Phase Transitions in Layered Diguanidinium Hexachlorostannate(IV)

Article pubs.acs.org/crystal

Phase Transitions in Layered Diguanidinium Hexachlorostannate(IV) Marek Szafrański*,† and Kenny Ståhl‡ †

Faculty of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Poznań, Poland Chemistry Department, Technical University of Denmark, Building 207, DK-2800 Lyngby, Denmark

‡

S Supporting Information *

ABSTRACT: Five crystalline phases of diguanidinium hexachlorostannate(IV), [C(NH2)3]2SnCl6, have been identified and characterized by calorimetric and dielectric measurements, single crystal X-ray diffraction at atmospheric and high pressure, and synchrotron X-ray powder diffraction. The crystal structures of all phases are built of similar layers in which the tin hexachloride anions are connected to the guanidinium cations by N−H···Cl hydrogen bonds, forming double H-bonded sheets. The layers, neutral as a whole, interact primarily by Coulombic forces between the ions from the opposing H-bonded sheets, and through the van der Waals contacts. From water solution the compound crystallizes at room temperature in phase III of space group C2/c. On heating, this phase transforms between 375 and 455 K to the hightemperature phase I of space group P1̅, either immediately or through the intermediate phase II of the same space group P1̅. The temperature range of phase II enhances meaningfully at elevated pressure, which made possible the high-pressure crystallization of this phase and determination of its structure. Different transition paths can be realized when the crystal is cooled from phase I: the reverse transition occurs in the monoclinic phase III or in the monoclinic phase IV (space group C2/m), or in the phase V of space group P1̅. In all phases the layered structure of the crystal is preserved, while the arrangement of the layers is different. The transitions involve also transformations in the networks of N−H···Cl hydrogen bonds. The large volume (∼3%) and entropy (∼R ln 3) change at the transition between phases II and III, and the giant pressure coefficient of −755 K GPa−1, indicate a great potential of this material for applications in solid-state cooling systems. are built from corner-sharing PbI6 octahedra.14 The onedimensional inorganic chains (SnCl4−2)n of square pyramids sharing corners are formed in diguanidinium tetrachlorostannate,15 [C(NH2)3]2SnCl4, whereas the crystals of guanidinium trichlorostannate, C(NH2)3SnCl3, crystallize in a perovskite-like structure with distorted SnCl6 octahedra sharing corners.16 A common feature of these crystals is a high susceptibility to thermodynamic parameters of temperature or pressure, modifying the interactions between the organic cations and inorganic framework, and resulting in structural phase transitions. The interest in this group of materials stems also from their potential photovoltaic applications. Recently reported studies17,18 have shown that guanidinium lead iodides can be considered as efficient absorbers for solar energy conversion. The structures presented here of diguanidinium hexachlorostannate, [C(NH2)3]2SnCl6, are topologically different from those of previously studied guanidinium compounds, as they contain separate SnCl6 octahedra linked into sheets through hydrogen bonding and electrostatic interactions with the guanidinium cations. To study the complex phase relations in

1. INTRODUCTION The guanidinium cation [C(NH2)3]+, with its planar and highly symmetric conformation and the six hydrogen atoms, serves as an applicative component in supramolecular chemistry. The capability of the cation to form hydrogen bonds has successfully been exploited for self-assembly of molecules into predictable structures with desired properties.1−11 Intense studies on this group of compounds have brought about new functional materials such as nanoporous crystals with adjustable porosity,2 smectic liquid crystals,4,5 and ferroelectrics.7−11 Various patterns of hydrogen bonds observed in the guanidinium complexes contribute in a different manner to the crystals properties. Of considerable interest are the guanidinium-based heterostructures with a low-dimensional architecture constrained by inorganic frameworks. An example is the structures with the anionic sublattices built of MX6 octahedra (M stands for tetravalent metal and X for halogen atom) sharing faces, edges, or corners. The layered sandwich-type crystals are the natural analogues of epitaxial layers, which are characterized by high layer homogenity, and therefore are convenient objects for studying electronic and optical phenomena resulting from sizerelated quantum effects.12,13 In the group of guanidinium salts such type of the structure has been reported for diguanidinium lead tetraiodide, [C(NH2)3]2PbI4, where the inorganic sheets © XXXX American Chemical Society

Received: December 28, 2015 Revised: February 17, 2016

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DOI: 10.1021/acs.cgd.5b01830 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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this material we employed calorimetric and dielectric methods at ambient pressure and at high hydrostatic pressures, as well as different X-ray diffraction techniques supported by highpressure crystallization.

