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Pressure-Induced Amorphization and Recrystallization of SnI2 Lu Wang, Xiaoli Huang, Da Li, Fangfei Li, Zhonglong Zhao, Wenbo Li, Yanping Huang, Gang Wu, Qiang Zhou, Bingbing Liu and Tian Cui* State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun, 130012, P. R. China
ABSTRACT : The high-pressure behavior of SnI2 has been investigated in a combined experimental and theoretical study by angle-dispersive X-ray diffraction, Raman scattering measurements and ab initio calculations. Both the Raman and XRD results confirm that the SnI2 crystal undergoes a gradual crystal to amorphous transition and subsequently recrystallizes to a new crystal structure upon compression. The intensity of the Sn-I symmetric stretching mode greatly decreases and manifests a red shift at 2.15 GPa in Raman spectra. The XRD patterns show a bonding break phenomenon before the pressure-induced amorphization. We propose that the bond breaking between neighboring layers leads to the formation of the amorphous phase under high pressure. The sample recrystallizes into a new high-pressure crystal phase at 33.18 GPa. The experimental and theoretical results provide a good candidate structure for the recrystallized phase with C2/m space group. Keywords: high pressure, Tin iodide (II) (SnI2), Synchrotron X-ray diffraction, Raman spectra
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INTRODUCTION
Tin iodide (II) (SnI2) is a layered luminescence semiconductor. The bright color and high photosensitivity properties of SnI2 make it useful in applications like electric arc lamps and photo-recording mediums1,2 The ambient structure of SnI2 contains two non-equivalent types of cation sites.3 This fact appeals to researchers to investigate the electronic structures and optical properties of SnI2. Although much work has reported about the band gap of SnI2, there is still a controversy regarding to the nature of the band gap.1-5 To understand the electronic structure in more detail, it is crucial to investigate the crystal structure of SnI2, since the electronic structure is closely related to the crystal structure. Up to now, no high-pressure research on SnI2 has been done and this study is the first to give structural information of SnI2 under high pressure. Pressure-induced phase transition has drawn great attention because of its fundamental importance in physics, chemistry, and materials. It is also a powerful way to synthesize new phases or novel properties which cannot be obtained under ambient pressure.6,7 Since the discovery of amorphization in ice (H2O) by Mishima et al,8 it has caused great interest to study the pressure-induced crystalline to non-crystalline transition which has been observed to occur for a range of structures.9-21 The pressure-induced amorphization (PIA) phases have been found to have many interesting properties, like anisotropy and memory effects.22-24 For compounds like SnI4 and GeO2, there exist phase transitions among the amorphous states.25,26 Group IV halides have been investigated widely to understand structural changes and amorphous mechanisms.27-30 It has been found that there are plentiful and interesting phenomena in the group IV halides under high pressure. In particular for SnI4 previous studies proposed that by the application of pressure (at ~10 GPa) the metallic high pressure amorphous phase of SnI4 is obtained,30 which recrystallizes to a non-molecular crystalline phase III (CP-III) at 61 GPa.25 Several models have been proposed, but the mechanism of pressure–induced amorphization is still controversial.25, 27, 28 To understand the structural behavior under pressure in group IV halides more detailed and comprehensive, it is insightful to study SnI2 as it exhibits 2
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variable valence metal halides. Numerous molecules will form dimers under high pressure31,32 and SnI2 is a partial component of SnI4, we may expect to learn more about its structural type under high pressure. These effects motivate our interest to study SnI2. We have performed in situ angle-dispersive synchrotron X-ray diffraction (XRD), Raman spectroscopy and ab initio calculations to study the crystal structure of SnI2 under high pressure. Our results show that the SnI2 undergoes two phase transitions: (1) a crystal to partial amorphous state transition and (2) a recrystallization of the amorphous phase. These observations give a new insight into the structural information of the binary compound SnI2 under high pressure.
