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The Pressure-Induced Amorphization of Strontium Azide Hongyang Zhu, Xue Han, Peifen Zhu, Xiaoxin Wu, Yanmei Chen, Miaoran Li, Xuefeng Li, and Qiliang Cui J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04446 • Publication Date (Web): 27 May 2016 Downloaded from http://pubs.acs.org on June 4, 2016

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The Journal of Physical Chemistry

The Pressure-induced Amorphization of Strontium Azide Hongyang Zhu†,*, Xue Han†, Peifen Zhu§, Xiaoxin Wu†, Yanmei Chen†, Miaoran Li†, Xuefeng Li†, Qiliang Cui† †

State Key Laboratory of Superhard Materials, College of Department of Materials Science and Engineering, Jilin University, Changchun, Jilin 130012, China

§

Department of Physics and Engineering Physics, the University of Tulsa, Tulsa, OK 74104, USA *Corresponding author: Tel: +8643185168881; fax: +8643185168881. E-mail address: [email protected];

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ABSTRACT Strontium azide (Sr(N3)2) has been studied by in-situ high pressure X-ray diffraction at room temperature. Sr(N3)2 exhibits an-isotropic compressibility due to the orientation and rotation of azide anions. The refinement results and spectra measurements reveal that Sr(N3)2 possesses bent azide ions at ambient conditions, differing from the linear azide ions of alkali azides. The bent azide ions further bend and rotate with increasing pressure. These unique properties of bent azide ions play a significant role in the process of electron orbit hybridization and greatly enhance the propensity of nitrogen polymerization. The bulk modulus of Sr(N3)2 is 49.1 GPa, which is larger than those of alkali azides and close to those of heavy metal azides. The larger bulk modulus is attributed to the partial covalent bonding character of Sr(N3)2. Sr(N3)2 transforms into an amorphous phase at relative low pressure compared with alkali azide. This property might induce that Sr(N3)2 transforms into polymeric nitrogen more readily than other inorganic azides.

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І. INTRODUCTOIN Inorganic azides have attracted considerable attentions in practical applications as nitrogen sources, initial explosives, and photographic materials,1-2 as well as significant scientific interest towards their structural stability, lattice dynamics, and electronic structure.3-4 The unique structures and properties of inorganic aides are mostly ascribed to the azide ions, which are linear anions bonded with double nitrogen bonds (N−=N+=N−). The orientation of linear azide ions changes during phase transitions of azides stimulated by either temperature or pressure and greatly affects their high pressure properties.5-10 For example, high-pressure studies of ammonium azide (NH4N3) observed a variation of hydrogen bond as result of rotation of azide ions with increasing pressure.11-12 High-pressure studies of inorganic azides are becoming special interest recently because of their use as precursors to form highly energetic polymeric nitrogen, a new generation of high density materials, which is pioneered by high-pressure studies of nitrogen.13-15 Compared to nitrogen, azide ion has been expected more readily forming polymeric nitrogen due to the lower bond energy of double nitrogen bond (418KJ/mol vs 954KJ/mol). Eremets et al have successfully transformed azide ions of NaN3 into polymeric nitrogen form at pressure above 120 GPa.15 A high-pressure study on LiN3 predicted the formation of polymeric nitrogen net under pressure beyond 60 GPa.16 Consequently, we further studied NH4N3,11 NaN3,10 KN3,17 RbN3,18 CsN3,8 and AgN319 under high pressure and found a series of phase transitions. Recently, a series of theoretical studies further revealed pressure dependence of 3



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electronic properties and band gap. Up to now, the high-pressure experimental studies of inorganic azides are mainly concentrated on alkali azides15-17 and other univalent azides (NH4N3, AgN3).11, 19-20 Accordingly, detailed study of this homologous series of compounds, alkaline earth azides, such as Sr(N3)2, offers the possibility of determining microscopic properties and energetic behavior of metal azides. As a starting material for synthesizing polymeric nitrogen, alkaline earth metal azides possess richer nitrogen mole content of nitrogen than univalent azides. In addition, alkaline earth azides have larger lattice energies than alkali azide, which might induce a different pressure behavior of structure and electron. Therefore, a study of high pressure behavior of alkaline earth azides, as an extension of studies of univalent azides, will provide more insight into the mechanism of formation of polymeric nitrogen. In this work, we have studied Sr(N3)2 under pressure and found the unique high pressure properties of Sr(N3)2 differing from univalent azides due to the unique bent azide ions. П. EXPERIMENTAL METHOD High-pressure experiments were performed in a symmetric diamond anvil cell with flat anvil of 400 µm in diameter. One of diamond anvils is mounted on a boron nitride seat for a large angular opening for X-ray scattering. T301 steel sheets served as gaskets with sample chambers of 100 µm in diameter and 50 µm in thickness. To accurately determine the phase transition pressures and compressibility of Sr(N3)2, two runs of XRD measurements were performed with two runs using a mixture of methanol and ethanol (M:E) with 4:1 volume ratio and one run using silicon oil as 4


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pressure transmitting medium, respectively. In-situ angle-dispersive synchrotron XRD experiments were conducted at 4W2 High-Pressure Station of Beijing Synchrotron Radiation Facility (BSRF) with use of beam wavelength 0.6199 Å, and at beamline B2 in the Cornell High Energy Synchrotron Source (CHESS) at Cornell University with a wavelength of 0.4860 Å.21 The structural refinement was performed through Rietveld method. Ш. RESULTS AND DISCUSSION Sr(N3)2 crystallizes in an face-centered orthorhombic structure with Fddd space group and eight molecules per unit cell (two molecules per primitive cell) at ambient conditions. The Wyckoff positions of Sr atoms, N(I) (end-N atoms) and N(II) (mid-N atoms) are assigned to 8a, 32h and 16e, respectively. Each Sr ion is surrounded by eight azide ions and each azide ion is surrounded by four Sr ions. Sr(N3)2 crystallizes in a layered structure, two Sr ions in one layer and four azide ions in the other layer within a unit cell. Each layer parallels to the (1 0 0) planes as shown in Figure 1.

Figure 1. Crystal structure of Sr(N3)2 along (a) a-b-c, (b) a-b, and (c) b-c axes. Blue color represents N atoms, purple color represents Sr atoms. 5



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Figure 2. Representative XRD patterns of Sr(N3)2 at selected pressures The XRD pattern obtained at ambient conditions was refined using Materials Studio software. The cell parameters are a = 11.81870 (121) Å, b= 11.53114 (145) Å, c = 6.11485 (68) Å. These results are in good agreement with the previous experimental study.22 The high-pressure synchrotron XRD was performed on Sr(N3)2 powder up to 33.5 GPa at room temperature. The representative XRD patterns are shown in Figure 2. The low angle diffraction peaks of (111) and (220) remain relatively stable to 19.2 GPa, while the peaks in high diffraction angle region exhibit a abundant variation. In order to investigate the variation, the high diffraction angle region is zoomed in Figure 2b and Figure 2c. As illustrated in Figure 2b and Figure 2c, the peaks (311) and (131), (202) and (022), (511) and (151), (531) and (351), (620) and (260), (313) and (133) are getting close with increasing pressure and finally merge each other at pressure 2.9 GPa. The orthorhombic phase is tetragonally degenerate at 2.9 GPa pressure instead of a orthorhombic-to-tetragonal phase 6


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transition. With further compress, the merged peaks subsequently split and exhibit abundant variations as illustrated in Figure 2b and Figure 2c. The variations are ascribed to the discrepancy of compression ratio of cell parameters.

Figure 3. The pressure dependence of cell parameters a, b, c of Sr(N3)2 (a), and variation of bending angle of azide ions (b)

The pressure dependences of cell parameters are illustrated in Figure 3a, indicating that a, b, and c axes shrink with different compression ratio of 87.29%, 98.85%, and 92.98%, respectively. The isotropic compressibility of Sr(N3)2 is attributed to the layer structure. There is no intermolecular interaction component along a axis of azide anion as illustrated in Figure 1. Therefore, a axis has the largest compressibility. Figure 1c shows that the intermolecular interaction of azide ions has the largest component along c axis. However, The lowest compressibility is along b 7



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axis, that because the intermolecular interaction component along b axis is increasing and the component along c axis is decreasing with increasing pressure as result of the rotation of azide anions as illustrated in Figure 1c. The rotation character of azide ions under high pressure is a common phenomenon, which has been observed in high-pressure studies on NH4N311, NaN310, RbN323, and CsN38. In the case of Sr(N3)2, the pressure dependence of rotation of azide ions is nonlinear, resulting in nonlinear variation of b axis as function of pressure. In the pressure range of 9.8~15.6 GPa, the b axis even abnormally expand, as shown in the inset of Figure 3a, corresponding to the shift to small angle of diffraction peaks of (040) and (151) as illustrated in parallelograms of Figure 2b,c. The abnormal expansion of b axis means that azide ions rotate faster in pressure range of 9.8~15.6 GPa than in other pressure range. At 23.9 GPa, all of the diffraction peaks vanished, which means Sr(N3)2 transforming into an amorphous phase. Upon pressure release to ambient conditions, the amorphous phase is retained. In the study of NaN3, the azide ions firstly transmit into an amorphouslike structure above 120 GPa, and then polymerized after laser heating.15 Nitrogen molecular also undergoes a nonmolecular or amorphous structure in the process of polymerization.14, 24 Consequently, amorphiziation can be considered playing a significant role in the polymerization of azide ions. So far, except for the report of amorphization of NaN3 above 100 GPa,15 there is no amorphization observed in other studies of alkali azides within the pressure of about 50 GPa (LiN3 to 60 GPa16, NaN3 to 50 GPa10, KN3 to 55 GPa9, RbN3 to 42 GPa18, and CsN3 to 55 GPa8). The significantly low amorphization pressure of Sr(N3)2 are ascribed to the 8


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unique bent azide ions, which might induce a low polymerization pressure. The mechanism is further discussed in the following texts.

Figure 4. Raman (a) and IR (b) spectra of Sr(N3)2 at ambient conditions. The bands below 400 cm-1 including Sr2+ translation , N3− translation, and N3− rotation are all external vibrational modes; ν1, ν2, and ν3 are symmetric stretching, bending, and antisymmetric stretching mode, respectively; The insets are the magnified view of peaks in the dash frames.

Table 1. The activities of the vibrational frequencies in linear and non-linear azide ions. Linear azide ions in AN3 (A=Na25, 9

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K , Rb , Cs , ν1

ν2

NH412) ν3

Bent azide ions in Sr(N3)2a

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Figure 5. Rietveld refinement of XRD pattern of Sr(N3)2 at ambient conditions. The observed diffraction intensities are represented by the red dots, and the calculated pattern is represented by the black solid line. Vertical bars indicate the positions of allowed Bragg reflections. The blue solid curve at the bottom represents the difference between the observed and the calculated intensities. Inset shows the Rietveld refined lattice structure of Sr(N3)2 at ambient conditions with a bent azide ions of 173.161 degree. The mean quality factors of the refinement are Rwp=6.90%, Rp=8.73%. By the group analysis of linear and symmetric azide ions (N=N=N), the symmetric stretching ν1 is Raman active, and bending ν2 and antisymmetric stretching modes ν3 are IR active. All the alkali azides with linear symmetric azide ions follow 10


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the rule analyzed above. However, in the case of Sr(N3)2, all of the ν1, ν2, and ν3 modes were observed in both Raman and IR spectra as shown in Figure 4. Therefore, we infer that the symmetry of azide ions in Sr(N3)2 is nonlinear and/or asymmetrical, differing from the linear and symmetric azide ions of alkali azide. The activities of the vibrational modes in different azides are summarized in Table 1. In order to study the structure of azide ions in Sr(N3)2, we refined the XRD pattern of Sr(N3)2 at ambient conditions. The refine result, as shown in Figure 5, indicates that the azide ions are slightly bent with a angle of 173.161 degree, identical to the spectra analysis of above. Therefore, both the point group analysis and experiments results confirm that the azide ions of Sr(N3)2 is bent. We believe the bent azide ions play a significant role in the process of amorphization and polymerization of Sr(N3)2. The theory studies predicted that the azide ions of metal azides transform into zig-zag chain or Benzene-like N6 polymeric nitrogen under high pressure, accompanied and driven by the change of electron orbit hybridization from sp to sp2 or sp3.4, 27 However, the hybridization of electron orbit hard to occur and the polymerization pressures of alkali azide are extremely high because the linear azide ions keep linearly until at extreme high pressure. In this respect, the bending azide ions chain might readily to approach each other and then hybridize and polymerize under pressure. Therefore, the bent azide ions might induce the azide ions of Sr(N3)2 to form a polymeric nitrogen network more readily than alkali azides. The study of variation of bent azide ions with increasing pressures can reveal the process and mechanism of polymerization of azide ions. We have tried to refine the XRD data to study the variation azide ions with 11



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compression, while the pressure dependence of variation of bending azide is of unreasonable fluctuation, because of the large contrast between X-ray scattering capacities of Sr and N, and the weak scattering of N atoms. It is thus difficult to determine the accurate positions of N atoms. Therefore, the pressure dependence of bent angle of azide ion was calculated using ab initio calculations with plane wave pseudo potential density functional computer code Cambridge Serial Total Energy Package (CASTEP) with Vanderbilt-type ultrasoft pseudopotentials.28 The correlation effects are described by the generalized-gradient-approximation (GGA) of Perdew-Burke-Ernzerhof (PBE).29 The cutoff energy of plane waves was set to 250.0.eV. The calculated results reflect the trend of changes of bending angle under pressure, though absolute values might not be not precise. The calculated angle as a function of pressure is plotted in Figure 3b, indicating that the angle of azide ions decreases with increasing pressure, which means the azide ions further bend under pressure. The adjacent azide ions rotate and bend to each other upon compression as shown in the schematic diagram Figure 6. The rotation and bending of adjacent azide ions and shrink of a axis result in the proximity of terminal nitrogen atoms of these adjacent azide ions. When the terminal nitrogen atoms close enough, the N electron orbits hybridize leading to the polymerization of azide ions to benzene like "N6" molecules, a phenomena has been predicted in theory studies.4, 27, 30-33 The variation and polymerization of azide ions under pressure is illustrated in schematic diagram Figure 6. Therefore, we believe the bending azide ions could greatly enhance the propensity of polymerization on N. The high pressure study of azides with bent azide 12


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ions might provide a significant step forward in the polymerization of azide ions. The variation of the azide ions under pressure is expected to be further confirmed by the study of vibration of azide ions.

Figure 6. The schematic diagram of variation and polymerization of azide ions The bulk modulus of Sr(N3)2 and its pressure derivative are fitted by the third-order Birch-Murnaghan equation of state.

3 P = K OT 2

7 5 2      3 3 3 V V V 3         ' 0  0 − 0  × 1 + ( K OT  − 4 ) ×   − 1       V   V    4  V       

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Figure 7. Unit cell volume dependence on pressure. The black and red lines demonstrate the fitting of the P-V data to the third-order and second-order Birch-Murnaghan equation of state, respectively. where V0 is the volume at ambient pressure, KOT is the bulk modulus at ambient pressure and temperature, and K'OT is the pressure derivative of KOT. The XRD peak positions are hard to be subtracted when pressure is above 20 GPa because of the weakening, broadening, and greatly overlap of XRD peaks. Therefore, the data below 18.2 were used to yield bulk modulus. The bulk modulus derived from experimental data is KOT = 49.1±1.1 GPa with pressure derivative, K'OT =2.5±0.2. When K'OT set as 4, the bulk modulus is KOT = 40.9±0.4 GPa. The bulk modulus yielded from CASTEP calculated volume is KOT = 45.7 GPa with K'OT =3.1, in good agreement with the experimental results. The consistency of experimental and computational results confirms calculation is reasonable. The experimental pressure dependence of unit cell volumes is plotted in Figure 7. In comparison with other metal azides, the bulk 14


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modulus of Sr(N3)2 is larger than that of alkali azides (NaN3 (17.3 GPa)10, KN3 (18.6 GPa)17, RbN3 (18.4 GPa)18, and CsN3 (18 GPa)8) and close to heavy metal azides (AgN3 (39 GPa)19 and α-Pb(N3)2 (41 GPa)34). Yedukondalu et al proposed that the covalent azides have a larger bulk modulus than ionic azide due to the strong directional bonding.3 Colton et al have reported that heavy metal azides are considerably more covalent than the alkali azides according to their X-ray electron spectroscopy studies.35 Hence, we infer Sr(N3)2 is not purely ionic compound and has covalent bonding character resulting in a larger bulk modulus. This inference is confirmed by the theory studies in which Zhu et al found a covalent character of Sr(N3)2.36

IV. CONCLUSION In conclusion, the high pressure in-situ synchrotron XRD study of Sr(N3)2 revealed a amorphization transition at relative low pressure. The amorphous phase transition is irreversible. With compression, the azide ions undergo a rotation and bending process. The process results in a proximity of adjacent azide ions, a phenomena which finally triggers the electron orbit hybridization and nitrogen polymerization. The high pressure study of azides with bent azide ions might provide a significant step forward in the polymerization of azide ions. The rotation of azide results in the lowest compressibility along b axis. The bulk modulus of Sr(N3)2 is determined to be KOT = 49.1±1.1 GPa with K'OT =2.5±0.2, which is larger than those

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of alkali azides and close to heavy metal azides. The larger bulk modulus is ascribed to the partial covalent bonding character.

ACKNOWLEDGMENTS We thank J Liu and Z Wang for their technical support with synchrotron XRD measurements at Beijing Synchrotron Radiation Facility and CHESS, Cornell University. CHESS is supported by the NSF & NIH/NIGMS via NSF award DMR-1332208. This work is supported by the National Natural Science Foundation of China (11304111, 11304139).

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1964, 40, 3195-3203. (26) Hathaway, C.E.; Temple, P.A. Raman spectra of the alkali azides: KN3, RbN3, CsN3. Phys. Rev. B 1971, 3, 3497-3503. (27) Wang, X.L.; Li, J.F.; Xu, N.; Zhu, H.Y.; Hu, Z.Y.; Chen, L. Layered polymeric nitrogen in RbN3 at high pressures. Sci. Rep. 2015, 5. (28) Segall, M. D.; Lindan, P. J. D.; Probert, M. J.; Pickard, C. J.; Hasnip, P. J.; Clark, S. J. Payne, M. C. First-principles simulation: Ideas, illustrations and the CASTEP code. J. Phys: Condens. Matter, 2002, 14, 2717-2744. (29) Perdew, J.P.;Burke, K.;Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865-3868.

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(30) Zhang, M.G.; Yan, H.Y.; Wei, Q.; Wang, H.; Wu, Z.J. Novel high-pressure phase with pseudo-benzene "N-6" molecule of LiN3. Epl 2013, 101, No. 26004. (31) Zhang, M.G.; Yin, K.T.; Zhang, X.X.; Wang, H.; Li, Q.; Wu, Z.J. Structural and electronic properties of sodium azide at high pressure: A first principles study. Solid State Commun. 2013, 161, 13-18. (32) Zhang, M.G.; Yan, H.Y.; Wei, Q.; Liu, H.Y. A new high-pressure polymeric nitrogen phase in potassium azide. Rsc Adv. 2015, 5, 11825-11830. (33) Zhang, J.; Zeng, Z.; Lin, H.-Q.; Li, Y.-L. Pressure-induced planar N(6) rings in potassium azide. Sci. Rep. 2014, 4, No. 4358. (34) Weir, C.E.; Block, S.; Piermarini, G.J. Compressibility of inorganic azides. J. Chem. Phys. 1970, 53, 4265-4269. (35) Colton, R.J.; Rabalais, J.W. Electronic structure of some inorganic azides from x

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TOC Graphic.

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Figure 1. Crystal structure of Sr(N3)2 along (a) a-b-c, (b) a-b, and (c) b-c axes. Blue color represents N atoms, purple color represents Sr atoms. 792x300mm (72 x 72 DPI)


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Figure 2. Representative XRD patterns of Sr(N3)2 at selected pressures 156x80mm (300 x 300 DPI)



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Figure 3. The pressure dependence of cell parameters a, b, c of Sr(N3)2 (a), and variation of bending angle of azide ions (b) 99x68mm (300 x 300 DPI)


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Figure 4. Raman (a) and IR (b) spectra of Sr(N3)2 at ambient conditions. The bands below 400 cm-1 including Sr2+ translation , N3− translation, and N3− rotation are all external vibrational modes; ν1, ν2, and ν3 are symmetric stretching, bending, and antisymmetric stretching mode, respectively; The insets are the magnified view of peaks in the dash frames. 80x71mm (300 x 300 DPI)



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Figure 5. Rietveld refinement of XRD pattern of Sr(N3)2 at ambient conditions. The observed diffraction intensities are represented by the red dots, and the calculated pattern is represented by the black solid line. Vertical bars indicate the positions of allowed Bragg reflections. The blue solid curve at the bottom represents the difference between the observed and the calculated intensities. Inset shows the Rietveld refined lattice structure of Sr(N3)2 at ambient conditions with a bent azide ions of 173.161 degree. The mean quality factors of the refinement are Rwp=6.90%, Rp=8.73%. 686x553mm (96 x 96 DPI)


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Figure 6. The schematic diagram of variation and polymerization of azide ions 597x476mm (72 x 72 DPI)



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Figure 7. Unit cell volume dependence on pressure. The black and red lines demonstrate the fitting of the PV data to the third-order and second-order Birch-Murnaghan equation of state, respectively. 99x133mm (300 x 300 DPI)


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Pressure-Induced Amorphization of Strontium Azide - The Journal of

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