2. EXPERIMENTAL SECTION 2.1. Synthesis and Crystal Growth. Diguanidinium hexachlorostannate was synthesized in aqueous solution by dissolving C(NH2)3Cl and SnCl4 in molar ratio 2:1. Large crystals were grown by slow evaporation at room temperature of the saturated solution acidified with a few drops of HCl. The crystals were colorless, of good optical quality, and stable in the temperature range up to about 520 K, where the onset of decay was observed (Figure S1). The crystals have a perfect cleavage plane parallel to (001) of the room-temperature phase, which hindered mechanical treatment of the samples. 2.2. DSC Measurements. Calorimetric studies were performed in the temperature range 95−490 K using a differential scanning calorimeter Q2000 (TA Instruments). Indium standard was used to calibrate temperature and enthalpy. The polycrystalline samples, prepared by grinding the crystals obtained from different crystallizations, were heated/cooled with a rate of 10 K/min. The representative results of DSC measurements are plotted in Figure 1. The three crystal phases, observed in the heating run of the freshly prepared sample, have been labeled as I, II, and III, beginning at the high-temperature phase. 2.3. Single Crystal X-ray Diffraction. The temperature-dependent single-crystal X-ray diffraction measurements were carried out with the graphite-monochromated Mo Kα radiation on a Gemini A Ultra diffractometer. The CrysAlisPro software19 was used for data collection and processing, and for absorption correction. The temperature of the crystals was stabilized within 0.1 K with a nitrogen stream generated by a Cryostream Plus (Oxford Cryosystems) attachment. SHELXS97 and SHELXTL97 programs were used for structure solution and refinement.20 Hydrogen positions were located in difference Fourier maps and refined without constrains for the room-temperature phase III, whereas for other phases the H atoms were located from the molecular geometry and refined in geometrically ideal positions after each cycle of the refinement, with isotropic thermal parameters set at 1.2 times Ueq of the carrier atoms. The single-crystal data together with experimental and refinement details are listed in Table 1. Selected distances and angles are collected in Tables S1−S8 (Supporting Information). The data for the crystal phase II were collected on the crystals grown in a high-pressure diamond anvil cell (DAC). The crystals were centered by the gasket-shadowing method.21 Because of the limited completeness of high-pressure data22 the anisotropic temperature factors were applied only for Sn and Cl atoms of SnCl62− octahedra, while for the guanidinium atoms N and C the isotropic thermal parameters were retained. The crystal data and structure refinements details for phase II at different pressures are collected in Table 2, and the selected distances and angles are listed in Tables S9−S14. Full documentation for all the structures has been deposited in the Cambridge Crystallographic Database Centre as CCDC nos. 1442688−1442694. 2.4. Synchrotron X-ray Powder Diffraction. The crossing of the transitions region usually lead to the fracturing of the samples. Moreover, at the reverse transition, on cooling to the low-temperature phase, the crystals broke into pieces scattered around, which indicated that a huge strain was generated at the transition point. Therefore, it was challenging to determine the high-temperature structures from single-crystal data. For those reasons we undertook parallel powderdiffraction studies. The data were collected at Beamline I711 at the MAX-II synchrotron23 in Lund, Sweden, using Huber G670 Guinier imaging strip camera.24 The high-temperature data were collected with a Huber G670.3 capillary furnace in the range 293−473 K. The wavelengths 1.18610(1) and 1.51080(1) Å were determined using a Si standard. The samples were contained in 0.3 mm quartz capillaries spinning during the measurements. On heating the sample through the phase transitions region only the high-temperature phase I could be

