Origin of Metastable Properties in the Ferroelectric ... - ACS Publications

Article pubs.acs.org/JPCC

Origin of Metastable Properties in the Ferroelectric Phase of Tetraguanidinium Dichloro-Sulfate Marek Szafrański,*,† Maria Połomska,‡ and Jacek Wolak‡ †

Faculty of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Poznań, Poland Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Poznań, Poland

‡

ABSTRACT: Single-crystal X-ray diffraction, calorimetry, and temperature- and timedependent Raman spectroscopy have been employed to study anomalous properties of ferroelectric tetraguanidinium dichloro-sulfate. A clearly metastable character of the changes in the Raman spectrum has been ascribed to the structural features of the crystal in the intermediate phase II. This phase of space group Fmm2 is strongly disordered, both in cationic and anionic sublattices, which results in a large number of configurations with local energy minima. This implies a quasistatic or long-time relaxation character of the configurations, which are separated by an energy barrier from the state corresponding to the global energy minimum in the room-temperature ferroelectric phase III. These features play a key role in the slow kinetics of the II/III phase transition. The structural evidence of the guanidinium cations disordering in phase II is very unusual because of the disordering which develops in low temperature from the high-temperature ordered state.

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INTRODUCTION Ferroelectrics are multifunctional materials of diverse technological applications.1,2 In the search for new materials with superior properties, several guanidinium compounds, with complex anionic sublattices formed of divalent and monovalent ions, have been synthesized. The main goal was to induce ferroelectric polarization through the interaction of the shape and valency differentiated anions with the planar monovalent guanidinium cations. This approach resulted in three new materials showing ferroelectric properties.3−5 Two of them, i.e., tetraguanidinium dichlorosulfate, [C(NH 2 ) 3 ] 4 Cl 2 SO 4 (G 4Cl 2 SO 4 ), and tetraguanidinium dibromosulfate, [C(NH2)3]4Br2SO4 (G4Br2SO4), are the above room-temperature ferroelectrics. Although their paraelectric phases are isostructural in space group I42̅ m, the sequence of phase transitions and the crystals properties below TC are considerably different. The crystal of G4Br2SO4 undergoes a single first-order ferroelectric-to-paraelectric phase transition at TC = 365.5 K, while G4Cl2SO4 on heating first enters the intermediate phase II around 352 K, and next at TC = 356 K reaches the paraelectric phase I. The sequence of these two first-order phase transitions depends on the thermal history of the sample and on the rate of temperature changes.3,6 Noteworthy is a huge entropy gain associated with these transitions. This property and the large crystal polarization and convenient temperature range suggest a potential usefulness of G4Cl2SO4 for solid-state cooling applications employing an electrocaloric effect.7,8 On the other hand, different crystal behavior on heating and on cooling through the transitions region may be a disadvantage. The explanation of these unusual crystal properties requires a better understanding of the transition mechanism. However, this is hindered because of a lack of the © 2014 American Chemical Society

crystal structure determination in the intermediate phase. Therefore, here we have undertaken this challenging task by using single-crystal X-ray diffraction. The structure determination is supported by Raman spectroscopic and calorimetric studies. Raman spectroscopy is a useful tool for studying the relationship between the macroscopic properties of the crystal and its molecular structure. In phase transitions of the displacement type, the soft modes can be observed in the Raman spectra, i.e., the vibrations whose frequencies critically tend to zero when the crystal approaches the phase transition temperature. Soft modes appear at very low frequencies, much lower than 100 cm−1. In the case of the order−disorder phase transitions, the soft modes are not observed, but the ordering process strongly affects the vibrations in which the disordered atoms or molecular groups are involved. The temperature dependencies of Raman spectra reveal changes in the bands position, in the spectral width of the lines, and in the lines intensity. Such changes are directly related to the mechanism of structural phase transitions.9 Therefore, the study of the dynamics of the molecular groups in G4Cl2SO4 should be helpful in better understanding of the transition mechanism and the crystal behavior at varied thermodynamic conditions.

