Pressure-Induced Phase Transition in Hydrogen-Bonded

Article pubs.acs.org/JPCC

Pressure-Induced Phase Transition in Hydrogen-Bonded Supramolecular Structure: Ammonium Formate Lei Kang,† Kai Wang,*,† Shourui Li,† Jing Liu,‡ Ke Yang,§ Bingbing Liu,† and Bo Zou*,† †

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China Beijing Synchrotron Radiation Laboratory, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China § Shanghai Synchrotron Radiation Facilities, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China ‡

S Supporting Information *

ABSTRACT: High-pressure behaviors of hydrogen-bonded supramolecular structure, ammonium formate (NH4+COOH−, AF), have been investigated under pressure by in situ synchrotron X-ray diffraction (XRD) and Raman spectroscopy up to 20 GPa. Under ambient conditions, AF exhibits three-dimensional hydrogen-bonded networks with two molecules crystallize in a monoclinic unit cell of space group Pc. A structural phase transition can be identified at around 1.8 GPa, as indicated by the abrupt changes in Raman spectra as well as the pressure dependence of major Raman modes. Furthermore, two new N−H stretching modes emerge, indicative of the construction of new hydrogen bonds. Rearrangement of the hydrogen-bonded networks is also deduced by the obvious changes of N−H stretching modes both in position and intensity. The reversible phase transition is confirmed by in situ synchrotron XRD experiments with the emergence of a new set of diffraction pattern. The high-pressure phase is found to have a structure with a monoclinic unit cell (space group P21) containing two molecules. The structural transformation is proposed to be a result of the rearrangement of the hydrogen-bonded networks. Detailed mechanism for the phase transition, high-pressure behaviors of hydrogen bonds, as well as the cooperativity of different noncovalent interactions are presented and discussed.

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INTRODUCTION In the past few decades, significant efforts have been devoted to understanding supramolecular chemistry, with an emphasis on the structure and functions of chemical entities formed through noncovalent interactions.1 These interactions, which include hydrogen-bond and electrostatic interactions, play an eminent role in building up large functional supramolecular architectures.2,3 In particular, the hydrogen bond is the most pervasive and extensively investigated noncovalent interaction because it widely exists in inorganic materials, organic materials, and biological systems, such as water and amino acids.4−7 Moreover, the structure and properties of supramolecular architectures are mainly determined by the intermolecular hydrogen bonds and other noncovalent interactions.8,9 Thus, the study of hydrogen bonds and the cooperativity of various noncovalent interactions is crucial for understanding the mechanisms of many processes, such as molecular selfassembly,10 molecular recognition, and complexation.11 It can also provide vital information for the design of supramolecular architectures with specific structures and functions.12 Pressure and temperature are two well-known basic thermodynamic parameters that are widely used to develop a better understanding of material laws. On the basis of equation of Gibbs free energy, which determines the structural © 2014 American Chemical Society

stabilization at different P−T thermodynamic conditions, pressure on a scale of thousands of atmospheres can be effective in inducing structural changes. Furthermore, the intermolecular distances can be easily modified by high pressure; thus, pressure can be a powerful tool for studying noncovalent interactions and their cooperativity.13 As we know, upon compression, the distances between molecules and atoms reduce; thus, they reorient themselves to counter steric hindrances and tend to achieve close packing, eventually resulting in new crystal structures.14 Moreover, compared with covalent bonds, the attraction interaction in hydrogen bonds is relatively weak,15−17 suggesting that the strength of hydrogen bonds can be tuned more easily by applying pressure. Pressure may also help to explore new structures that cannot be accessible at ambient conditions. Recently, pressure has proven to be a powerful tool for studying hydrogen bonds in supramolecular structures, as small changes induced by external pressure in the hydrogen bond can result in large variations in intermolecular separations, which may cause dramatic structural rearrangement.18,19 Besides, the balance between hydrogenReceived: December 10, 2013 Revised: March 24, 2014 Published: March 26, 2014 8521

dx.doi.org/10.1021/jp412112g | J. Phys. Chem. C 2014, 118, 8521−8530

The Journal of Physical Chemistry C

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bonding and electrostatic interactions can be easily tuned by pressure, which may lead to structural changes.20 Thus, pressure is an ideal tool for tuning and understanding noncovalent interactions, especially the hydrogen bond, which is of fundamental importance for scientific and potential practical applications with respect to supramolecular chemistry. In recent years, a number of structural transitions induced by high pressure in hydrogen-bonded structures have been reported.21−25 Variations in hydrogen bonds play a key role in structural transition in these hydrogen-bonded structures. For example, Martins et al.24 studied the proton transfer in a hydrogen-bonding structure formed by squaric acid and bipridine and found two high-pressure phases at 1.8 and 2.6 GPa. Fabbiani et al.25 conducted high-pressure studies on piracetam and observed a reversible single-crystal to singlecrystal phase transition between 0.45 and 0.70 GPa. Recently, we have performed high-pressure experiments on guanidinium nitrate, and the collapse of two-dimensional hydrogen-bonding networks was found to result in a structural transformation at around 1 GPa,26 which was also confirmed by single-crystal Xray diffraction.27 For guanidinium perchlorate, the competition between hydrogen bonds and close packing resulted in a new structure at 4.5 GPa.28 However, compared with other condensed materials, there is still limited research reported on supramolecular architectures applied by high pressure, and systematic experimental investigations are expected to provide more insight into the nature of noncovalent interactions and their cooperative relationships. Ammonium formate (AF) is one of the simplest hydrogenbonded supramolecular architectures and can be regarded as a model system for exploring the behaviors of hydrogen-bonded structures under high pressure. AF is widely used in scientific research and the pharmaceuticals industry and has attracted much attention due to its important applications and unique properties.29−31 Under ambient conditions, AF crystallizes in the monoclinic space group Pc with Z = 2 in a unit cell, and the unit cell parameters are a = 3.81(4) Å, b = 4.67(9) Å, c = 9.11(2) Å, and β = 91.17(1)°.32 The crystal structure of AF is shown in Figure 1. Four hydrogen bonds N−H···O in a tetrahedral arrangement are formed between the ammonium ion and the adjacent oxygen atoms, two of which are significantly longer than the other two. Each oxygen atom of the carboxylate groups accepts two hydrogen bonds thus forming three-dimensional hydrogen-bonded networks with puckered layers inside. The N−H···O hydrogen bonds, together with electrostatic interactions, exist among these molecules to support the structure. Therefore, a detailed highpressure study of AF, especially its structural evolution, is expected to provide a deeper insight into the chemical nature of hydrogen-bond and electrostatic interactions. Actually, AF has attracted significant attention from high-pressure scientists. For instance, the first high-pressure experiment was performed by Bridgman, who selected AF as a model compound in his systematic high-pressure research, up to about 2.0 GPa, the result displayed a large volume change at 1.2 GPa.33 Later Hamann et al.34,35 found that this transition was accompanied by an increase of N−H stretching frequencies through infrared spectra study. However, these experiments were carried out over a very limited pressure range, and there is still no structural information on the high-pressure phase available due to lack of X-ray diffraction (XRD) data, which is important for understanding the mechansim of phase changes. Thus, a thorough

Figure 1. Crystal structure of AF with space group Pc under ambient conditions: (a) the unit cell; (b) the hydrogen-bonded networks projected on the bc plane. The hydrogen bonds are marked as dashed lines.

study of AF combining synchrotron XRD and vibrational spectroscopy is necessary. In situ synchrotron XRD patterns with high intensity synchrotron radiation are irreplaceable in probing the structural information at high pressures. High-pressure Raman spectroscopy has proven to be powerful tool for monitoring modifications in hydrogen bonds and molecular arrangements. In this study, we report a detailed high-pressure study of the crystal structure of AF combining synchrotron XRD and Raman scattering spectroscopy techniques up to 20 GPa. Detailed analysis of the mechanism for the phase transition, structural behaviors, and the cooperativity of noncovalent interactions has been presented. The primary goal of this work is to provide some insight into the chemical nature of the hydrogen bond as well as the cooperative effects of different noncovalent interactions.