EXPERIMENTAL AND THEORETICAL METHODS SnI2 of 99 % purity was purchased from the J&K company and the crystals were ground into fine powders. The sample was placed into a cavity of 120 µm diameter drilled in a T-301 stainless steel gasket of 53 µm thick, and loaded into the modified Mao-Bell-type diamond anvil cell. A small ruby chip was put into the hole together with the powder sample to determine the pressure inside the Diamond Anvil Cell (DAC) by the standard ruby fluorescence method. To avoid other unnecessary chemical reactions, no pressure transmitting medium was used in our experiments. The complete sample preparation and the loading process were done in a nitrogen atmosphere glove box because of the moisture-sensitivity of SnI2. The in situ angle-dispersive X-ray diffraction (XRD) measurements were carried out at the 4W2 beam line of the Beijing Synchrotron Radiation Facility (BSRF) at room temperature. An image plate detector was used to collect the diffraction patterns. The sample−detector distance and the geometric parameters were calibrated using a CeO2 standard. The average acquisition time for each diffraction pattern to obtain sufficient intensity was 300 s. To obtain the 1D intensity versus the diffraction angle 2θ spectra, FIT2D was used to analyze the 2D XRD data. Further analysis of the XRD pattern was completed by means of the Reflex Module of the Materials Studio 3
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Program.33,34 The Raman spectra were measured in the back scattering geometry with a T64000 spectrograph system in the DAC. A solid-state, diode pumped, frequency-doubled Nd: vanadate laser (λ = 532 nm) was used as excitation laser. Prior to each measurement, the spectrometer was calibrated using a Si line. The Raman spectra were collected in the range of 20−420 cm−1 as a function of pressure. The average acquisition time was about 300 s. Ab initio calculations were performed with the pseudopotential plane-wave method based on density functional theory implemented in the CASTEP code.35 A plane-wave cutoff energy of 800 eV was employed for norm-conserving pseudopotentials. The generalized gradient approximation (GGA) with the Perdew−Burke–Ernzerhof (PBE) exchange-correlation functional was used in the calculation of the crystal vibration spectrum. The details of the convergence tests had been described elsewhere.36
RESULTS AND DISCUSSION It is well known that Raman spectroscopy is an important analytical technique to gain information about the structural changes in a material. Table 1. The comparison of the main peak frequency (cm-1) assignments in the Raman spectra with a previous study.
Heilmann
Lattice modes Eg Eg Ag ν ν ν 23.6 27.5 36.3
Eg2 ν 52.6
Fg4 ν 65.4
This work
25.4
49.6
64.1
29.4
36.8
Internal modes Ag1 Fg3 ν ν 147.0 208.4 147.4
208.8
Fg3 ν 215.1 215.6
The spectra of the powder SnI2 are recorded at room temperature at pressures up to 42 GPa. Figure 1(a) shows the typical Raman spectra of SnI2. The observed crystal vibrational modes are given in Table 1, and are compared with previous studies.28,37
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Figure 1. (a) Raman spectra of SnI2 measured as a function of pressure. The acquisition time of the spectra is 300s; the dashed line indicates the red shift in lattice vibrational modes and the inset shows the linear fit results (b) Pressure dependence of the main Raman frequencies of SnI2; the solid circles and pentacles represent the first and the second runs of Raman spectrum measurements respectively. The inset represents the bond breaking of Sn-I between the adjacent layers of the ambient structure. The Raman spectra can be divided into two groups: (1) Frequencies below 45 cm-1 which mainly originate from the lattice vibrational modes and (2) the Raman active internal vibrational modes ranging from 50 cm-1 to 300 cm-1. As shown in Figure 1(a), the scattering intensity decreases and especially the relative intensity in the low-energy region changed drastically with increasing pressure. From the previous study,we can assign the peaks at 25 cm-1 and 29 cm-1 as the intermolecular Eg modes, and the peak at 36 cm-1 as the intermolecular Ag mode. These vibration modes reflect the translation and rotation motion of the molecules in the lattice, but the combined vibration corresponds to the actual motion in the lattice modes. The enormous changes are due to the severe compression and distortion of the crystal lattice. The intensity of all internal vibration peaks decreases with increasing pressure and shows a regular blue shift, except for the modes centered at 147 cm-1 and 120 cm-1 which show unique variations with pressure. Because the SnI2 molecule has three non-equivalent iodine atoms, the bands center at 147 cm-1 and 120 cm-1 are both 5
