Hydrogen Bond in Compressed Solid Hydrazine - American Chemical

Jan 15, 2014 - symmetry of phase I is confirmed to be space group P21 by the peak ... hydrogen atom and hydrogen bond in solid hydrazine under pressur...
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Hydrogen Bond in Compressed Solid Hydrazine Shuqing Jiang, Xiaoli Huang, Defang Duan, Songkuan Zheng, Fangfei Li, Xue Yang, Qiang Zhou, Bingbing Liu, and Tian Cui* State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, People’s Republic of China ABSTRACT: The high-pressure behavior of hydrazine has been investigated by in situ Raman spectroscopy and synchrotron X-ray diffraction experiments under pressure up to 46.5 and 33.0 GPa, respectively. It is found that the liquid hydrazine solidifies into phase I at about 1.2 GPa. The symmetry of phase I is confirmed to be space group P21 by the peak assignment, group theory analysis, and Rietveld refinement of XRD patterns. A solid−solid transition from phase I to phase II is observed in both Raman spectroscopy and XRD experiments at about 2.4 GPa, which is ascribed to the formation of new hydrogen bonds between hydrazine molecules. At 18.4 GPa, an isostructural transition from phase II to the final phase III is observed. The pressure-induced adjustment of bifurcated hydrogen bond is first researched and regarded as the origin of the isostructural transition. Above 20.6 GPa, a clear softening behavior occurs in the NH2 symmetric stretching mode. The coupling of optical vibrations derived from enhancement of the hydrogen bond is proposed as a crucial role in this softening process.

I. INTRODUCTION

in the diamond anvil cell (DAC) gives a reference in the storing of compressed hydrazine in the rocket. In previous works, the molecular structure of hydrazine is confirmed to be gauche conformation with C2 symmetry with 12 fundamental vibrations as shown in Figure 1.25,26 Pure

The solid hydrogen, an electrical insulator, was predicted to become an alkali metal under extreme compression.1 Since Ashcroft pointed out that the metal hydrogen would be a roomtemperature superconductor in 1968,2 the metallization of hydrogen has drawn tremendous attention as a significant scientific problem so far. Ceperley and Alder reported that the metallic pressure of hydrogen was about 3.0 ± 0.4 Mbar by the Quantum Monte Carlo calculation.3 However, the recent experimental result showed that the solid hydrogen remained transparent until 360 GPa.4 In view of such an ultrahigh pressure in pressurizing pure hydrogen directly, it is proposed that the role of chemical precompression could lower the pressure of metallization.5 Therefore, the hydrogen-rich simple molecular systems under high pressure have been studied extensively, such as CH4, NH3, H2O, and HX (X = Cl, Br).6−14 And the hydrogen atom shows various behaviors and properties in these systems under pressure. Furthermore, it is suggested that the pressure-induced change in the hydrogen bond plays an important role in the phase transitions of hydrogen-rich materials. The hydrogen-bond symmetrization has been observed in solid water,15−17 ammonia,18−20 and halogen hydride.21−23 And the symmetric hydrogen bond in the simple compounds of the hydrogen and nitrogen, oxygen, and halogen atoms can achieve no more than 60 GPa. As an important and typical simple molecule, the hydrazine (N2H4) is composed of two N atoms and four H atoms, which contains as high as 12.6 wt % of hydrogen. Thus, as a hydrogenrich and high-energy material, the hydrazine has been applied greatly including as one major fuel in the aerospace field, etc.24 What is more, the study of high-pressure behavior of hydrazine © 2014 American Chemical Society

Figure 1. Fundamental vibrations of hydrazine molecule. Received: October 16, 2013 Revised: January 14, 2014 Published: January 15, 2014 3236

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Figure 2. (a) Raman spectra in the region of 0−800 cm−1 from 0.6 to 46.5 GPa. (b) Peak positions as a function of pressure in three phases. The detailed peak fit of the lattice peaks in the pressure range from 1.8 to 17.0 GPa is inserted in (b). The new peak is marked with the symbol *. The two dotted lines represent boundaries of different phases.