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EXPERIMENTAL SECTION The crystals of G4Cl2SO4 were synthesized by dissolving C(NH2)3Cl and [C(NH2)3]2SO4 in a molar ratio 2:1 in water. The substance obtained after evaporating the solvent was Received: February 24, 2014 Revised: June 24, 2014 Published: June 26, 2014 15556

dx.doi.org/10.1021/jp501920x | J. Phys. Chem. C 2014, 118, 15556−15564

The Journal of Physical Chemistry C

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RESULTS AND DISCUSSION DSC Measurements. Detailed calorimetric studies, which have been done previously using the differential thermal analysis (DTA) method, have shown different behavior of the crystal in the heating and cooling runs, and provided information on the time-dependent evolution of the thermal effects associated with the phase transitions.3 Here we have reinvestigated the thermal anomalies observed in G4Cl2SO4 by using the DSC method, which is better suited for the thermodynamic parameters determination. As shown in Figure 1 the thermal anomaly recorded on heating the sample is

purified by 3-fold recrystallization from water solution. Colorless and transparent single crystals were grown by slow evaporation of a saturated aqueous solution at room temperature.3 Fourier transform Raman spectra were obtained with a Bruker ISF66 FTIR spectrometer equipped with FRA 106 Raman module. The polycrystalline samples were excited at 1064 nm by a diode pumped Nd:YAG laser with maximum output power of ∼300 mW. Raman spectra were measured at 180° geometry with a spectral resolution of 2 cm−1. A Linkam cooling/heating stage was used for temperature-dependent studies in the temperature range 100−450 K. The temperature of the sample between the measuring points was changed at a rate of 0.5 K/min. Each spectrum was recorded for 15 min at the temperature stabilized within ±0.1 K. The position of the Raman bands and their full width at half-maximum (FWHM) were derived by using the OPUS software from Bruker. Calorimetric measurements were performed by differential scanning calorimetry (DSC) on a Q2000 (TA Instruments) instrument. The DSC runs were recorded on heating and on cooling the polycrystalline sample prepared by grinding a single crystal. The temperature was changed at a rate of 10 K/min. Pure indium was used for temperature and enthalpy calibration. The single-crystal X-ray diffraction data were collected on a Gemini A Ultra diffractometer operating with graphitemonochromated MoKα radiation. The CrysAlisPro software10 was used for the data collection and processing. The data were measured at 348 K. The temperature of the sample was stabilized within 0.1 K with a nitrogen stream using a Cryostream Plus (Oxford Cryosystems) attachment. The crystal structure was solved by direct methods with SHELXS97 program11 and refined by full-matrix least-squares method on all intensity (F2’s) data using the SHELXL97 program.12 All the heavy atoms were refined with anisotropic thermal parameters. The H atoms were located from geometry and refined with isotropic thermal parameters Uiso, assumed as 1.2 times Ueq of their closest heavy atoms. The crystal data together with experimental and refinements details are listed in Table 1. The final atomic coordinates and full documentation have been deposited in the Cambridge Crystallographic Database Centre as supplementary publication CCDC 966421.

Figure 1. DSC heating and cooling runs of the powdered G4Cl2SO4 sample, measured at a temperature rate of 10 K/min.

associated with the entropy gain of 33.3 J mol−1 K−1, while on cooling the entropy change is only of 7.6 J mol−1 K−1. Noteworthy is that no additional thermal anomaly has been observed on cooling the sample up to the lowest accessible temperature of 95 K. This indicates that on cooling from the high-temperature phase I a huge entropy is retained in the crystal lattice, and the crystal remains in the high-entropy state. The most probable scenario is that the reverse phase transition is realized between phases I and II.6 Thus, having determined the total entropy change concomitant with both phase transitions, and the transition entropy ΔSI/II, the entropy difference between phases III and II could be roughly estimated as ΔSII/III = 25.7 J mol−1 K−1. Structure in the Intermediate Phase and Crystal Disorder. A slow kinetics of the transition between the phases III and II has been observed in dielectric and optical experiments.6 This feature is the main reason for the coexistence of the three crystal phases in the narrow temperature range of phase II. Therefore, it is extremely difficult to obtain a pure phase II by heating the roomtemperature phase III, even though the heating is very slow. On the other hand, when the crystal is cooled from the hightemperature phase I, it becomes twinned at the reverse phase transition, which usually impedes the structure determination. The temperature range for phase II stability, between 352 and 356 K, was estimated from the previous studies.3 Therefore, in this experiment the crystal was heated to 352.5 K, and it was kept at this temperature for 24 h before the data collection.