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attributed to the Sn-I symmetric stretching vibration. Below 2.15 GPa, the peak centered at 120 cm-1 shows an apparent red shift. We have fitted the change in frequency of Sn-I symmetric stretching mode around 120 cm-1 with a linear fit and the fitted slopes are -3.08 and 3.93 cm-1 / GPa below and above 2.15 GPa, respectively (Figure 1 (b)). Above 2.15 GPa, the peak centered at 36.6 cm-1 which is attributed to the lattice vibrational modes shows a red shift and the intensity of the peak increase sharply up to 7.75 GPa. The variation in lattice vibrational modes has been fitted linearly and the fitted slopes are 1.07 and -1.08 cm-1 / GPa below and above 2.15 GPa, respectively (Figure 1(a) inset). The continuous red shift in the two specific vibrational modes suggests a big change in the units of SnI2. Previous research has found that there is a unique layer crystal structure of SnI2 shown in Figure 2. The most tightly bonded pairs of Sn-I ions are packed in puckered sheets along the (201) plane with weaker bonds linking adjacent layers. Thus, the crystal can be easily cleaved along this plane.3,5 If so, the high pressure amorphization may be expected to yield a disordered chain of SnI2 molecules. The chained layer of SnI2 molecules is shown in Figure 1(b). The observed unusual phenomena in the Raman spectra may be due to the pressure-induced non-uniformity between the adjacent layers and produces a relative slide. If this is the situation, then the relatively slide probably leads to a bond breaking between the adjacent layers and this is a new view to account for the pressure-induced amorphization. Two peaks centered at 82 cm-1 and 88 cm-1 are attributed to the bending modes of the SnI2 molecules in the lattice. At 5.28 GPa the intensity of the lattice modes and the bending modes display a rising trend. This implies that SnI2 molecules undergo a strong distortion. At 6.13 GPa, the Sn-I symmetric stretching vibration is essentially as intense as the bending vibration and subsequently the intensity of Sn-I symmetric stretching vibration increases. It means that the SnI2 crystal is no longer deformed but just the bond length has shortened. Up to 9.76 GPa, the lattice modes and most of the internal modes become broaden and diffuse. Above 11.2 GPa, nearly all of the peaks disappear, which indicates that the sample begins to transform into the amorphous state. 6
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Figure 2. The crystal structure of monoclinic SnI2 (space group C2/m) at atmospheric pressure, the structure in (a) is the polyhedron type and in (b) is viewed along the (201) plane. The big grey balls represent Sn atoms and the small purple balls represent I atoms. The parallel slashes indicate the distinct layers. At ambient conditions, the crystal structure of SnI2 has monoclinic symmetry (space group C2/m), as shown in Figure 2(a). The metal atoms occupy two distinct sites: One third of the Sn atoms (see Figure 2 (a)) are surrounded almost octahedrally by six iodine atoms as in the case for PbI2. The remaining tin atoms occupy sites similar to those in SnCl2 (PbCl2 type).3 The monoclinic structure has two inequivalent positions for the tin atoms and three inequivalent positions for the iodine atoms with distinct layers, which was verified by nuclear quadrupole resonance frequency measurements.3,38 In this structure, along the (201) plane, every four SnI2 molecules are connected with each other and arranged parallel and periodically, this kind of arrangement forms puckered sheets. Two such puckered sheets exist per unit cell of SnI2 and are depicted in Figure 2(b).