The backscattering Raman spectra in the DAC are recorded in the backscattering geometry, using the system with a 532 nm laser excited by doubled solid-state Diode Nd: Yanadate laser (Coherent Company), and the maximum laser power is 2 W. The in situ high-pressure angle scattering X-ray diffraction measurement is carried out on the wiggler beamline (4W2) of the Beijing Synchrotron Radiation Facility. An image plate detector (MAR-3450) is used to collect diffraction patterns, and the two-dimensional XRD images are analyzed using the FIT2D software, yielding one-dimensional intensity versus diffraction angle 2θ patterns. The XRD patterns are fitted by Rietveld profile matching using the Material Studio program.

hydrazine exists as a liquid at ambient conditions, and the strong hydrogen bonds form between molecules as in the case of water.27 The structure of solid hydrazine has been mainly researched at low temperatures.28−31 The peak assignment in the Raman and infrared (IR) spectra is uniform in these experiments, although some debates exist in the low-temperature structural confirmation. Recently, solid hydrazine has been researched by Raman spectroscopy under pressure, and two phase transitions are observed in the pressure range of 0− 19 GPa.32 However, no peak below 500 cm−1 in the lattice region is collected in the high-pressure Raman spectra, which is unreasonable in such a long hydrogen-bond linked molecular crystal. In addition, to the best of our knowledge, there is no report on the structure of solid hydrazine under pressure. Therefore, it is very significant to uncover the structure and property of solid hydrazine, especially the behaviors of hydrogen atom and hydrogen bond in solid hydrazine under pressure. In this work, we have performed in situ Raman spectroscopy, synchrotron angle-dispersive X-ray diffraction (XRD), and group theory analysis to explore the high-pressure behaviors of hydrazine. Our results show that the solid hydrazine undergoes two solid−solid phase transitions arising from pressure-induced rearrangement of hydrogen atoms related to hydrogen-bond formation and adjustment directly.

III. RESULT AND DISCUSSION The in situ high-pressure Raman spectra of hydrazine are collected from the pressure of 0.6 GPa. As shown in Figure 2a and 3c, the lattice peak at 188 cm−1 is assigned to the transitional vibration of hydrazine dimer in liquid state. Four weak and broad peaks at 3181, 3203, 3273, and 3334 cm−1 are attributed to NH2 symmetric and antisymmetric stretching modes, indicating that the hydrazine molecules rotate freely and are orientationally disordered in liquid state. All the stretching peaks shift to low frequency with increasing pressure up to 1.1 GPa. Such red-shift behavior resulted from the strengthening hydrogen bonds like those reported in liquid water.33 Phase I. Editor: At about 1.2 GPa, the liquid hydrazine solidifies into translucent white crystal with a decrease in pressure to 0.5 GPa. However, the solid hydrazine (phase I) has not melted back to liquid. It is deduced that new short and strong hydrogen bond forms during the solidification process, providing stronger attraction between molecules. Moreover, as shown in Figures 2 and 3, remarkable changes appear in the spectra. The peaks located in the low-frequency region from 0 to 500 cm−1 are assigned to the modes of lattice vibration. Raman peaks are recognized below 200 cm−1, indicating that the molecular arrangements are long-range ordering at low

II. EXPERIMENTAL DETAILS The liquid hydrazine is purchased from Sigma-Aldrich (purity >98%). The DAC with a culet face of 400 μm in diameter is used to generate high pressure for in situ Raman scattering and angle-dispersive XRD measurements. A T301 stainless steel gasket is preindented to a thickness of 60 μm and a center hole with a diameter of 100 μm is drilled as the sample chamber. The moisture-sensitive sample is loaded into the gasket hole in a vacuum glovebox (nitrogen atmosphere). The pressure in the DAC is calibrated by the ruby fluorescence method. 3237

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Figure 3. Raman spectra at various pressures in the ranges of (a) 800−1300 cm−1, (b) 1450−1950 cm−1, and (c) 3000−3500 cm−1. Pressure dependence of the optical peaks frequency is shown in (d).

pressures. Two peaks at 213 and 280 cm−1 are related to the lattice vibration too. At higher frequency, a weak peak at 521 cm−1 is rather isolated from the lattice peaks, which is assigned to the torsion vibration (V7). As shown in Table 1, the difference in the torsion peak frequency between the experimental and calculated results is attributed to the calculated deviation of weak interactions in molecular crystal.34 The lattice peaks move slowly to high frequency with intensity increasing, implying that the crystal is slightly compressed up to 1.2 GPa. The peaks in the region of 800−3500 cm−1 are attributed to the intramolecular vibrational modes of solid hydrazine. In Figure 3a, a sharp peak appears at 891 cm−1 with a weak shoulder at 911 cm−1 at 0.5 GPa; the two peaks are assigned to the NH2 rocking modes (V6 and V12). Another sharp peak at 1118 cm−1 is attributed to the N−N stretching mode (V5). Besides, a weak crystal split is observed at 1137 cm−1. The deformational vibrations locate in the region from 1500 to 1900 cm−1. As shown in Figure 3b, two peaks at 1637 and 1662 cm−1 are assigned to the NH2 deformational mode (V10). In addition, the NH2 wagging peaks are not observed, which are covered by

the strong first-order Raman peak of diamond (at about 1331 cm−1). In Figure 3c, the NH2 stretching peaks locate in the region from 3000 to 3500 cm−1. During the solidification process, the mixed peaks separate into three intense and wellresolved peaks. Two peaks at 3168 and 3226 cm−1 are assigned to NH2 symmetric stretching mode (V9 and V2), and the highfrequency peak at 3304 cm −1 is attributed to NH 2 antisymmetric stretching mode (V1). It is concluded that the hydrazine molecules are no more orientationally disordered in the solid. With increasing pressure, the symmetric stretching peaks shift to low frequency, while the antisymmetric stretching peak shifts normally to high frequency. The red-shift behavior in symmetric stretching peaks is attributed to the strengthening hydrogen bond, which weakens the covalent bond and leads to a negative pressure shift of the NH2 symmetric stretching frequency. As shown in Table 1, the assignment of optical peaks in phase I is listed with a comparison of the calculated results. Phase II. At about 1.8 GPa, several new peaks appear in the spectra, suggesting that phase transition occurs from phase I to phase II. Concretely, three weak peaks appear in the lattice vibrational region as the strongest peak vanishing at around 200 3238

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Table 1. Comparison of Raman Peaks between Experimental Measurement and Calculated Results

Figure 4. (a) Frequency shifts of the NH2 symmetric stretching peak and deformational peak as a function of pressure. (b) and (c) illustrate the detailed evolution of the two peaks under pressures.

cm−1 in the new phase as shown in Figure 2. Simultaneously, two close peaks at 127 and 128 cm−1 appear as the pressure reaches 1.8 GPa. The detailed peak fit of the lattice peaks is inserted in Figure 2b. It is noted that many weak overlapped peaks are collected, which is attributed to various polarizations in the polar molecular crystal such as ammonia.35 The remaining peaks in the new phase show continuous shift to high frequency. In the optical vibrational region, as shown in Figure 3, a new intense peak appears abruptly at 1605 cm−1, which is assigned to the NH2 deformational mode. At the same time, the intense antisymmetric stretching peak splits into two mixed peaks at 1.8 GPa, and then separates into two entirely different peaks centered at 3345 and 3367 cm−1 up to 29.9 GPa. It is suggested that the splits in the deformational and

antisymmetric stretching modes are caused by the distortion of crystal above 1.8 GPa. With increasing pressure, the new NH2 deformational and stretching peaks shift to low frequency with the strengthening hydrogen bond. Phase III. The frequency shift of the new deformational peak as a function of pressure and the selected spectra are shown in Figure 4a,c. The deformational peak shows persistent red shift up to 18.4 GPa, and then its frequency turns to increase up to the highest pressure of 46.5 GPa. Synchronously, the peak intensity increases rapidly up to 18.4 GPa and then begins to drop continuously at higher pressures. In particular, in the pressure range from 18.4 to 29.9 GPa, the peak frequency shows a very small shift range of 1.0 cm−1. Above 29.9 GPa, it begins to shift to high frequency with a ratio of 0.8 cm−1 per 3239

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GPa up to 46.5 GPa. The similar dramatic spectral change of the deformational vibration has been observed by Ninet et al.35 In the high-pressure Raman spectra of solid ammonia, it is found that the deformational mode (V4) is either constant or slightly increasing in the pressure range from 6 to 11 GPa. After the neutron diffraction study is performed, the origin of change in V4 of ND3 is ascribed to a slight displacement of D atom in the D-bond.21 On the other hand, the calculated result shows that the phase transition is attributed to a crossover of the offdiagonal moduli C12 and C13.7 In general, this conversion of mode V4 is interpreted as an isostructural transition in solid ammonia. An obvious softening behavior is observed in the NH2 symmetric stretching mode with increasing pressure. As shown in Figure 4, the frequency shift of the intense symmetric stretching peak is plotted as a function of pressure. It is noted that the NH2 stretching peak weakens and broadens rapidly above 20.6 GPa. Such softening behavior is called Fermi resonance, which results from the coupling of optical vibrations.36,37 As mentioned above, the pressure-induced softening in the H−X (X = N, O, Br, Cl) stretching mode is a significant phenomenon in the hydrogen-bond symmetrization process in hydrogen-rich molecular crystals.15−23 Furthermore, most stretching peaks disappear at about 40 GPa, and then the hydrogen-bond symmetrization achieves no more than 60 GPa. In solid hydrazine, the vibrations coupling occurs between the NH2 symmetric stretching mode and other optical vibrations by the strong hydrogen-bond interaction. With increasing pressure, the NH2 symmetric stretching peak completely disappears at 29.9 GPa. The disappearance pressure is lower than those in the solid water and ammonia. According to the double Morse potential (DMP) model of hydrogenbond symmetrization,38 it is proposed that the hydrogen-bond symmetrization in hydrazine may occur below 60 GPa. In phase III, the other peaks shift normally to high frequency and no new peak appears until 46.5 GPa. Upon decompression, the phase transitions are reversible and the hydrazine molecules survive in pressure cycling up to 46.5 GPa. Space Group Analysis. The space group of phase I in solid hydrazine is discussed by a comparison with the structures suggested in previous studies. Although the previous highpressure Raman study did not provide direct structural information of solid hydrazine, several alternative structures at low temperature are suggested.28−31 In the Raman spectrum at 1.2 GPa, seven Raman peaks are observed in the lattice vibrational region from 0 to 500 cm−1. Meanwhile, at least eight peaks are collected in the optical vibrational region. The irreducible representation of proposed space group C22h is given as28,31

Γ(OL) = 3A + 3B

Although some Raman peaks have not been observed in our experiment owing to the limitation of experimental installations and precision, the collected peaks are well assigned into this space group. Furthermore, we have calculated the Raman spectra of C22 using the CASTEP code.39 The calculated results fit well with the experimental spectra as shown in Figure 5.

Figure 5. Raman spectra of solid hydrazine ranging 0−3400 cm−1 in phase I by experiment measurement (above) and calculation (below).

XRD Measurement. To further explore the crystal structure of hydrazine, we have carried out in situ high-pressure XRD experiment from 1.6 to 33.0 GPa. Selected XRD patterns of hydrazine as a function of pressure upon compression are shown in Figure 6. At 2.4 GPa, a new weak peak emerges close to the two strongest peaks in the middle degree region. The small change in the XRD patterns during the phase transition is consistent with the Raman experimental result. With increasing pressure, the intensities of peaks decrease rapidly, and no new peaks appear until 33.0 GPa. It is in accord with the isostructural transition in the Raman experiment above. As shown in Figure 7, The Rietveld refinement of measured XRD pattern shows good agreement with the calculated result, indicating that the space group P21 matches phase I very well, and the Rwp = 0.13, Rp = 0.26. Because the sample in the chamber is more like the polycrystal rather than perfect powder sample, a small number of diffraction points appearing in the rings affect the real peak intensities. This is a main reason that the values of R factors are relatively high. But the calculated peak positions have fitted the experimental results perfectly. Therefore, we conclude that values are authentic and in the acceptable range. Hence, the space group of phase I is proved to be P21 by both the Raman and XRD measurements; the refined lattice parameters are a = 4.49 (3) Å, b = 5.57 (6) Å, c = 3.35 (6) Å, and β = 110.27(3)° with unit cell volume V = 78.88(1) Å3. The fractional coordinates of N and H atoms are listed in Table 2. In the transition from phase I to phase II, only a few new peaks appear in the spectra both in Raman and XRD experiments while most peaks remain unchanged, implying that the phase transition is caused by distortion in structure rather than reconstruction. So it is inferred that phase II is still the monoclinic structure. Hydrogen Bond. The pressure-induced changes in hydrogen atom and hydrogen bond play a key role in the phase transitions of hydrogen-rich materials. Therefore, the behavior

Γ(vib) = 2A u + 3Ag + 3Bg + Bu Γ(OT) = 2Ag + Bg Γ(OL) = Ag + 2Bg + 2A u + Bu

The peaks in phase I cannot be assigned into the 12 modes, so the C22h symmetry is ruled out. The alternative space group is C22.29,30 Its irreducible representation is given as

Γ(OT) = 2A + B Γ(vib) = 5A + 4B 3240

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of hydrogen atom and hydrogen bond in hydrazine is emphatically discussed. In the solidification process, a decrease in pressure from 1.2 to 0.5 GPa is observed. However, the hydrazine stays in the solid state owing to the strong hydrogenbond attraction between molecules in the crystal. The decrease in pressure in the solidification process is usually accompanied by a collapse of the sample volume. Meanwhile, a clear Raman spectrum of the hydrazine crystal is collected down to 1.0 GPa upon decompression. It means that the solid hydrazine could preserve at 1.0 GPa, even 0.5 GPa. It is of great significance in storing and reducing volume of the hydrazine fuel in the rocket. In phase I, as shown in Figure 8, there are two hydrazine molecules in the unit cell. In solid hydrazine, the molecules

Figure 6. Representative XRD patterns of solid hydrazine in the pressure range from 1.6 to 33.0 GPa at room temperature with background subtracted (incident wavelength λ = 0.6199 Ǻ ). The new peak is marked with the symbol *. The detailed peak fit of the peaks in the 2θ range of 12°−16° is inserted in the blank area at the top.

Figure 8. Crystal structure P21 at 1.6 GPa simulated by the Material Studio program. The hydrogen bonds are marked as dashed lines, and the length and angle of bifurcated hydrogen bond are calculated in detail.

with different spatial configurations form stagger layers along the B axis. The H atoms between two layers are mainly attracted by two nearest N atoms. At 1.6 GPa, the lengths of Hbonds in N−H1...N1 and N−H1...N2 branches are 2.107 and 2.727 Å, and the bond angles are 125.793° and 120.931°, respectively. It is recognized as the typical configuration of the bifurcated hydrogen bond (aad model).40 Another H atom between layers is also surrounded by two N atoms; the Hbonds lengths in the N−H2...N1 and N−H2...N2 branches are 1.864 and 2.297 Å, and the angles are 155.137° and 126.849°, respectively. In addition, other two H atoms would like to form H-bonds in the monolayer, and the shortest H-bond length is 1.618 Å. With increasing pressure, the hydrogen bonds become shorter and stronger in the condensed crystal. Meanwhile, the H and N atoms with long distance begin to participate in the hydrogen-bonds network at high pressure. In the transition from phase I to phase II, the new optical peaks are assigned to the split of the deformational and antisymmetric vibrations. It is attributed to the degeneracy of optical modes accompanying the distortion of the crystal induced by the formation of hydrogen bond. In the isostructural transition, the remarkable change in the deformational peak is observed in the Raman spectra. The displacement of hydrogen atom and the crossover of elastic moduli C12 and C13 are obtained in solid ammonia by