Table 1. Crystal Data and Structural Refinement Details for G4Cl2SO4 in the Intermediate Phase II at 348 K temperature (K) crystal system space group unit cell: a (Å) b (Å) c (Å) V (Å3) Z, calc density (g/cm3) θ range (deg) refl collected/unique/Rint data/restraints/parameters goodness-of-fit on F2 indices R1/wR2 (I > 2σI) R1/wR2 (all data) largest peak and hole (e·Å−3)

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348 orthorhombic Fmm2 9.4219(4) 14.1922(5) 14.6060(6) 1953.08(14) 4, 1.385 2.79−32.27 15384/1813/0.0229 1813/1/111 1.355 0.0923/0.2846 0.0948/0.2890 0.471 and −1.035 15557



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sites are situated on the surface of 3.5−6 Å in diameter, which resembles a distorted ball. It is interesting that the space in the interior of the “guanidinium ball” is empty. The SO42− and Cl− anions occupy sites in the voids between the “balls”. The packing of the crystal is shown in Figure 2b. Obviously this picture represents an average model of the structure, which can be a result of a dynamic disorder as well as a static one. Although it is not possible to judge from X-ray diffraction study between these two options, we believe that the static or quasistatic disorder is more probable in the case of G4Cl2SO4 phase II. This is supported by the slow kinetics of the transition between phases III and II, and by the metastable properties of the crystal when it is cooled from the paraelectric phase through the phase transitions region.3,6 In Figure 2a two possible configurations of guanidinium cations are marked by light and dark colors. The number of configurations is limited by steric reasons. But, on the other hand, locally in the crystal structure the “guanidinium balls” can be arranged randomly. Thus, in this respect phase II is reminiscent of glassy systems. The high degree of disorder in phase II is consistent with the results of calorimetric measurements. The experimental value of the transition entropy ΔSII/III = 25.7 J mol−1 K−1 is close to R ln 22 (R is a gas constant), which is in agreement with a large number of configurations predicted in phase II on the basis of the structural model. Noteworthy is that the disorder of the cationic sublattice in phase II is surprising in the light of the high-temperature crystal structure. In phase I the guanidinium cations are clearly ordered,3 and thus only one of the two guanidinium configurations, shown in Figure 2a, is realized. Despite the crystal ordering within the cationic sublattice the total entropy rises at the transition to the paraelectric phase. This puzzling issue can be explained by taking into account a higher degree of disorder of the SO42− anions in phase I. The 2-fold disorder of the sulfate anion, which is observed in phase II, would also satisfy the crystal symmetry in the tetragonal phase. However, the modeling of the high-temperature structure has shown that the oxygen atoms of the anion occupy many more sites than in phase II, and it has been postulated that the anions can rotate about their gravity centers.3 Thus, the triggering of free or almost free rotations of the sulfate anions can compensate, with an excess, the crystal entropy loss due to the ordering of the cations. Raman Studies. The room-temperature G4Cl2SO4 structure belongs to the orthorhombic class mm2, space group Cmc21 (C2v12). The crystal unit cell contains four chemical formula units.3 The factor-group analysis13 has shown that the dynamics of the crystal can be expressed in terms of 564 phonon modes. These modes (Γopt, Γac) are distributed among the symmetry species in the following form.

