Structural and Electronic Changes of SnBr4 under High Pressure

Article pubs.acs.org/JPCC

Structural and Electronic Changes of SnBr4 under High Pressure Xiaoli Huang, Defang Duan, Kai Wang, Xinyi Yang, Shuqing Jiang, Wenbo Li, Fangfei Li, Qiang Zhou, Xilian Jin, Bo Zou, Bingbing Liu, and Tian Cui* State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China S Supporting Information *

ABSTRACT: The high-pressure behavior of SnBr4 has been explored by angle-dispersive X-ray diffraction, Raman spectroscopy, optical absorption/ transmission measurements, and ab initio calculations. The joint of experimental and theoretical results shows that there is a crystal−crystal transition induced by molecular dimerization, which begins at ∼10.8 GPa and completes at 21.5 GPa. Above 21.5 GPa, the gradual crystal−amorphous transition induced by the molecular dissociation appears, and the sample remains in partial amorphous state up to 43.8 GPa. At ∼40.3 GPa, the possible insulator−metal transformation is discovered by in situ optical transmission measurements. It is proposed that the observed amorphous phase is a nonmolecular phase by theoretical calculations.

I. INTRODUCTION Since the discovery of amorphization of ice (H2O) under high pressure,1 studies of pressure-induced transitions from crystalline to noncrystalline have been performed extensively, ranging from simple molecular systems to open framework structures.2−14 Some of the interesting properties, such as anisotropy and memory effects, have been found in pressureinduced amorphization (PIA) phases, even the possibility of the existence of structurally different amorphous phases for a given compound.15−17 It is known that the decrease in molecular distance in a molecular crystal causes various novel phenomena, for example, superconductivity in Si2H6,18 molecular dissociation in solid I2,19 and hydrogen-bond symmetrization in HBr and HCl.20 As important molecular crystals, group IV halides have recently been investigated for understanding structural changes and amorphous mechanism.21−23 Although several models have been proposed, the mechanism of PIA observed in group IV halides is still under debate.21,22,24,25 With regard to SnBr4, a previous Raman study showed that a crystal-toamorphous phase transition occurred in the pressure range of 13.0−15.0 GPa and the amorphous phase was thought to consist of randomly oriented chains of SnBr4,26 but the Mössbauer spectroscopy studies indicated that the (SnBr4)2 dimers were the basic unit of the amorphous phase.27 Therefore, more experimental and theoretical researches are urgently needed for confirming and uncovering the PIA phenomenon. Recently, it has been found that most group IV halides simultaneously undergo pressure-induced metallization and amorphization.24,28 However, there is no experimental evidence to confirm such phenomenon for SnBr4. As an effective probe, the optical measurement is of vital importance for exploring the electronic structure at high pressure.29,30 Moreover, the SnBr4 © 2013 American Chemical Society

crystal shows the optical absorption in the ultraviolet (UV) region at ambient pressure, so it is of significance to investigate the UV−visible (VIS)−near-infrared (NIR) absorption/transmission for exploring the electronic structure changes in the molecular SnBr4 solid. In this article, we have carried out in situ synchrotron angledispersive X-ray diffraction (XRD), Raman spectra, absorption/ transmission measurements, and ab initio calculations to study SnBr4 crystal under high pressure. Our results show that the SnBr4 crystal transforms into a nonmolecular partial amorphous phase and then a semiconductor-metal phase transition appears under high pressure. These results provide new insight into the structural and electronic changes of SnBr4.

II. EXPERIMENTAL AND THEORETICAL METHODS SnBr4 sample was purchased from Sigma Aldrich (purity >99%). The moisture-sensitive sample loadings were performed in a glovebox (nitrogen atmosphere). Pressure inside the diamond anvil cell (DAC) was determined by the standard ruby fluorescence method.31 SnBr4 is a soft crystal with melting point of 304 K under ambient conditions. So, the ruby lines were found to be sharp and well-separated up to 40 GPa and quasi-hydrostatic conditions over the whole pressure range were confirmed. The XRD measurements at room temperature were performed at the 4W2 beamline of Beijing Synchrotron Radiation Facility (BSRF). An image plate detector (MAR3450) was used to collect diffraction patterns, and the 2D XRD images were analyzed using the FIT2D software, yielding 1D intensity versus diffraction angle 2θ patterns.32 The average Received: November 1, 2012 Revised: March 29, 2013 Published: March 29, 2013 8381

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optical absorption spectra (Figure S2b in the Supporting Information). The unit cell parameters and volume are reliably measured up to 9.4 GPa. Figure 3a shows the fitted lattice constants with the following relationship between the linear compressibility: kc (0.106 GPa−1) > ka (0.104 GPa−1) > kb (0.058 GPa−1). Thus the SnBr4 sample undergoes anisotropic compression. The experimental pressure-volume data in Figure 3b are fitted by third-order Birch−Murnaghan equation of state34

acquisition time was 600 s. The sample−detector distance and geometric parameters were calibrated using a CeO2 standard. The fitted XRD patterns were completed by means of the Reflex module in the Material Studio Program. Raman spectra in the DAC were 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 was 2 W. Prior to each measurement, the spectrometer was calibrated using the Si line. The Raman spectra were collected in the range of 0−420 cm−1 as a function of pressure. The average acquisition time was about 2 and 60 s before and after the 15.0 GPa, respectively. UV−vis optical transmission was measured up to 21.5 GPa using the optics of Ocean Optics QE65000, and the sample was placed in a 120 μm diameter hole drilled on a 40 μm thick T-301 stainless-steel gasket and inserted between the diamonds in a symmetric type DAC. The UV−vis-NIR transmission spectra were measured by a Shimadzu UV-3150 spectrometer up to 40.3 GPa. The ab initio calculations were carried out by the local density first-principles CASTEP methods within the generalized gradient approximation (GGA).

−7/3 ⎡ ⎛ V ⎞−5/3⎤ 3B0 ⎢⎛ V ⎞ ⎥ P= −⎜ ⎟ ⎜ ⎟ ⎥⎦ 2 ⎢⎣⎝ V0 ⎠ ⎝ V0 ⎠

⎧ ⎡⎛ ⎞−2/3 ⎤⎫ ⎪ ⎪ V 3 ⎢ ⎨1 + (B0′ − 4) ⎜ ⎟ − 1⎥⎬ ⎢ ⎥ ⎪ 4 ⎣⎝ V0 ⎠ ⎦⎪ ⎩ ⎭

where V0 is the volume per unit cell at ambient pressure, V is the volume per unit cell at pressure P given in GPa, B0 is the isothermal bulk modulus, and B0′ is the first pressure derivative of the bulk modulus. The volume of ambient pressure is fitted to be 778.14 (11) Å3, which is in good agreement with those in previous literature.33 A bulk modulus of 11.9 (1.2) GPa is obtained with a pressure derivative of 3.2 (0.1). The low bulk modulus shows that the sample is easily to be compressed under high pressure. So the quasi-hydrostatic condition is confirmed in the whole sample chamber, which excludes its important effect on the width of the XRD patterns in amorphous phase. Raman spectroscopy is an important analytical technique for obtaining structural information on the studies of PIA.35−37 Group theory predicts the following representation for the optical vibration modes of q = 0 (center of Brillouin zone) with point group C2h for SnBr4 crystal. The irreducible representation is given as

III. RESULTS AND DISCUSSION Under ambient conditions, SnBr4 is a tetrahedral molecule, described by the point group Td. Its crystal structure with space group P21/c has four molecules in the unit cell.33 The schematic representation of its crystal structure is shown in Figure 1a. The refinement of SnBr4 has been carried out on an

Γ = 15A g + 15Bg + 15A u + 15Bu

Ag and Bg represent Raman-active modes and Au and Bu are infrared-active modes. Figure 4a shows the measured Raman spectrum at 0.3 GPa, which is in good agreement with the calculated one as well as with that of the previous report.26 The Raman spectra can be roughly divided into two parts: (1) Spectral bands below 60 cm−1 mainly from the lattice vibration and (2) the Ramanactive internal vibration regions from 80 to 300 cm−1. The strongest band at 220.5 cm−1 is attributed to the Sn−Br symmetric stretch vibration (vibration mode illustrated in Figure S3a in the Supporting Information), while three other higher frequency peaks at 272, 280, and 288 cm−1 are triply degenerate asymmetric stretch vibrations. Two peaks centered at 89 and 81 cm−1 are attributed to the distortion vibrations of the SnBr4 tetrahedral molecules. Here we focus on the internal mode vibration regions. We have measured in situ Raman spectra of SnBr4 up to 60.4 GPa. As shown in Figure 5a, the principal changes concern the appearance of additional peaks in the 50−80 cm−1 region up to 8.0 GPa, in which the Raman peaks cannot be observed at ambient pressure owing to the limitations of our Raman spectroscopy. The previous Raman study has found a phase transition at 4.0 GPa according to the appearance of new peaks.26 However, there is no sign of changes in the XRD patterns up to 10.8 GPa, showing no phase transition at ∼4.0 GPa, which has been confirmed by our Raman spectra

Figure 1. Crystal structures of (a) monoclinic SnBr4 (space group P21/c)) and (b) P-1 phase with dimeric molecules are viewed along the a−c plane.

XRD pattern obtained at ∼2.1 GPa (Figure S1 in the Supporting Information). The refined lattice constants are a = 9.7554 (4) Å, b = 6.4927 (2) Å, c = 9.8473 (6) Å, and β = 101.488 (8)° with unit cell volume V = 611.222 Å3. To examine the changes in the crystal structure under high pressure, the XRD measurements are carried out (Figure 2a). With increasing pressure, some peaks disappear and widen compared with the initial sharp peaks. The intensities of the strong peaks decrease with increasing pressure. At ∼10.8 GPa, there are two weak peaks emerging at very low angles, indicating the appearance of the phase transition. Above 31.3 GPa, the broad diffuse peaks dominate the pattern and the sample converges on a predominantly amorphous phase until the highest pressure of 43.7 GPa. Upon decompression, the amorphous sample transforms back to the original crystal structure below 2.0 GPa proved by XRD patterns (Figure 2b), Raman spectra (Figure S2a in the Supporting Information), and 8382



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Figure 2. (a) Selected XRD patterns of SnBr4 as a function of pressure on compression up to 43.7 GPa at room temperature and (b) decompression patterns down to the full release of pressure. The wavelength of the incident X-ray beam is 0.6199 Å.

Figure 4. Experimental and calculated Raman spectra of SnBr4 (a) at ambient pressure and (b) at ∼5.0 GPa.

Figure 3. (a) Lattice parameters and (b) pressure−volume data with error bars of monoclinic SnBr4. Solid black symbols represent experimental data, and solid black lines are fitted data.

At ambient pressure, SnBr4 is known as a transparent insulator with an energy gap of 3.4 eV. In Figure 6a, the absorption spectra are indicated with solid lines. There are two remarkable features: (1) the width of the absorption edge narrows under pressure and (2) a long tail ranging to visible or near-infrared region. The changes of the sample color are consistent with the red shift of absorption spectra in Figure 6b, which first becomes orange, then red, and at last black with increasing pressure. Apparently, the spectra exhibit the broadened, step-like spectral shapes predicted by the Elliott− Toyozawa theory of excitonic optical absorption in direct semiconductors.38 So the absorption edge of SnBr4 has been analyzed using the following form for α (ν) near the optical gap39

calculations (Figure 4b). When the pressure increased to 8.0 GPa, a weak peak appears at 200 cm−1 (Figure 5a). Another new peak centered at 273 cm−1 emerges at ∼8.7 GPa. With increasing pressure, the two new peaks strongly increase in intensity. Up to 15.2 GPa, the initial 200 cm−1 peak is essentially as intense as the Sn−Br symmetric stretch vibration mode. In particular, the strongest peak initially at 220 cm−1 is nearly invisible at 21.2 GPa. This indicates that the original Sn− Br bonds are strongly affected and the new structure has been completely formed. In contrast, the observed new peaks at 200 and 273 cm−1 continue to broaden and remain the most intense up to 34.7 GPa. Besides, the original mode at 96 cm−1 shifts discontinuously to higher frequency at about 21.2 GPa and retains up to 34.7 GPa. Above 34.7 GPa (Figure 5b), nearly of all the peaks disappear except a weak peak centered around 189 cm−1, which is too weak to read its position accurately.

α∝ 8383

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Figure 5. (a) Raman spectra of SnBr4 measured upon compression. The acquisition time of the spectra below 10.0 GPa is ∼5 s and that of above 10.0 GPa is ∼60 s. The new peaks in both profiles are indicated by black star symbols. (b) Pressure dependence of main Raman frequencies of SnBr4 (open red circles and cyan diamonds); the phase changes are marked by dashed-dotted lines.

intramolecular and intermolecular distances of SnBr4 using ab initio density functional calculations, as illustrated in Figure 7. It is noticed that the uniform decrease in the two intramolecular Sn−Br distances is interrupted and a small jump takes place above 15.0 GPa. This is consistent with the crystal structural transition observed in the XRD patterns. At ∼20.0 GPa, a larger jump of the intramolecular Br−Sn distances appears and the intermolecular Br−Sn distances decrease quickly. Up to 30.0 GPa, the intermolecular Br−Sn distances are nearly equal with the intramolecular ones, indicating the full dimerization of the molecules. Above 40.0 GPa, the two intermolecular Br−Br distances decrease by ∼1.0 Å with respect to the values at low pressures, which is also same with the intramolecular distances. So the dimeric phase undergoes further molecular dissociation. Moreover, the crystal structure of the dimeric phase is obtained by the theoretical calculations with space group P-1 (Figure 1b). The calculated Raman spectrum and XRD pattern of the new structure are compared with the experimental data, respectively. The refinement of the measured XRD pattern at 21.5 GPa shows good agreement with the calculated results, which can be seen from Figure 8a. Additionally, the calculated Raman spectra at 20.0 GPa are in good agreement with the experimental one at 21.2 GPa (Figure 8b). As for the phenomena observed in the Raman spectra, the two new Raman peaks centered at 202 and 273 cm−1 in Figure 4b grow and predominantly occupy the spectra up to 21.2 GPa. On the basis of the calculated results, the two new strong peaks near 202 and 273 cm−1 are considered as new Sn−Br symmetric stretch vibration modes of the dimeric (SnBr4)2 (Figure S3 in the Supporting Information). Above 34.7 GPa, nearly all of the intramolecular vibration modes are invisible, indicating that the Sn−Br bonds in a tetrahedral molecule change and a nonmolecular crystal forms. This is consistent with the calculated results. Combined with the phenomena of pressure-induced metallization at ∼40.3 GPa, it is suggested

The energy gap is given by the intersection between the horizontal axis and a line obtained from the (αhν) 2−hν plot. The absorption coefficient α is approximately obtained by OD/ d, where OD and d are the optical density and thickness of the sample, respectively. Although the thickness d varies with pressure, it is believed that the maximum uncertainty in d is ∼10% because the rate of decrease in d is not large in the pressure region of ∼20 GPa.40 So the original thickness of the sample (d = 40 μm) is used for calculation with different pressures. The obtained pressure dependence of the optical gap Eg is shown in Figure 6d. The Eg decreases almost linearly to 1.5 eV at ∼21.0 GPa except for a small jump around 10.0 GPa. This jump corresponds to the small change in the original crystal structure, which is consistent with the Raman and XRD results. The transparent and colorless ambient phase becomes black and opaque, suggesting a large decrease in the band gap energy. This possible precursor of metallization motivates us to study SnBr4 up to 40.3 GPa for a metallic phase. As illustrated in Figure 6c, the transmission spectra are measured above 21.0 GPa. At 21.5 GPa, the observed absorption edge indicates that valence and conduction bands are not overlapping, showing that samples are not metallic. However, the spectra exhibit a nearly straight line with increasing pressure, which indicates that the sample becomes completely opaque. By 40.3 GPa, the transmission extends to cover the entire spectral region measured. This phenomenon suggests possible band gap closure or metallization. Therefore, the possible semiconductor−metal transition occurs at ∼40.3 GPa. Although previous experimental studies have proposed different structure models for the high-pressure amorphous phase of SnBr4,26,27 their experimental pressure is so low that they did not comprehensively observe the whole amorphous process and reach an exact conclusion. To better understand the process of amorphization, we have calculated the 8384



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Figure 6. (a) Absorption spectra of SnBr4 obtained at various pressures from 0.1 to 21.0 GPa. (b) Microphotographs of SnBr4 sample at different pressures. (c) Transmission spectra measured with increasing pressure from 21.5 to 40.3 GPa. (d) Pressure dependence of energy gap (closed blue circles) as a function of pressure up to 21.0 GPa, and the prolonged dotted line shows the possible metallization pressure.

The theoretical results agree with the experimentally observed changing trends, although the calculated transition pressure is a little higher than our experimental one. In addition, there is a potential barrier that prevents the structural change observed by theoretically structural relaxation at lower pressure. It is noted that the transition phenomenon is more important than the resultant structure because an amorphous state could not be obtained directly from our original structure by geometry optimization. Therefore, more experiments and theoretical calculations are needed to explore the microscopic structure of the high-pressure amorphous phase.

IV. CONCLUSIONS In summary, we have performed XRD, Raman spectroscopy, absorption/transmission measurements, and ab initio calculations of SnBr4 under high pressure. At ∼10.8 GPa, a crystal− crystal transition is observed in X-ray pattern. Combining the Raman spectroscopy results with theoretical calculations, it is found that the crystal−crystal transition happens due to the molecular dimerization. The gradual amorphization induced by molecular distortion begins at ∼34.7 GPa, and the amorphization process does not complete until 43.8 GPa. The joint of experimental and theoretical results shows that the SnBr4 molecular crystal becomes the nonmolecular phase upon compression. Our experimentally optical transmission spectra

Figure 7. