.39 Our study shows analogies and specific features of NH+···N bonded crystals with classical OH···O bonded (KDP-type) ferroelectrics. It appears that in


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NH···N bonded crystals their properties can be engineered by changing the composition, environment and dimensions of their NH+···N bonds.

ASSOCIATED CONTENT Supporting Information: structural drawings, disorder of ions, plots, comparisons of crystal phases. AUTHOR INFORMATION Corresponding Author *[email protected] ACKNOWLEDGMENT We are grateful to Prof. Garry McIntyre for his assistance in the neutron-diffraction experiments performed at Institut Laue-Langevin in Grenoble. REFERENCES (1)

Katrusiak A.; Szafrański M. Ferroelectricity in NH···N Hydrogen Bonded Crystals. Phys. Rev. Lett. 1999, 82, 576–579.

(2)

Szafrański, M.; Katrusiak, A. Thermodynamic Behaviour of Bistable NH+···N Hydrogen Bonds in Monosalts of 1,4-Diazabicyclo[2.2.2]octane. Chem. Phys. Lett. 2000, 318, 427– 432.

(3)

Katrusiak, A. Proton Dynamics in NH+⋯N Hydrogen Bond in the Paraelectric Structure of 1,4-Diazabicyclo[2.2.2]octane Perchlorate. J. Mol. Struct. 2000, 552, 159–164.


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Katrusiak, A.; Ratajczak-Sitarz, M.; Grech, E. Stereochemistry and Transformations of NH+···N Hydrogen Bonds Part II. Proton Stability in the Monosalts of 1,4Diazabicyclo[2.2.2]octane. J. Mol. Struct. 1999, 474, 135–141.

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Olejniczak, A.; Anioła, M.; Szafrański, M.; Budzianowski, A.; Katrusiak, A. New Polar Phases of 1,4-Diazabicyclo[2.2.2]octane Perchlorate, An NH+···N Hydrogen-Bonded Ferroelectric. Cryst. Growth Des. 2013, 13, 2872–2879.

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Anioła, M.; Olejniczak, A.; Katrusiak, A. Pressure-Induced Hydration of 1,4Diazabicyclo[2.2.2]octane Hydroiodide (dabcoHI). Cryst. Growth Des. 2014, 14, 2187– 2191.

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Szafrański, M. Low-Temperature and High-Pressure Phase Transitions in Ferroelectric DabcoHBF4. J. Phys. Condens. Matter., 2004, 16, 6053–6062.

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Szafrański, M.; Katrusiak, A. Short Range Ferroelectric Order Induced by Proton TransferMediated Ionicity. J. Phys. Chem. 2004, B108, 15709–15713.

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Szafrański, M. A. High-pressure Crystallography. Kluwer, Dordrecht 2004, 295–310.

(10) Budzianowski, A.; Katrusiak A.; Szafrański M. Anomalous Protonic-Glass Evolution from Ordered Phase in NH···N Hydrogen-Bonded DabcoHBF4 Ferroelectric. J. Phys. Chem. B 2008, 112, 16619–16625. (11) Szafrański, M.; Katrusiak, A.; McIntyre, G. Ferroelectric Order of Parallel Bistable Hydrogen Bonds. Phys. Rev. Lett. 2002, 89, 2155021–2155074.


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(12) Budzianowski, A.; Katrusiak A. Pressure Tuning between NH···N Hydrogen-Bonded Ice Analogue and NH···Br Polar dabcoHBr Complexes. J. Phys. Chem. B 2006, 110, 9755– 9758. (13) Olejniczak, A.; Katrusiak, A.; Szafrański, M. Ten Polymorphs of NH+···N HydrogenBonded 1,4-Diazabicyclo[2.2.2]octane Complexes: Supramolecular Origin of Giant Anisotropic Dielectric Response in Polymorph V. Cryst. Growth Des. 2010, 10, 3537– 3546. (14) Nowicki, W.; Olejniczak, A.; Andrzejewski, M.; Katrusiak, A. Reverse Sequence of Transitions in Prototypic Relaxor 1,4-Diazabicyclo[2.2.2]octane. CrystEng Comm 2012, 14, 6428–6434. (15) Merrill, L.; Bassett, W. A. Miniature Diamond Anvil Pressure Cell for Single Crystal XRay Diffraction Studies. Rev. Sci. Instrum. 1974, 45, 290–294. (16) Piermarini, G. J.; Block, S.; Barnett, J. D.; Forman, R. A. Calibration of the Pressure Dependence of the R1 Ruby Fluorescence Line to 195 kbar. J. Appl. Phys. 1975, 46, 2774– 2780. (17) Mao, H. K.; Xu, J.; Bell, P. M. Calibration of the Ruby Pressure Gauge to 800 kbar under Quasi‐Hydrostatic Conditions J. Geophys. Res. 1986, 91, 4673–4676. (18) Xcalibur CCD System, CrysAlisPro Software System, version 1.171.33; Oxford Diffraction Ltd.: Wrocław, Poland, 2009. (19) Budzianowski, A.; Katrusiak A. High-pressure Crystallography. Kluwer, Dordrecht 2004, 101–112.


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For Table of Contents Use Only

Pressure-temperature phase diagrams and transitions mechanisms of hybrid organic-inorganic NH--N bonded ferroelectrics Anna Olejniczak, Marek Szafrański and Andrzej Katrusiak

Synopsis: Structural transformations of dabcoHX crystals (dabco=1,4-diazacyclo[2.2.2]octane; X=ClO4, BF4) at TC in their ferroelectric and paraelectric phases are correlated with the dielectric and calorimetric properties.


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Pressure-temperature phase diagrams and transitions mechanisms of

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