However, it appeared that the reflections collected could be indexed using the high-temperature tetragonal unit cell of phase I. Thus, the transition temperature to the paraelectric phase was by about 4 K lower than expected. Taking this into account, the subsequent crystals were heated to a slightly lower temperature, while the time of temperature stabilization before the data collection was increased. The best data set was collected at 348 K after the crystal temperature stabilization for 48 h. No any trace of phase transition has been observed during the 12-h data collection. Therefore, we can conclude that the narrow temperature range of phase II is situated around 348 K, which is by several degrees lower than reported previously.3,6 It is plausible that this discrepancy originates from a different thermal treatment of the crystal in this study and in the previous works. The rate of temperature changes affects not only the transition temperatures but also the temperature range of the transitions.6 The structure was solved in orthorhombic space group Fmm2, consistent with the predictions.3 But the structure refinement should be carried out very carefully because of a large extent of the crystal disorder. The oxygen atoms of the SO42− ions were found to be split, each in two sites with the occupancy factors 0.5. The refinement procedure has shown that also guanidinium cations occupy a doubled number of sites at the halved occupancy factors. As shown in Figure 2a these

Γtotal = (A1 + B1 + B2 )ac + (187A1 + 94A 2 + 187B1 + 93B2 )opt

The sulfate anions of approximate tetragonal symmetry Td, and the guanidinium cations only slightly distorted from the hexagonal symmetry D3h may be considered as molecular entities relatively weakly bonded to the crystal lattice through the system of N−H···O hydrogen bonds. The Raman spectrum of G4Cl2SO4 measured at 103 K is presented in Figure 3. At low temperature the lines were narrow, and their centers could be precisely determined. Table 2 shows the Raman bands positions for three different

Figure 2. (a) Eight disordered guanidinium cations (each with the occupancy factor 0.5) forming a distorted “ball”, the disordered SO42− and the ordered Cl− anions, which constitute the formula unit of G4Cl2SO4. The disordered sites are highlighted with light and dark colors. The thermal ellipsoids have been drawn at the 50% probability level. (b) Crystal structure in the intermediate phase II at 348 K viewed along the crystallographic a direction. 15558



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The temperature evolution of the external vibrations is shown in Figure 4a. Generally, at room temperature and also in

Figure 3. Raman spectrum of G4Cl2SO4 measured at 103 K.

Table 2. Frequencies of Raman Bands (in cm−1) at 103, 300, and 480 K, and Tentative Assignments of Vibrationsa 103 K

300 K

480 K

96 (vs) 117.9 (s) 158 (sh) 457 (w) 520

95 (vs) 119.1(s)

105.2 (s)

455 (w)

453 (w)

531(s) 549 (s) 619 (w) 733.2 (vw)

532 (s) 547 (s) 616 (w)

532 (s) 620.8 (w)

ν4 (SO4) bending (CN3) angle deformation (out-of-plane)

979 (s) 1010.7 (vs)

980 (s) 1005.9 (vs)

ν1 (SO4) symmetric stretching νs (CN3) symmetric stretching ν3 (SO4) stretching

745.6 (vw) 773 (vw) 980 (s) 1011 (vs) 1080 (vw) 1463 (vw) 1548.6 (w) 1570 (w) 1575

1547 (w) 1567.8 (w) 1582 sh

1573.3 (w) 1574

1640 (w)

1641(w)

1643 (w)

1650 (w) 3203 (w) 3243 (w)

tentative assignment lattice vibrations lattice vibrations lattice vibrations ν2 (SO4) bending (CN3) angle deformation (inplane) (NH2) wagging

Figure 4. Temperature evolution of Raman spectrum in the range of external modes (a) and the temperature dependence of the external modes positions (b).

δ(NH2) bending δ(NH2) bending ν(CN3) stretching and δ(NH2) bending νas (CN3) asymmetric stretching

low temperatures, two broad bands are observed, but it is difficult to assign particular bands to the librational and transitional modes of guanidinium and sulfate ions. As the temperature rises, the bands broaden and merge close to the phase transition (Figure 4a). The temperature dependences of the external modes frequencies are plotted in Figure 4b. Above TC the position of these Raman lines is almost temperature independent. The lines observed above 200 cm−1 are related to the internal vibrations of guanidinium and sulfate ions. The most intense lines arise from the guanidinium ion vibrations (see Table 2). The evolution of Raman spectrum in the frequency range of internal vibrations of SO42− and C(NH2)3+ is presented in Figure 5. Because of the limitations of the pyroelectric detector applied in the spectrometer, we were not able to analyze the temperature behavior of the stretching CH2 modes in the frequency range 3000−3400 cm−1. Internal Vibrations of SO42−. The free SO42− ion has Td symmetry. Its four fundamental vibrations have the following