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Figure 3. (a) Selected typical XRD patterns of SnI2 as a function of pressure on compression up to 48.1 GPa at room temperature (incident wavelength λ = 0.6199 Å). The new weak peaks are marked by solid rhombus. (b) Variation of the d-spacings of main peaks under high pressure. The dashed line is used to divide the three-phase field. We have carried out XRD measurements to explore the structural changes of SnI2 under high pressure. The typical XRD patterns under pressure are shown in Figure 3(a). The XRD spectrum of SnI2 obtained at 2.97 GPa has been refined. The result of the structural refinement is shown in Figure 5(a). The refined lattice constants are a = 13.49 Å, b = 4.39 Å, c = 10.27 Å, and β = 93.05° with unit cell volume V = 607.16 Å3. In accordance with the contraction of corresponding d-spacings, the diffraction peaks shift to bigger angles with increasing pressure. Some peaks disappear or become broader compared with the sharp initial ones. The intensity of the strong peaks decrease upon compression. Because the shift rate of the peaks is different, two shoulder peaks arise at about 12° (2θ) at ~7.36 GPa, but the new peaks still belong to the ambient phase. Above 8.22 GPa, the intenstiy of the strong peaks decreases dramatically and all diffraction peaks become broad and diffuse indicating the formation of a predominantly amorphous phase until 23.48 GPa. At about 33.18 GPa, the intensity of the weak peak suddenly becomes stronger and some new weak peaks emerge at the same time. These phenomena indicate the apperance of a new crystal phase. With continuous compression, all peaks become 8
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stronger until the highest pressure in this experiment of 48.1 GPa.
Figure 4. (a) Lattice parameters and (b) pressure-volume data of monoclinic SnI2. The solid symbols represent the experimental data, and solid black lines are the fitted data. The accurate lattice parameters and volume of the ambient structure are measured up to 7.36 GPa. Figure 4(a) shows the lattice parameters of the ambient structure as a function of pressure. Figure 4(b) shows the pressure dependence of the volume obtained from experimental data for the ambient phase structure. The experimental pressure-volume data is fitted with the third-order Birch – Murnaghan (BM) equation of state (EOS): −7 −5 −2 3 3 3B0 V V V 3 3 P= − × 1 + ( B0 '− 4 ) − 1 2 V0 V 4 V 0 0
where V0 is the volume per unit cell at ambient pressure, V is the volume per unit at a pressure P given in GPa, B0 is the isothermal bulk modulus, and B0′ is the first pressure derivative of the bulk modulus. We have set B0′ to 4 and a bulk modulus of 22 ± 3 GPa is obtained. V0 is found to be 113 ± 2 Å3, which is in good agreement with values reported in previous studies. The evolutions of lattice parameters of SnI2 under pressure are plotted in Figure 4, indicating the anisotropic compressibility along different directions. The lattice parameters have been fitted linearly. The fitted slopes are -0.07, -0.04, -0.09 cm-1 / GPa correspond to a, b and c, respectively. The c axis was found to be more compressible than a axis and b axis, thus the volume shrinkages 9
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mainly contribute to the compression of the c axis. The larger compressibility in c axis is the main reason for the bond breakage between the adjacent layers. This deduction testifies the assumption which has been proposed in the Ranman discussion part.
Figure 5. (a) and (b) are the Rietveld refinements of the ambient phase with respect to the synchrotron XRD patterns of 2.97 GPa and 7.36 GPa respectively. The new high pressure structure is fitted good for the XRD pattern shown in (c). The X-ray results provide strong evidence for pressure induced phase transitions in SnI2. As shown in Figure 3(b), based on the variation of the d-spacings at the whole pressure range, the pressure range can be divided into three parts: the crystal phase I (crystal phase I designated as CP-I ), the amorphous phase and the new crystal phase II (CP-II). To obtain more crystal strcture information, ab initio calculations are performed employing the pseudopotential plane-wave method based on density functional theory implemented in the CASTEP code. The theoretical results indicate three potential structures with low enthalpy. To identify the new crystal structure, the Reflex Module in the Materials Studio program is used to compare the theoretical and expetimental results. The refined result identifies the symmetry of the new high pressure phase to be the space group C2/m, which is shown in Figure 5(c).