Figure 7. Rietveld refinement of the XRD pattern at 1.6 GPa. The refinement is conducted without the background. The black lines denote the difference between the observed and the simulated profiles. The inset represents the calculated structure P21.

Table 2. Fractional Coordinates of Atoms of Solid Hydrazine in Phase I at 1.6 GPa

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experimental and calculated methods, respectively.7,21 It is wellknown that the hydrogen atom position variation is closely related to the angle and length of hydrogen bond. In other words, it means that the displacement of hydrogen atom is determined by the total force of related hydrogen bonds. Therefore, the hydrogen-bonding environment should be taken into consideration in interpretation of the isostructural transition. It is proposed that the pressure-induced selfadjustment of the bifurcated hydrogen bond plays a leading role in the isostructural transition. As shown in Figure 8, in the bifurcated hydrogen bond, the H1 bond holds dominant in the movement of the H1 atom for it is much stronger than the H2 bond below 18.4 GPa. With increasing pressure, the H1 atom moves nonlinearly to the N1 atom and the NH2 deformational peak shows monotonically red shift to low frequency in this process. On the other hand, as mentioned above, the abnormal changes of the elastic moduli are observed simultaneously.7 Concretely, in phase II, because of the existence of the broad vacant field between the H1 and N2 atoms, the elastic moduli in this direction (recorded as C2) is much smaller than it is in the H1−N1 direction (recorded as C1). Therefore, the hydrogen bond between H1 and N2 is compressed at a faster rate. At 18.4 GPa, the N2 atom moves to a specific position where the H1 bond and H2 bond equally dominate the movement of the H1 atom. The isostructural phase transition happens with a crossover of the elastic moduli C1 and C2 at this pressure. Above 18.4 GPa, the H2 bond plays a leading role in the movement of H1 atom, and the H1 atom moves to the N2 atom slowly. In this process, the inner angle of the NH2 group decreases first and then increases in the new phase.21 Therefore, the deformational peak shows a transition from red shift to blue shift during the isostructural transition. The pressure-induced self-adjustment of the two branches in bifurcated hydrogen bond is proposed as the original reason.

and Innovative Research Team in University (No. IRT1132), National Natural Science Foundation of China (Nos. 51032001, 11074090, 10979001, 51025206, 11204100, 11274137, 11004074), and National Found for Fostering Talents of Basic Science (No. J1103202).

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IV. CONCLUSIONS In summary, the behavior of solid hydrazine has been studied by in situ Raman and XRD measurements under pressure. It is found that liquid hydrazine solidifies to solid phase I at 1.2 GPa. And then a solid−solid transition from phase I to phase II occurs at 2.4 GPa, which is arising from the formation of hydrogen bonds. At about 18.4 GPa, further isostructural transition from phase II to phase III is observed in Raman spectroscopy. The origin of phase transitions is proved to be the pressure-induced self-adjustment of bifurcated hydrogen bond. Clear softening behavior in the NH2 stretching modes has been observed above 20.6 GPa, which is regarded as an important signal in the hydrogen-bond symmetrization process.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86-431-85168825. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Xiaodong Li, Yanchun Li, and Lun Xiong for their help during the experimental research. ADXRD experiments of this work were performed at 4W2 HP-Station, BSRF assistance in the synchrotron measurement. This work was supported by the National Basic Research Program of China (No. 2011CB808200), Program for Changjiang Scholars 3242

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