Figure 1. DSC runs measured for different polycrystalline samples (a) and the transition entropy change derived from the heating run D (b). The arrows in the chart (a) indicate the heating/cooling runs; the crystal phases are labeled by the Roman numbers. obtained in a pure form because of the coexistence of phases. Owing to a huge temperature hysteresis, the diffraction pattern of this phase could be collected in a wide temperature range. On the other hand, by quenching the high-temperature phase I in liquid nitrogen a new phase IV was obtained. The powder diffraction patterns were collected in the 2θ range from 3 to 100° in steps of 0.05°. The data were accumulated for 120 s in phase I and V, and for 60 s in phase IV. Indexing was performed with program ITO.25 The crystal structures were solved with EXPO.26,27 The Rietveld refinements28 utilized a locally modified and Windows adapted version of the LHMP program.29 The parameter set included: 2θ-zero and Chebyshev background; pseudo-Voigt half-width, peak shape, asymmetry and preferred orientation; and scale factor, unit cell, fractional coordinates and thermal parameters. Hydrogen positions were added in calculated B



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Table 1. Selected Data Collection and Refinement Parameters for [C(NH2)3]2SnCl6 phase

I

I

method wavelength (Å) temperature (K) space group Z no. of reflections no. of parameters R1/wR2 (I > 2σI) (%) R1/wR2 (all) (%) S/GOF Rp/Rwp/RBragg (%) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) ρ (g/cm3) μ (mm−1)

single-crystal 0.71073 450.0(1) P1̅ 1 1687 70 5.80/13.30 8.92/15.49 1.029

single-crystal 0.71073 330.0(1) P1̅ 1 1634 70 4.47/10.69 5.20/11.31 1.062

7.4073(10) 7.4131(11) 7.9241(22) 99.807(12) 105.595(12) 107.007(13) 385.86(9) 1.943 2.676

7.2751(4) 7.2778(5) 7.8739(6) 100.640(6) 104.055(6) 106.022(6) 374.28(5) 2.003 2.759

V powder 1.51080(1) 297(2) P1̅ 1 794 66

27.6 2.4/3.3/1.03 7.23805(12) 7.23945(12) 7.85711(14) 100.7899(12) 103.9132(10) 105.8118(15) 370.16(1) 2.025 21.93

III single-crystal 0.71073 297(2) C2/c 4 2164 93 1.65/3.93 2.07/4.17 1.137 15.1376(5) 8.6683(2) 13.6252(4) 90 121.003(4) 90 1532.46(7) 1.957 2.695

IV

V

powder 1.18610(1) 297(2) C2/m 2 937 46

single-crystal 0.71073 270.0(1) P1̅ 1 1941 70 4.08/9.09 4.69/9.56 1.069

18.6 4.49/6.1/3.06 11.7120(3) 8.74100(19) 9.8751(2) 90 130.8286(11) 90 764.96(3) 1.960 11.61

7.2114(4) 7.2144(4) 7.8282(5) 100.729(5) 103.813(5) 105.624(5) 366.96(4) 2.043 2.814

Table 2. Selected Crystallographic Data and Refinement Parameters for [C(NH2)3]2SnCl6 in Phase II at 0.11, 0.28, and 0.60 GPa (T = 293 K) pressure (GPa)

0.11

0.28

0.60

space group Z a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) ρcalc (g cm−3) refl. coll./unique Rint completeness (%) R1/wR2 (I > 2σI) R1/wR2 (all data)

P1̅ 1 7.2698(6) 7.2777(7) 7.625(4) 78.36(2) 88.212(19) 65.951(8) 360.25(19) 2.081 1516/248 0.0991 13.7 0.0492/0.1218 0.0497/0.1227

P1̅ 1 7.2006(18) 7.2134(8) 7.5845(19) 88.309(13) 78.53(2) 66.494(16) 353.49(13) 2.121 1512/237 0.0656 13.6 0.0259/0.0609 0.0266/0.0614

P1̅ 1 7.1088(14) 7.1317(8) 7.5306(15) 88.469(12) 78.589(17) 67.043(14) 344.06(10) 2.179 1419/237 0.0588 13.4 0.0308/0.0851 0.0309/0.0852

positions with N−H distances of 0.85 Å. Thermal parameters were refined freely for Sn and coupled for Cl and guanidinium ions. The powder diffractograms are shown in Figure 2, the data collection and refinement summary is given in Table 1, and the selection of distances and angles is listed in Table S15. It should be pointed out that the relatively high goodness-of-fits (18.6 and 27.6) is a consequence of the systematic nature of the peak misfits in combination with the very high intensities obtained from the synchrotron (cf. discussion in ref 30). The powder-diffraction data for phase IV at 297 K have been deposited in the International Centre for Diffraction Data, reference code I06019. These data and the data for phase V at 297 K are available in the form of data deposited with the CCDC. 2.5. High-Pressure Experiments. The T−p phase diagram of [C(NH2)3]2SnCl6 was studied by high-pressure differential thermal analysis (DTA) and high-pressure electric permittivity measurements.31 The studies were performed under pressures from 0.1 to 300 MPa in the temperature range between 100 and 420 K. The pressure was generated by a GCA-10 Unipress compressor using helium gas as a transmitting medium. A manganin gauge was applied for the pressure calibration with an accuracy of ±2 MPa. The DTA runs were recorded at the temperature rate of 2 K/min. Electric permittivity was measured with a Hewlett-Packard 4192A impedance analyzer at 100 kHz frequency of measuring field. For dielectric

measurements the sample was prepared in a form of cylindrical capacitor filled with a fine grounded polycrystalline material. The temperature of the sample was measured by a copper-konstantan thermocouple placed inside the high-pressure cell. The transition temperatures were determined as the onsets of the thermal or dielectric anomalies. The single crystal high-pressure diffraction studies were performed on the samples in situ crystallized in a modified Merrill-Bassett diamond anvil cell.32 The cell was equipped with 0.8 mm culet diamonds supported on steel backing plates. The gasket was made of 0.3 mm thick tungsten foil with a spark-eroded hole of 0.35 mm in diameter. Pressure in the DAC was calibrated by the ruby fluorescence method33 with a precision of ±0.05 GPa. The crystallizations of (C(NH2)3)2SnCl6 were performed in slightly acidified water/methanol (2:1) mixture. The pressure chamber was filled with the saturated solution and a few crystal grains of the compound. Several small ruby chips were placed in the cell for pressure calibration. After filling the DAC was sealed and pressure was increased. Then the cell was slowly heated until the compound dissolved. A careful lowering of temperature usually resulted in a nucleation of several crystals. To remove the unwanted seeds the cell was heated until a single grain remained. The crystal was grown by slow cooling of the DAC to room C



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Figure 2. Powder diffractograms for [C(NH2)3]2SnCl6 in phases V (a) and IV (b), recorded at 297 K. The red solid lines correspond to the best Rietveld fits to the experimental data (solid black line). The difference between the model and experimental diffraction pattern is shown at the bottom of each plot. In part (a) the bottom vertical bars correspond to α-quartz added as an internal standard. temperature. An exemplary crystallization at 0.6 GPa (final pressure at room temperature) is shown in Figure 3.

heating, and around 300−305 K on cooling the sample, whereas the second anomaly occurred in the temperature range 420−430 K and 350−365 K, respectively, on heating and on cooling. This indicates that the first-order phase transitions in [C(NH2)3]2SnCl6 exhibit extremely large thermal hysteresis of 50−100 K. However, the analysis of the calorimetric result may also suggest that the sequence of phase transitions in the heating and cooling runs is different. In order to estimate the transition entropies we selected the first DSC heating run of the fresh sample with clearly visible two thermal anomalies (run D in Figure 1a). The changes in the entropy in the temperature region of both phase transitions are plotted in Figure 1b. According to the relation ΔSIII/II = R ln(NII/NIII), where R is the gas constant and NII, NIII are the numbers of configurations, respectively, in phases II and III, the transition entropy ΔSIII/II = 8.5 J mol−1 K−1 is close to R ln 2.8. This could suggest an order−disorder contribution to the

3. RESULTS AND DISCUSSION 3.1. Calorimetry and Thermal Expansion. Calorimetric measurements revealed a very uncommon phase situation in [C(NH2)3]2SnCl6. The DSC runs presented in Figure 1a show that on heating the sample to 490 K either a single broad endothermic anomaly (runs A and B) or two successive anomalies (runs C and D) occur. These results indicate that the scenario of the phase transitions in [C(NH2)3]2SnCl6 can vary from sample to sample and also between the different heating/ cooling runs. As seen in Figure 1a, the sequence of phase transitions in the cooling runs is irrespective of that observed on heating the sample. Moreover, the onsets of the anomalies occur in the extremely wide temperature ranges: the first anomaly was mostly observed in the range 375−405 K on D