νs (NH2) symmetric stretching νas (NH2) asymmetric stretching

3330 (w) a

Abbreviations: vs - very strong, s - strong, w - weak, vw - very weak, sh - shoulder.

temperatures and the tentative assignments of vibrations. The Raman lines were assigned using earlier Raman studies of materials containing guanidinium ion.14−19 The bands observed in the low energy range between 50 and 200 cm−1 arise from the external vibrations related to the translational and librational modes of molecular groups. 15559



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Figure 5. (a−c) Temperature evolution of Raman spectrum of G4Cl2SO4 in the vicinity of phase transitions for the three selected ranges of frequencies.

frequencies:20 the nondegenerate ν1(A) = 981 cm−1 and triply degenerate ν3 (F2) = 1104 cm−1 stretching vibrations, and the doubly degenerate ν2(E) = 451 cm−1 and triply degenerate ν4(F2) = 613 cm−1 bending vibrations. As it is seen in Figure 5 and Table 2, the frequencies of the sulfate ion vibrations are very close to those reported for “free” ion. The most intense line corresponds the nondegenerate symmetric stretching vibration ν1 at ∼978 cm−1 (Figure 5b). The doubly degenerate bending vibration ν2 generates a broad band with weak intensity, centered around 455 cm−1. The triply degenerate bending vibration ν4 gives also a weak and broad band centered at ∼616 cm−1 (Figure 5a). Similarly the triply degenerate stretching vibration ν3 is observed as a very weak band centered at ∼1080 cm−1. Raman intensities of the all bands assigned to the internal vibrations of SO42− ion decrease with increasing temperature (Figure 5a,b). The temperature dependences of the frequency and spectral width of the vibration ν1(SO4) are plotted in Figure 6. The frequency ν1 increases significantly in the temperature range of the two successive phase transitions,

reaching a value close to 981 cm−1 in the paraelectric phase, which is characteristic of the free SO42− anion. This confirms that in the high-temperature phase the anions occupy the sites with a potential energy allowing their almost free rotations. Noteworthy is also the spectral width of the line, which exhibits a maximum near 365 K. No similar dependences for other internal vibrations of SO42− could be analyzed because of the low intensity and significant broadening of the corresponding bands. Internal Vibrations of C(NH2)3+. The internal vibrations of guanidinium cation arise mainly from the vibrations of CN3 and CH2 entities. The most intense line at ∼1009 cm−1 in the Raman spectrum of G4Cl2SO4 (Figure 5b) is generated by the stretching vibration of CN3 skeleton. The temperature dependences of the frequency of this mode and of the band spectral width, are plotted in Figure 7. The frequency of νs(CN3) decreases conspicuously in the vicinity of the phase transitions, but it changes only slightly above TC. The line broadens monotonously with increasing temperature, except

Figure 6. Temperature dependence of the symmetric vibration ν1(SO4) frequency and spectral width.

Figure 7. Temperature dependence of the symmetric vibration νs(CN3) frequency and spectral width. 15560



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for the transition region, where its FWHM exhibits a clear maximum around 365 K. The stretching asymmetric vibration νas(CN3) does not show such spectacular changes, but its frequency only slightly decreases with rising temperature, as shown in Figure 8.

Figure 8. Temperature dependences of Raman bands positions of νas(CN3) asymmetric vibration of guanidinium skeleton and δ(NH2) bending vibration.