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EXPERIMENT SECTION Commercially available AF powder (purity 99%) was purchased from Alfa Company and used without further purification. Symmetric diamond anvil cells (DACs) with diamond anvils of 400 μm in diameter were employed for in situ high-pressure Raman and XRD experiments. The powder was loaded into the sample chamber of 130 μm in diameter and 40 μm in thickness in the center of T301 steel gasket. As AF powder is very water and air sensitive, the sample loading was performed in a glovebox (nitrogen atmosphere). The pressures in the DAC were calibrate by ruby fluorescence method,36 and a quasihydrostatic pressure condition in the sample chamber was confirmed by the sharp and well-separated ruby fluorescence peaks. All of the experiments were performed at room temperature. High-pressure Raman spectra were recorded in the standard backscattering geometry using an Acton SpectraPro 2500 8522



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spectrograph (500 mm focal length) equipped with a liquid nitrogen-cooled CCD (PyLoN:100B). A diode-pumped solidstate laser with wavelength of 532 nm was applied as the excitation source, and the output power was 10 mW. The spectral resolution of the Raman system was around 1 cm−1. Raman profiles were fitted using Gaussian and Lorentzian functions. In situ synchrotron XRD experiments were conducted at the 4W2 beamline at the High Pressure Station of the Beijing Synchrotron Radiation Facility (BSRF) with a wavelength of 0.6199 Å. The average exposure time was 300 s. The geometric parameters were calibrated using a CeO2 standard before data collection. A MAR345 image plate detector was used to collect the Bragg diffraction rings, and the obtained two-dimensional XRD images were converted to one-dimensional intensity versus 2θ patterns using the software FIT2D.37 Part of the synchrotron XRD measurements were carried out at the 15U1 beamline of Shanghai Synchrotron Radiation Facility (SSRF) (wavelength 0.6199 Å). High-pressure structural information was obtained using the Rietveld refinement method combined in the Material Studio 5.5 program. During refinement cycles, the scale factors, background parameters, profile function, lattice parameters, fractional coordinates, and isotropic thermal parameters were optimized.38 The first-principles calculations were carried out with the CASTEP code,39 based on density functional theory (DFT) method using norm-conserving pseudopotential.40 The local density approximation (LDA) with the with the CA-PZ exchange-correlation functional was used in the high-pressure calculations. Structural optimizations under high-pressure conditions, including lattice constants and atomic positions were performed using the Broyden−Fletcher−Goldfarb− Shanno algorithm (BFGS).41

Figure 2. Raman spectra of AF powder samples at atmospheric pressure. For clarity the spectra have been divided into three parts: (a) 50−1200 cm−1; (b) 1400−3200 cm−1.

pressure derivative (dω/dP), and mode assignments of the Raman spectra of the AF at two different phases in the 0−20 GPa range are summarized in Table 1. Pressure-induced Raman spectra changes of AF in the frequency ranges 50−500, 700−1200, 1400−2200, and 2700− 3200 cm−1 are depicted in Figure 3a−d, respectively. The Raman spectra have been measured at various pressures up to ∼20 GPa. As shown in Figure 3a−d, a structural transformation (phase I−phase II) can be identified around 1.8 GPa, as indicated by the dramatic changes in the pattern profile as well as the pressure dependence of the major Raman modes. The Raman spectra show no further significant changes above 1.8 GPa, indicating the high-pressure phase is stable up to 20 GPa. Although it is difficult to determine the structure of the highpressure phase from Raman spectra, the observed changes in Raman spectra allow the understanding of the local structure and chemical bond, which can give some information on symmetry change. The evolution of the lattice modes can provide useful information on structural changes. As seen in Figure 3a, there are eight lattice distinguished modes, which are very sensitive to changes on application of pressure because of weak interionic interactions.44,45 With increasing pressure, some external modes exhibit ordinary blue shifts, which indicate the enhancement of ionic interactions due to the reduction of interionic distances.17,46,47 Meanwhile, a remarkable change is observed in the relative intensities among the first three modes marked with rhombus (59, 66, and 103 cm−1). Below 1.8 GPa, the mode at 66 cm−1 has the lowest intensity among them; however, at 1.8 GPa, this mode increases to be the highest intensity. Meanwhile, the intensities of the modes at 59 and 103 cm−1 significantly decrease at 1.8 GPa and eventually lose their intensities at higher pressures. Furthermore, there are two new

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RESULTS AND DISCUSSION The Raman spectrum of AF collected at room temperature and ambient pressure is shown in Figure 2. The factor group of AF is Cs(m). Because there are two molecules in each unit cell, every molecule of AF consists of 9 atoms, that is, NH4+ (5 atoms) and HCOO− (4 atoms); thus, there are 54 crystal lattice vibration modes. The irreducible representation is ΓCs(m) = 27A′ + 27A″

(1)

of which there are three acoustic modes Γacoustic = 2A′ + A″

(2)

and 51 optic modes Γoptic = 25A′ + 26A″

(3)

According to the Raman selection rule, the 51 optic modes are all Raman active, consisting of 21 intermolecular modes (lattice modes) and 30 intramolecular modes (internal modes); however, some of these modes could not be detected in our experiment because of the weak intensity. In the low frequency region of the Raman spectra are the lattice modes, which represent the relative movement among molecules or ions. Vibrations induced by molecular deformation in the high frequency region are assigned as internal modes. Because there is no information available on the assignments of AF Raman modes, we propose tentative assignments of the Raman modes under ambient condition based on the reported spectra of ammonium and formic acid.42,43 The mode frequencies (ω), 8523



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1743, and 2138 cm−1) disappear accompanied by the emergence of some new peaks marked with asterisk (751, 755, 1064, and 1773 cm−1). The pressure dependence of internal modes related to N−H stretching modes is helpful for understanding the changes in hydrogen-bonded networks. Figure 3d shows the Raman patterns of N−H stretching modes together with two C−H vibration modes at various pressures. The detailed decomposition of bands corresponding to the N−H stretching vibrations at 0.1 and 1.8 GPa is shown in Figure 3e. At 1.8 GPa, the mode related to C−H deformation motion disappears, and the C−H stretching modes (2803 and 2814 cm−1) merge into the mode at 2798 cm−1, indicating that the phase transition lowers the frequencies of C−H stretching modes. There are four N−H stretching modes (2870, 3010, 3075, and 3190 cm−1) in this region of the spectrum. The N−H stretching modes at the lower frequencies (2870 and 3010 cm−1) show blue shifts all the way up to 1.8 GPa, which indicates that the strong hydrogen bonds in AF continue to be strengthened with increasing pressure.35 Meanwhile, the mode at 3010 cm−1 disappears above 1.8 GPa, indicating a breakdown of the strong hydrogen bonds through the phase transition. However, the N−H stretching modes at the higher frequencies (3075 and 3190 cm−1) display red shifts with increasing pressure, indicating the strengthening of the weak or moderate N−H··· O hydrogen bonds in AF.35,50 For these hydrogen bonds, because the distance between the hydrogen atoms and oxygen atoms is reduced with pressure, the electrostatic attraction between them is enhanced, leading to an elongated N−H distance.51 At 1.8 GPa, two new N−H stretching modes (2879 and 3086 cm−1) emerge, indicative of the construction of new hydrogen bonds. Meanwhile, the N−H stretching modes show obvious changes both in position and intensity, indicating that the hydrogen-bonded networks rearranged across the transition. With the application of pressure, the ions or molecular fragments and even the hydrogen-bonding networks may be rotated because of the increased energy of interionic interactions.52 Considerable rearrangement of hydrogen bonds can be inferred by the abrupt changes of NH stretching vibration modes at ∼1.8 GPa. The frequency shifts of selected Raman modes as a function of pressure are presented in Figure 4a−c. At ∼1.8 GPa, most vibrational modes become discontinued. At the same time, abrupt changes in shift rates are observed in some vibrational modes. Furthermore, several new modes emerge, together with the splitting of internal mode at this pressure, strongly suggesting a phase transition. As hydrogen bonds play an eminent role in determining supramolecular structure, the N− H stretching modes can be helpful for understanding the changes in noncovalent interactions.5 The disappearance and emergence of N−H stretching modes indicated that this phase transition is accompanied by arrangement of hydrogen bonded networks. To confirm the pressure-induced phase transition and provide detailed structural information on AF under highpressure conditions, we have performed the ADXRD experiment, which is considered to provide the most reliable evidence for phase transitions. Figure 5 depicts the evolution of representative XRD patterns of AF up to 20.2 GPa. With increasing pressure, all of the diffraction peaks shift to higher angles, due to the expected decrease of interplanar distances of crystal planes.53 At ∼1.8 GPa, a pressure-induced phase transition can be concluded, evidenced by the abrupt changes

Table 1. Frequency (ω) and Its Pressure Derivative (dω/dP) for the Main Raman Modes of AF in Two Different Phases and Their Mode Assignments phase I (0 GPa) −1