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Figure 3. Isochoric growth of [C(NH2)3]2SnCl6 in the DAC: (a) the crystal at 360 K, (b) at 345 K, (c) at 335 K, and (d) the final result of crystallization at 293 K and 0.6 GPa. The letter “R” in part (a) marks the ruby chip.

transition mechanism. On the other hand, the X-ray diffraction studies did not provide any evidence of structural disorder in the high-temperature phases II and I. The second thermal anomaly is associated with the entropy change ΔSII/I = 1.3 J mol−1 K−1 (≈ R ln 1.15), indicating a displacive character of this transition. It is worth noting that the values of the transition enthalpies in the subsequent runs were significantly different, with a decreasing tendency up to about 20%. Such behavior of the crystal can be related with a huge lattice strain accompanying the strongly first-order phase transitions. The crossing of the transition region results most probably in a generation of a large number of lattice defects, which strongly influence or even locally impede the transformation process in the next temperature cycle. The other important reason for the lack of good reproducibility in the calorimetric study can be a diversification of the transition paths, evidenced by the X-ray single-crystal and powder-diffraction experiments. The high-temperature phase I has originally been solved and refined in space group P1̅, from the powder-diffraction data. The anomalous symmetry reduction from monoclinic to triclinic on heating the crystal was then confirmed by singlecrystal X-ray diffraction (see Table 1). In fact, most of the numerous attempts to collect the single-crystal data in phase I were unsuccessful because of the crystal twinning and fracturing in the phase transitions region. But, in one of the heating cycles the single-crystal sample survived the transition quite well, and most importantly, the diffraction in phase I was predominated by one prevailing component. This allowed us to collect good data in phase I at 450 K (Rint = 1.52%) and also on cooling the sample. The temperature dependences of the unit-cell parameters of [C(NH2)3]2SnCl6 in phase III up to the transition point are plotted in Figure 4. Of note is a negative thermal expansion of the crystal along [010] above ca. 310 K. The linear thermal expansion coefficients derived at 350 K assume the values αa = 6.15 × 10−5 K−1, αb = −1.03 × 10−5 K−1, and αc = 1.68 × 10−4 K−1. A strong anisotropy in the thermal expansion originates

Figure 4. Temperature dependence of the unit-cell dimensions in phase III (a), and of the unit-cell volume per [C(NH2)3]2SnCl6 formula unit in phases III, I, and V (b). The arrows indicate heating and cooling runs, the vertical dashed lines mark the phase transitions.

from the layered architecture of the crystal. The largest magnitude of the linear thermal expansion coefficient along c is a consequence of weak van der Waals forces between the Hbonded layers. Similar properties were observed for the crystals of guanidinium nitrate,34 monoguanidinium dioxonium trinitrate,35 and guanidinium ethoxysulfonate,9 where the supramolecular sheets are formed of H-bonded ions. The strong anisotropy in the thermal expansion is a general property of the layered crystal structures directed by the two-dimensional networks of hydrogen bonds (see for example refs 36−38). In the heating cycle presented in Figure 4a the transition onset was observed near 405 K, but owing to a large temperature range of the coexistence of phases, the crystal was first heated to 450 K where the data were collected, and then the measurements were performed on lowering the E