In contrast, the frequency of NH2 bending vibration increases remarkably when the crystal crosses the transition region (Figure 8). The continuous character of the changes observed in the temperature dependence of vibrations is not compatible with a first-order character of both phase transitions in G4Cl2SO4. However, this can be reconciled by taking into account a slow kinetics of the phase transition between phases III and II, and a coexistence of phases. It is worth noticing that in the dielectric measurements performed with a heating rate of 0.4−1.0 K/ min3,6 a qualitatively similar character of the transition anomalies was observed in the same temperature range of 360−370 K. The transition temperatures are significantly lowered when the crystal is heated in a long-term isothermallike manner, as evidenced by the present X-ray diffraction experiments. Effect of Thermal Treatment on the Molecular Dynamics in G4Cl2SO4. As it has been shown previously,3,6 the properties of G4Cl2SO4 strongly depend on the thermal history of the sample. To characterize the influence of thermal treatment on the molecular groups dynamics, Raman spectra have been measured in different heating/cooling cycles. In Figure 9 the temperature changes of νs(CN3) and ν1(SO4) vibrations during the consecutive heating/cooling cycles are presented. In the first run, the sample prepared from the as “grown” crystal was heated to 373 K and then cooled to 350 K. The temperature changes in both Raman bands positions are very unusual. The frequency νs(CN3) decreases significantly when the temperature approaches 373 K, but surprisingly, it continues to decrease also when the sample is cooled (Figure 9a). The only explanation is that at 373 K the transition was incomplete and still proceeded when the temperature was lowered. The crystal transformation during the cooling run was possible within the temperature hysteresis region, as the reverse phase transition from the paraelectric phase I to the ferroelectric phase II occurs around 354 K. This indicates that not only temperature but also

Figure 9. Temperature dependences of νS(CN3) (a) and ν1(SO4) (b) Raman bands of G4Cl2SO4 for different heating/cooling runs. The sample was heated to 373 K for 1 and 2 runs and to 393 K for 3 and 4 runs.

time factor plays a prominent role in the crystal transformation. At the end of this cycle at 350 K, the νs(CN3) frequency reaches a value that is about 2 cm−1 lower than at the starting point. Similar behavior was observed for the vibration ν1(SO4), as illustrated in Figure 9b, but in this case the frequency increases with rising temperature. The second heating was performed after keeping the crystal at room temperature for 24 h. It turned out that after this period of time the vibrations restored their initial frequencies, suggesting that the crystal returned to the low-temperature phase III. However, on heating the crystal the changes in the νs(CN3) frequency occurred at lower temperatures, which indicates a lowering of the phase transition temperature in the second measuring cycle. The third heating/cooling cycle was performed after the sample staying for 288 h at room temperature. At 350 K the frequency of νs(CN3) vibration was practically the same as for the first and second heating runs, while its temperature changes were similar to those observed in the second heating cycle. The 15561