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Figure 6. (a) The bond breaking of Sn-I between the adjacent layers of the ambient structure and (b) the new high pressure phase with a distinct layer structure along the ac plane. By means of Rietveld refinement, the XRD pattern at ~7.36 GPa is refined with the atmospheric structure. The calculated Sn-I bond lengths between the adjacent layers are found to be 3.437 Å and 4.212 Å at 2.97 and 7.36 GPa, respectively. The bond length increases with increasing pressure and the refined structure provides the evidence of the bond breaking shown in Figure 6(a). The observation of the bond breaking is due to the effect of the pressure, which causes the two adjacent layers to slide relatively. This suggestion is in accordance with our Raman spectra results. In previous studies, they always report a phase transition before appearance of the amorphous state
31, 32
but we didn’t find evidence for the phase transition before
amorphization. Based on our research we suggest that the SnI2 crystal gradually transforms to the partial amorphous state directly. If this is the case, we expect anomalous atomic movements in CP-I as the structural disordering develop with increasing pressure. At 33.18 GPa, the amorphous state transforms into a crystalline phase CP-II giving rise to the simple diffraction pattern shown in Figure 3(a). Combining the experimental data with the theoretical results indicates a possible new crystal structure of monoclinic with space group C2/m (see Figure 5(c)). The refined lattice constants for CP-II are a = 9.93 Å, b = 2.86 Å, c = 5.51 Å, and β = 71.94° with unit cell volume V = 148.57 Å3. The significant Sn-I distances are all in the range 1.71 - 2.36 Å. The new high pressure structure consists of chained SnI2 molecules parallel to the axis a as shown in Figure 6(b). Along axis a, the arrangement of two linked molecules in every single layer is similar to mirror symmetry. One unit cell contains 11
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four SnI2 molecules with each Sn atom coordinating with five I atoms. For the new high pressure phase, we have no insight into its properties. Therefore, more experimental and theoretical studies are required. CONCLUSION In summary, we have performed a combined experimental and theoretical study employing XRD, Raman spectroscopy measurements and ab initio calculations of SnI2 under high pressure. Both the Raman and XRD results indicate a gradual crystal-amorphous transition. The XRD patterns show bond breaking in the ambient crystal structure at 7.36 GPa. The intensity of Sn-I symmetric stretching mode decreases drastically and shifts to low frequencies below 2.15 GPa indicating an abnormal increase of the bond length. Partial amorphization is observed which is probably induced by the bond breaking of the neighboring layers. At 33.18 GPa, SnI2 recrystallizes into a new high-pressure crystal phase with a distinct layer structure (C2/m space group).