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temperature. The temperature dependence of the unit-cell volume per one formula unit, V/Z(T), is shown in Figure 4b. In this plot the most striking is a large negative change in the crystal volume between phases III and I. The high-temperature phase I is definitely more densely packed as the crystal volume in this phase is reduced by about 3% when compared to phase III. On cooling, the crystal remained in phase I up to about 320 K, where a first-order phase transition was observed. The transition to phase V is associated with an abrupt change in the lattice parameters (Figure S3) and a stepwise volume decrease by 2.5 Å3 (0.7%), but the crystal retains the same symmetry of space group P1̅ as in phase I. Thus, this transition can be classified as isostructural. The lowering of temperature below 260 K caused broadening and splitting of the diffraction spots, which most probably indicated a reverse transition to the roomtemperature monoclinic phase (III or IV). Unfortunately, the powder-like diffraction made impossible the determination of the resulting phase. It is worth noticing that the small thermal anomalies observed in the cooling runs B and D in Figure 1a, in the temperature range 310−360 K, can correspond to the transition between the isostructural phases I and V. On the other hand, the powder-diffraction experiments with synchrotron radiation evidenced that on quick cooling in liquid nitrogen, phase I can transform into monoclinic phase IV of space group C2/m. This phase was stable at room temperature and in low temperatures, while on heating it transformed gradually back into phase I without any signs of phase III (see Figure S2). In contrast, when phase I was cooled with lower rates, usually a mixture of phases with prevailing component of phase III, was formed at room temperature. 3.2. High-Pressure Studies. The operating temperature range of the high-pressure cell used for dielectric and DTA investigations was limited to 420 K. Therefore, only the phase boundary between phases III and II could be examined. In the heating cycles this phase transition was usually associated with a broad anomaly, both in the dielectric and calorimetric measurements (see Figures 1 and 5), whereas on cooling the sample the transition proceeded more sharply. Therefore, the transition temperature could be determined more precisely from the cooling cycles. The results of the high-pressure measurements are summarized in the p−T phase diagram in Figure 6. The transition temperature TII/III strongly decreases

Figure 6. p−T phase diagram of [C(NH2)3]2SnCl6.

with increasing pressure. The solid line fitted to the experimental data measured on cooling the samples corresponds to the pressure coefficient dTII/III/dp = −0.755 K/MPa. According to the Claussius-Clapeyron equation, dT/dp = ΔV/ ΔS, where ΔV and ΔS are the volume and entropy changes at the transition temperature, respectively, valid for first-order phase transitions, the negative value of the pressure coefficient implies a negative volume change at the transition point. The pressure coefficient dTII/III/dp and the transition entropy ΔSIII/II (Figure 1) were used for an estimation of the crystal volume change ΔVIII/II = −6.46 × 10−6 m3/mol. This value indicates that the crystal volume in the intermediate phase II is by about 3% reduced when compared to the low-temperature phase III. The other striking feature of the p−T phase diagram presented in Figure 4 is a huge pressure-induced change in the temperature hysteresis observed for the phase transition III/II. In the pressure range below 80 MPa the hysteresis is very large, of about 80−90 K, whereas for pressures above 120 MPa it decreases to about 20 K. A strong susceptibility of the crystal to hydrostatic pressure can be ascribed to the squeezing of space between the layers, where the interactions are dominated by van der Waals forces and thus have a short-range nature. It is plausible that at low pressure the interlayer interactions are very weak, which hinders a cooperative response of the crystal to thermodynamic stimuli. The application of relatively low pressure strongly modifies the intersheet contacts which implies a strong pressure dependence of the crystal properties. The boundary between the phases I and II was sketched in Figure 6 on the basis of an estimation made using the Clausius−Clapeyron equation. In order to assess the volume difference between the phases II and I, the unit-cell volume VII in phase II (Figure 7) was approximated to ambient pressure, and similarly VI (Figure 4) was approximated to 293 K. The roughly estimated ΔVII/I = 3.2 Å3 (1.927 × 10−6 m3 mol−1) and the transition entropy ΔSII/I = 1.3 J mol−1 K−1 correspond to dTI/II/dp = 1.48 K/MPa. The application of pressure apparently extends the temperature region of the phase II stability, which allowed us to grow [C(NH2)3]2SnCl6 immediately in this phase by in situ crystallization in DAC, and to determine the crystal structure. The high-pressure crystallizations at 0.11, 0.28, 0.60, and 1.13

Figure 5. Electric permittivity measured for the electric field frequency of 500 kHz at 0.1 and 260 MPa. The arrows indicate heating/cooling runs. F