ω (cm )

dω/dP

57 62

−1.2 9.7

103 121 133 152

1.4 4.0 2.3 0.7

208 224 780 782

9.2 14.2 3.0 7.4

phase II (1.8 GPa) ω (cm−1) 86 87 106

0.1 0.7 4.8

158 165 192 224

2.9 3.7 9.3 10.2

0.9 −0.6 −0.8

1706 1742

−1.9 4.3

2138 2710 2808 2813 2870

2.6 −2.5 8.2 6.5 27.0

3010 3075

8.7 −9.6

3190

−19.6

tentative mode assignment lattice lattice lattice lattice lattice lattice lattice lattice

modes modes modes modes modes modes modes modes

N−H bending N−H bending 751 755 1064

1071 1437 1467

dω/dP

1.6 2.0 3.7

1456 1481 1702

3.6 −0.4 4.8

1773

2.7

2798 2913 2879

4.7 2.2 4.0

3056 3086

3.4 2.7

C−H deformation N−H deformation N−H deformation asymmetric CO stretching symmetric CO stretching

C−H deformation C−H stretching C−H stretching N−H stretching N−H stretching N−H stretching N−H stretching N−H stretching N−H stretching

modes arising at 165 and 192 cm−1 marked with asterisk, with the two modes at 119 and 211 cm−1 disappearing. With further compression to 20 GPa, the spectrum remains essentially similar to that at 1.8 GPa except for a general increase in the frequency of the Raman modes. The abrupt changes in the Raman spectra demonstrate a pressure-induced structural transition at around 1.8 GPa. The internal modes are sensitive to high pressure and can probe local variations of the chemical environment around specific groups efficiently.28 Figure 3b,c illustrates the selected Raman spectra in the frequency range 700−2200 cm−1. With increasing pressure, all internal modes gradually shift toward higher frequencies due to the decrease of bond distances as well as the increase of effective force constants.48 However, there are remarkable characteristic changes in the Raman pattern at 1.8 GPa, for example, the mode at 1464 cm−1 relating to N−H deformation motion shows an obvious split. The splitting of Raman internal modes is considered as an indication of lowering crystalline symmetry due to the phase transition.49 At the same time, the modes (780, 782, and 1071 cm−1) relating to N−H bending and C−H deformation motion suddenly reduce their intensity and eventually disappear at higher pressures. Furthermore, some Raman modes (1430, 1640, 8524



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Figure 3. Selected Raman spectra of AF crystal at various pressures and the decomposition of the N−H stretching vibrations bands. For clarity the spectra have been divided into four parts: (a) 50−500 cm−1; (b) 700−1200 cm−1; (c) 1400−2200 cm−1; (d) 2700−3200 cm−1; (e) is the decomposition of the N−H stretching bands. The peaks marked by rhombus are three lattice modes, and the new peaks are marked by asterisk; more detailed descriptions of these peaks are presented in the main text.

listed in Table S1 (Supporting Information). The indexed lattice constants are a = 3.47(5) Å, b = 10.80(5) Å, c = 3.46(9) Å, β = 90.08(7)°, and Z = 2 with unit cell volume V = 130.27(5) Å3. The crystal structures of ambient and highpressure phase are given in Figure 7. We can conclude that the ions in phase II are more closely packed, evidenced by the considerable reduction in unit cell volume, consistent with the results of Bridgman.33 Furthermore, the symmetry of the highpressure phase II is lower than that of phase I, in line with the splitting of Raman bands in phase II. The pressure dependences of the lattice parameters and volume are presented in Figure 8a,b. As is shown, with increasing pressure below 1.8 GPa, the lattice parameters and volume gradually decrease as expected, and the a axis is more compressible than the c axis, due to the larger space between (100) than that of (001). Moreover, the compression of the b

in the pattern profile as well as the peak positions. These changes in the diffraction pattern are in the same pressure region that shows significant modification in the Raman spectra. With further compression, no obvious change is observed from the diffraction profile, indicating that the new high-pressure structure is stable up to 20.2 GPa. Furthermore, upon release of pressure, the diffraction pattern returns back to its original state, indicating the phase transition is fully reversible. To obtain the structural information for phase II, we carried out the Rietveld refinement of the diffraction pattern at 1.8 GPa, as shown in Figure 6. The Rietveld refinement of the high-pressure phase II shows good agreement with the P21 space group, belonging to monoclinic system, and the fit qualities of the XRD refinement are Rwp = 0.70% and Rp = 1.25%. The fractional coordinates of C, H, O, and N atoms are 8525



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Figure 4. Frequency shifts of the Raman modes as a function of pressure. The vertical dashed lines stand for the boundary of the two phases. For clarity the modes have been divided into three parts: (a) 50−450 cm−1; (b) 700−2200 cm−1; (c) 2700−3200 cm−1.

Figure 6. Rietveld refinement of the ADXRD pattern collected at 1.8 GPa. The inset shows the crystal structure of phase P21.The black line shows the difference between the observed (pink) and the simulated (blue) profiles.

mechanism of the pressure-induced phase transition is proposed. Under ambient conditions, hydrogen bonds and electrostatic interactions are the two dominant interactions in the AF crystal structure. As pressure is increased, the distances between the adjacent ions are gradually reduced, which lead to the enhanced electrostatic interactions between neighboring NH4+ and HCOO− groups. The hydrogen-bonding interaction is also strengthened because of the reduced length of the hydrogen bonds. With further compression, the crystal structure becomes unstable because of the increase in total Gibbs free energy and the disturbed balance between hydrogen bonds and electrostatic interactions. When pressure is increased to 1.8 GPa, the NH4+ and HCOO− groups adopt new orientations, and the hydrogen-bonded networks rearrange to release free energy, resulting in the phase transition. Therefore, the cooperation of hydrogen-bonding and electrostatic interactions is responsible for this pressure-induced phase transition at 1.8 GPa. The inferred mechanism of the phase transition is consistent with features observed in the Raman and XRD results. The abrupt changes in the lattice and internal modes at 1.8 GPa indicate a structural phase transition, which is also confirmed by the different diffraction patterns of phases I and II. The Rietveld

Figure 5. Representative ADXRD diffraction patterns of AF at different pressures.

axis is the smallest because of the compact ions along the b axis. Furthermore, there is a considerable contraction of volume across the phase transition (∼12%), indicating that the highpressure phase is a more compact structure. We performed the first-principles calculations with the CASTEP code39 to gain more insight into the mechanism of the phase transition. As is illustrated in Figure 9, the calculated enthalpies show that the ambient structure I transform into high-pressure structure II at around 1.4 GPa, which is in the same pressure region of the experimental phase transition. On the basis of the experimental and calculation results, the 8526



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Figure 7. Crystal structures of AF: (a) ambient phase Pc and (b) high-pressure phase P21, respectively. This figure reflects arrangements of ions on the bc plane. The color codes for atoms are red (oxygen), gray (hydrogen), dark gray (carbon), and blue (nitrogen). Hydrogen bonds N−H···O are marked as dashed lines.

Figure 8. Variations in (a) lattice parameters and (b) unit cell volume as a function of pressure. The vertical dotted lines represent the boundary of the two phases.

for instance, pressure. In many high-pressure studies of hydrogen bonds, hydrogen-bonded structures undergo phase transitions at relatively low-pressure conditions.14,25,54 During phase transitions, there are large changes in hydrogen bonds and arrangement of molecules, including distortion,53 break and formation,51,55,56 and rearrangement of hydrogen-bonded networks.14,56 For example, urea experiences the first phase transition at 0.48 GPa, and three subsequent transitions at about 0.6, 2.8, and 7.2 GPa.57,58 During the phase transitions of urea, they found that one of the four hydrogen bonds broken, and interestingly, the capacity of the H-acceptor carbonyl oxygen restored when pressure reached 2.8 GPa.55 However, for electrostatic interaction existing solely structures, because the ionic bond (150−400 kJ/mol) is sufficiently strong that can be comparable to covalent bond, as well as its long-range force characteristics, ionic crystals are generally more stable than hydrogen-bonded crystals; therefore, pressures needed for phase transitions of these structures are generally higher than that of hydrogen-bonded structures.59−61 For example, sodium chloride is found to have a first phase transition at about 30

refinements manifest that the high-pressure phase II shows good agreement with space group P21, lower symmetry than the ambient phase, which is consistent with the splitting of the internal mode in the Raman spectra of phase II. In addition, the application of pressure induces the rearrangement of hydrogen bonds, as evidenced by the remarkable redistribution both in the intensity and position of N−H stretching modes as well as the large discontinuities in N−H stretching modes across the phase transition. Furthermore, the emergence of N−H stretching modes at 1.8 GPa indicates that new hydrogenbonded networks have been reconstructed. To further understand the cooperative effect of hydrogenbond and electrostatic interactions in the phase transition, it is worthwhile to compare the high-pressure behaviors of AF with that of some other chemical systems in which these two interactions do not coexist. Compared with the strength of covalent bonds (150−400 kJ/mol) in molecules, hydrogen bonds are far weaker (8−50 kJ/mol) and a short-range force; thus, the bond geometry parameters of hydrogen bonds (bond strength, bond length) can be easily changed by external effects, 8527