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heating of the sample to 393 K ensured a completeness of the transition to the paraelectric phase. The cooling run, similarly as in the previous cycles, was essentially different from the heating run. The fourth cycle was performed after 24 h. Of note is that both νs(CN3) and ν1(SO4) vibrations exhibited, respectively, higher/lower frequencies at 350 K than in all previous cycles. Hence, it can be anticipated that after the reverse phase transition from the paraelectric phase I, the reconstruction of phase III required a much longer time than in a case when the transition was incomplete. Thus, it is apparent that when the crystal is cooled from the temperature region of the coexisting phases I, II, and III the presence of the seeds of phase III accelerates the restoration of this phase at room temperature. Such a conclusion can be drawn despite the similarities in the frequencies of particular vibrations, observed in the temperature region 350−360 K during cooling the sample from different temperatures (cf. Figure 9). Time Evolution of the Raman Spectra. The unusual changes in the Raman spectrum of G4Cl2SO4, observed during different heating/cooling runs through the phase transitions region, prompted us to perform time-dependent studies. The spectrum of a virgin sample was recorded at 298 K, and next the sample was heated to the paraelectric phase, where its spectrum was measured at 393 K. Next the sample was cooled to room temperature, and the measurements were performed as a function of time. The time evolution of Raman spectra in the frequency range of external vibrations is shown in Figure 10a. It is seen that just after cooling the sample from the paraelectric phase to 298 K the band centered around 120 cm−1 appears as a shoulder, but it progressively grows over the course of time. A similar behavior was found in the frequency range of internal vibrations, where the most distinct time-dependent changes were observed in the range of NH2 deformation vibrations. As it is evident from the plots in Figure 10b, the changes in the range 1500−1600 cm−1 proceed continuously, but a full restoration of the initial state requires at least several days. At the reverse phase transition from the paraelectric phase I, the crystal enters the multiconfigurational phase II, which at room temperature slowly recrystallizes into phase III. Thus, immediately after cooling the crystal from the paraelectric phase, the Raman spectrum is a superposition of two spectra characteristic of phases II and III. The changes observed over the course of time reflect progressive changes in the ratio of both phases in the sample. The process of phase III restoration is also clearly seen in the time dependence of νs(CN3) and ν1(SO4) frequencies, plotted in Figure 11a. Of note is the opposite direction of changes when compared to those observed during the sample heating through the transition region (see Figures 6 and 7). It is also interesting that the bands spectral width increased for several hours, reaching a maximum after about 5 h, and then it started to decrease. An exemplary plot of FWHM for νs(CN3) is shown in Figure 11b. The broadening/narrowing of the band as a function of time resembles the temperature-induced changes in FWHMs presented in Figure 7. Indeed, the mechanism of this behavior is the same. In both cases, the anomaly in FWHM is a consequence of the coexistence of different crystal phases, for which the frequencies of vibrations are different. The observed bands result from a convolution of the lines corresponding to the vibrations of the coexisting phases.

Figure 10. Time evolution of the Raman spectra of G4Cl2SO4 in the range of external vibrations (a) and in the range of bands related to bending vibration of NH2 group and asymmetric stretching vibration of CN3 group (b). Dashed lines present Raman bands measured for sample “as grown” at room temperature and after heating to the paraelectric phase at 393 K.

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CONCLUSIONS The intriguing properties of ferroelectric G4Cl2SO4 have successfully been explained on the basis of the structural model determined for the crystal intermediate phase II, and the extensive Raman spectroscopic studies. The intermediate phase of the crystal is strongly disordered, but it retains polar symmetry in space group Fmm2 and ferroelectric features. A large number of configurations in this phase is crucial for the crystal properties. It is highly probable that different configurations have similar energies separated by energy barriers. The crystal behavior strongly suggests that the minimum of Gibbs free energy of phase III is also separated by a barrier from the local energy minima of phase II. Such a situation, characteristic of a metastable state,21 hinders a smooth transformation between phases II and III and implies a long reconstruction time of the structure when the crystal is cooled from the paraelectric phase. Hence, slow kinetics of the 15562



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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was partly supported by Polish Ministry of Science and Higher Education, Grant N N202 032240.

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Figure 11. Time dependence of the frequency of stretching symmetric vibration νs(CN3) and ν1(SO4) of G4Cl2SO4 (a) and of νs(CN3) spectral width (b).

phase transition, and the metastable behavior of the crystal beyond the stability region of phase II, were observed in the present Raman experiments, as well as in the previous calorimetric and dielectric studies.3,6 Such properties have not been observed for the isostructural (in paraelectric phase) G4Br2SO4, for which the intermediate disordered ferroelectric phase is not realized.4 The sequence of phases in G4Cl2SO4 is very unusual also with respect to the order−disorder processes within the cationic sublattice. The disordering of the guanidinium cations occurs in ferroelectric phase II on cooling from the paraelectric phase I where the cations are ordered. On further cooling, the cations order again in the room-temperature ferroelectric phase III. Such crystal behavior as a function of temperature is uncommonly rare. The previous similar observations for 1,4diazabicyclo[2.2.2]octane tetrafluoroborate concerned the protonic glass formation from the high-temperature ordered phase,22 whereas in the present case the disordering involves much heavier units.

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REFERENCES

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Bonded DabcoHBF4 Ferroelectric. J. Phys. Chem. B 2008, 112, 16619− 16625.

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Origin of Metastable Properties in the Ferroelectric ... - ACS Publications

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