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guanidinium organodisulfonates.2 However, because of the different properties of the organodisulfonate and haxachlorostannate anions the patterns of hydrogen bonds formed show substantial differences. The transitions between phases in [C(NH2)3]2SnCl6 preserve the layered architecture of the crystal, but the structure is modified due to the changes in the arrangement of the layers and in the packing within the double H-bonded sheets. The crystal packing can be characterized by the spacing between the layers d, and by the area S of the Hbonded sheet per one formula unit of the crystal. These parameters are drawn in Figure 9. The interlayer spacing is the same in the monoclinic phases III and IV, and moreover, in these phases also the packing of the ions within the layers and the patterns of hydrogen bonds are very similar. In phase III the interlayer spacing rise remarkably with increasing temperature which well corresponds to the high thermal expansion of the crystal along the c axis (Figure 4a). The transition between the room-temperature phase III and the intermediate phase II pertains to the most unusual, because at elevated temperature the crystal lowers its symmetry from monoclinic, space group C2/c, to triclinic, space group P1̅, whereas usually the high-temperature phases adopt higher symmetry. The transition involves significant changes in the Hbonded network and in the stacking of the layers. The crystal volume in phase II is the lowest (Figure 7) among all the crystal phases of [C(NH2)3]2SnCl6, but on the other hand the interlayer spacing is the largest, as illustrated in Figure 9a (see also Figure 10). It looks seemingly contradictory, but can be reconciled by taking into account a close packing of the ions within the double sheets, as indicated by the lowest value of the parameter S (Figure 9b). It is worth noticing that d in phase II becomes comparable with the interlayer distances in other phases after the application of pressure higher than 0.7 GPa (Figure 10). The other peculiarity of the transition between phases III and II is the high transition entropy ΔSIII/II = R ln 2.8, suggestive a disorder in phase II, whereas the structural study has evidenced that this phase is well-ordered. The plausible explanation is, that the entropy gain results from the large negative change in the crystal volume at the transition point. The transition between the phases II and I has an isostructural character. The crystal volume increases slightly at the transition to the high-temperature phase I, but the structure remains more densely packed than in the monoclinic phases III and IV (cf. Figure 4). The small changes in the crystal density strongly contrast with a considerable decrease in the interlayer spacing in phase I, as illustrated in Figure 9a. However, this effect is compensated for the rearrangement of the ions within the sheets due to the reconstruction of the hydrogen bonding, which is reflected in the high value of S (cf. Figure 9b). The reverse transition from the high-temperature phase I can be realized either directly to the monoclinic phases III or IV of similar packing of the crystal structures, or through the intermediate phase V. Both monoclinic phases have almost identical V/Z, d and S, and therefore it is justified to assume that the entropy change associated with the transition is roughly irrespective of the transition path. The magnitude of thermal anomalies, observed during the cooling DSC runs, indicated that the prevailing part of the polycrystalline sample returned to the low-density phase (both monoclinic phases are less densely packed than the phases I and V). The transition between phases I and V is of isostructural type. The necessary

Figure 7. Unit-cell volume per formula unit of the crystal, V/Z, in phases I, III, IV and V, and pressure dependence of V/Z in phase II.

GPa all yielded single crystals in phase II, in accordance with the p-T phase diagram presented in Figure 4. The pressure dependence of the unit-cell volume shown in Figure 7 clearly indicates that at room temperature the crystal remains in phase II up to at least 1.13 GPa. This plot also shows that phase II is the most densely packed phase of the crystal. 3.3. Description of the Structures and Phase Transitions. The [C(NH2)3]2SnCl6 crystal exhibits rich polymorphism, but the five crystal structures determined in this study are characterized by the same topology. As illustrated in Figure 8 all phases are built from neutral layers formed by electrostatic and N−H···Cl hydrogen bonding between SnCl62− and C(NH2)3+ ions. The layers are composed of two sheets of guanidinium cations H-bonded to the chlorine atoms. In the monoclinic phases III and IV, each of the six Cl atoms of the SnCl62− octahedron is linked by two N−H···Cl hydrogen bonds to the guanidinium cations (Figure 8c,d, Tables S6 and S15). All Cl atoms are also involved in hydrogen bonds in the triclinic phases I and V, but in this case the pattern of hydrogen bonds is different: two chlorine atoms of the octahedron participate in three bonds, while the remaining four Cl atoms are linked to the guanidinium cations by two hydrogen bonds (Figures 8a,e). Thus, each of the guanidinium cations is tied up with the seven H-bonds, six of them are involved in one sheet, while the seventh links the two sheets forming the layer. The later bond is created as a result of three-centered interaction between the N(3)−H(3B) group and the chlorine atoms Cl(2) and Cl(3). The temperature induced changes in the geometrical parameters of this bifurcated bond (see Tables S2, S4, S8, and S15) indicate that with the lowering temperature the bond N(3)−H(3B)···Cl(3) gradually weakens, whereas the strength of N(3)−H(3B)···Cl(2) increases. In the triclinic phase II the H-bonded network is significantly different from the other phases. In this case only four Cl atoms of the octahedron are involved in hydrogen bonding, but each of them participate in three bonds (Figure 8b, Tables S11, S13, and S15). The two-sheet motif of the layers in [C(NH2)3]2SnCl6 resembles those observed in the numerous structures of G