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] Tel./Fax:+86-431-85168825
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENTS We are grateful to Xiaodong Li, Yanchun Li, and Chuanlong Lin for their help during the experimental research. Angledispersive XRD experiments of this work were performed at 4W2 HP-Station, BSRF assistance in the synchrotron measurement. 12
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This work was supported by the National Basic Research Program of China (No. 2011CB808200), the National Natural Science Foundation of China (Nos. 51032001, 11074090, 10979001, 11274137, 11204100, 51025206, 91014004, 11004074), Changjiang Scholar and Innovative Research Team in University (No. IRT1132), project 20121035 supported by Graduate Innovation Fund of Jilin University, and China Postdoctoral Science Foundation (2015M570265). Supporting Information Available: The supporting information file includes the references with author names more than ten. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1) Ravindran, P.; Delin, A.; Ahuja, R.; Johansson, B.; Auluck, S.; Wills, J. M.; Eriksson,O. Optical Properties of Monoclinic SnI2 from Relativistic First-Principles Theory. Phys. Rev. B 1997, 56, 6851 − 6861. (2) Kostyshin, M. T.; Kostko, V. S.; Indutnyi, I. Z.;and Kosarev, V. M. Optical Constants of Tin Diiodide at the Fundamental Absorption Edge .Opt. Spektrosk. 1982, 52, 108-110. (3) Howie, R.A.; Moser, W.; and Trevena, I. C. The Crystal Structure of Tin (II) Iodide. Acta Cryst. 1972, B28, 2965-2971 (4) Doni, E.; Grosso, G.; and Ladiana. I. A Layer Model for the Band Structure of SnI2. Physica B + C.1980, 99, 281-286. (5) Fujita, M.; Hayakawa, K.; Fukui, K.; Kitaura, M.; Nakagawa, H.; and Miyanaga, T. Optical Spectra of SnI2 Crystal. J. Phys. Soc. Jpn. 1996, 65, 606-609. (6) Lü, X.; Hu, Q.; Yang, W.; Bai, L.; Sheng, H.; Wang, L.; Huang, F.; Wen, J.; Dean J. Miller; and Zhao, Y. Pressure-Induced Amorphization in Single-Crystal Ta2O5 Nanowires: A Kinetic Mechanism and Improved Electrical Conductivity. J. Am. Chem. Soc. 2013, 135, 13947−13953 (7) Lü, X.; Yang, W.; Quan, Z.; Lin, T.; Bai, L.; Wang, L.; Huang, F.; and Zhao, Y. Enhanced Electron Transport in Nb-Doped TiO2 Nanoparticles via Pressure-Induced Phase Transitions. J. Am. Chem. Soc. 2014, 136, 419−426 (8) Mishima, O.; Calvert, L. D.; Whalley, E. An Apparently First-Order Transition 13
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2733 −2736. (30) Fujii, Y.; Kowaka, M.; Onodera, A. The Pressure-Induced Metallic Amorphous State of SnI4 : I. A Novel Crystal-to-Amorphous Transition Studied by X-Ray Scattering. A. J. Phys. C 1985, 18, 789. (31) Huang, X.; Duan, D.; Wang, K.; Yang, X.; Jiang, S.; Li, W.; Li, F.; Zhou, Q.; Cui, T.; et al. Structural and Electronic Changes of SnBr4 under High Pressure. J. Phys. Chem. C 2013, 117, 8381 −8387 (32) Li, W.; Duan, D.; Huang, X.; Jin, X.; Yang, X.; Li, S.; Jiang, S.; Cui T.; et al. Pressure-Induced Diversity of π‑Stacking Motifs and Amorphous Polymerization in Pyrrole. J. Phys. Chem. C 2014, 118, 12420 −12427. (33) Accelrys Inc., San Diego, CA, USA, http://accelrys.com. (34) Young, R. A. The Rietveld method. IUCr monographs on crystallography 5; Oxford University Press: Oxford, 1993. (35) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. J.; Refson, K.; Payne, M. C. First Principles Methods using CASTEP. Z. Kristallogr. 2005, 220, 567−570. (36) Li, D.; Bao, K.; Tian, F.; Zeng, Z.; He, Z.; Liu B.; Cui, T. Lowest Enthalpy Polymorph of Cold-Compressed Graphite Phase. Physical Chemistry Chemical Physics, 2012, 14, 4347-4350. (37)
Heilmann, I. U.; Lockwood, D. J.; Pawle, G. S. The Raman Spectrum of Tin
Tetraiodide: Temperature and Pressure Measurements and Lattice Dynamical Calculations. J. Phys. C: Solid State Phys., 1978, 11, 1699-1716. (38) Bastow T. J.; Whitfield. H. J. NQR in Metal Dihalides MX2. J. Magn. Reson. 1980, 37, 269-274.
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