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nitrate experienced a structural collapse at a low pressure of 0.6 GPa.27 The interactions supporting the AF crystal structure are hydrogen-bonding and electrostatic interactions, so highpressure behaviors of it are expected to originate from the cooperative effect of them. AF experiences a phase transition at a relatively low pressure of 1.8 GPa; thus, we propose that this transition is mainly attributed to large changes of hydrogen bonds, including the breaking of hydrogen bonds and the rearrangement of hydrogen-bonded networks. High-pressure studies of AF are expected to provide us some insight into the chemical nature of the hydrogen bond as well as the cooperative effect between hydrogen-bond and electrostatic interactions.

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CONCLUSION In summary, we have carried out high-pressure Raman and synchrotron XRD studies of AF. Abrupt changes in the pattern profile as well as the pressure dependence of the major Raman modes indicate the occurrence of a phase transitions at ∼1.8 GPa. The phase transition has been confirmed by synchrotron XRD experiment. Moreover, the high-pressure phase with P21 symmetry is stable up to 20 GPa in this study, and the pressureinduced phase transition is completely reversible. In addition, we propose that the phase transition is accompanied by rearrangements of the hydrogen-bonded networks. Highpressure studies of the AF supramolecular crystal can provide some new insight into the chemical nature of the hydrogen bond as well as the cooperative effects of different noncovalent interactions, which is of great importance for crystal engineering.

Figure 9. Enthalpy difference of per formula unit as a function of pressure for AF in the ambient (full line) and high-pressure (dashed line) crystal structures. The enthalpy of the ambient structure is taken as the reference energy.

GPa.59 Moreover, there may be metallization of some ionic structures under high-pressure conditions because of changes in the band structures induced by high pressures.62,63 When interactions of hydrogen-bonding and electrostatic interactions coexist together, behaviors of the structures are expected to stem from the cooperativity of these two noncovalent interactions. Hydrogen-bonding and electrostatic interactions will be strengthened as the distances between the neighboring molecules reduce with increasing pressure; however, they are not expected to make equal contributions to the stability and the structural transitions because of their intrinsic characteristics. Some structures are found to have phase transitions in the relatively high-pressure region, because the energy barriers for the molecular rearrangement of these structures are high and changes in the hydrogen bonds may not provide sufficient energy for the rearrangement with applied pressure, even under high-pressure conditions. However, when pressure is sufficiently high, the strength of the electrostatic interaction may increase sufficiently to break through the energy barriers, eventually inducing large changes in the crystal structures and resistance state; thus, we can tentatively attribute those phase transitions mainly to electrostatic interaction. For instance, the phase transitions of ammonium halides are found at about 15 GPa for NH4Br, 27 GPa for NH4Cl, and 42 GPa for NH4F.64 For structures that have relatively low pressures of phase transition, the energy barriers for these structural changes are low. In the low-pressure region, although electrostatic interactions will become stronger, the change may not be as large as for hydrogen bonds. Meanwhile, because of their weakness and sensitivity, the strength of hydrogen bonds will change significantly; thus, changes of hydrogen bonds are expected to play a more important role (e.g., guanidinium nitrate, ammonium perchlorate).27,65 For example, a highpressure study found that the layered structure guanidinium

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ASSOCIATED CONTENT

S Supporting Information *

Atomic coordinates for the high-pressure phase (II) of AF at 1.8 GPa. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(K.W.) E-mail: [email protected]. *(B.Z.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Dr. Xiaojia Chen and Dr. Bin Chen at High Pressure Science and Technology Advanced Research (HPSTAR, Shanghai) for helpful discussions and instructions. This work is supported by NSFC (nos. 91227202 and 11204101), China Postdoctoral Science Foundation (no. 2012M511327), RFDP (no. 20120061130006), National Basic Research Program of China (no. 2011CB808200), and the Graduate Innovation Fund of Jilin University (no. 20121041). XRD measurement was performed at the 4W2 beamline, Beijing Synchrotron Radiation Facility (BSRF) that is supported by Chinese Academy of Sciences (nos. KJCX2-SWN03 and KJCX2-SW-N20). Portions of this work were performed at the 15U1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF). 8528



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