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Figure 8. Crystal structure of [C(NH2)3]2SnCl6 in the phases: I at 330 K (a), II at 0.28 GPa and 293 K (b), III at 297 K (c), IV at 297 K (d), and V at 270 K (e), projected down the main crystallographic directions (left drawings) and the best views of the single hydrogen bonded sheets (right drawings).

condition for that case, i.e., a first-order character of the transition, is evidenced by the stepwise change in the crystal volume (cf. Figure 4). Discontinuous changes are also observed in the parameters d and S (Figure 9), which characterize the

stacking of the layers and packing of the ions within the double sheets. However, these changes are relatively small when compared to other phase transitions in [C(NH2)3]2SnCl6. H



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Figure 10. Pressure dependence of the interlayer spacing in phase II. The interlayer spacings in phases I, III, IV, and V are shown for comparison.

can adopt the structure of at least three phases, which indicates that the minima of the Gibbs free energies of phases III, IV, and V are very similar.39,40 But on the other hand, no phase transition has been observed between the phases III and IV, which testifies that they are separated by a high energy barrier. The control of the transition paths in [C(NH2)3]2SnCl6 is fairly limited because of a large number of factors which can affect the nucleation of new phases. Apart from the lattice defects and the associated internal strain, which are generated at the transition points, a very important role can be played by the temperature gradients inside the crystals, by the size of crystalline grains,41 and even by the sample humidity.42 The large entropy gain associated with the first-order phase transition between the low- and high-density phases, together with the exceptionally large pressure coefficient dTII/III/dp, strongly suggest that giant barocaloric effect can be expected in [C(NH2)3]2SnCl6. The pressure coefficient of −755 K/GPa is more than 10 times higher when compared to the best barocaloric materials,43 and also the temperature range, close to the room temperature, can prove advantageous. These properties make diguanidinium hexachlorostannate(IV) a potentially useful material for solid-state refrigeration.

Figure 9. Parameters characterizing the crystal packing in phases I−V: (a) the interlayer distance d, and (b) the area S of the H-bonded sheet per one formula unit. The label (p-d) denotes powder-diffraction data.

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4. CONCLUSIONS The [C(NH2)3]2SnCl6 crystals are composed of isolated octahedral [SnCl6]2− anions and planar guanidinium cations [C(NH2)3]+. The ions are assembled into double sheets linked by N−H···Cl hydrogen bonds. The interactions between the layers are very weak which has significant implications for the crystal properties. In particular, this is one of the possible reasons of diversification of the transition paths, and large temperature hysteresis between the transition points in the heating/cooling runs. The phase transition mechanism requires a cooperative behavior of the crystal structure, which may be hindered in a case of weakly interacting layers. The structures of the five polymorphs determined in this study show that the layered architecture of the crystal is preserved in all crystal phases, whereas the stacking of the layers, the packing of the ions within the double sheets, and the hydrogen bonding, are modified. Moreover, at room temperature [C(NH2)3]2SnCl6

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01830. Thermogravimetric analysis (Figure S1), powder-diffraction pattern as a function of temperature (Figure S2), the lattice parameters in the phases I and V (Figure S3), pressure dependence of parameter S (Figure S4), selected bonds and angles (Tables S1−S15) (PDF) Accession Codes

CCDC 1442688−1442694, 1455610, and 455611 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. I



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Article

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AUTHOR INFORMATION

Corresponding Author

*Phone: + 48 61 8295094; fax: + 48 61 8295155; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Phase Transitions in Layered Diguanidinium Hexachlorostannate(IV)

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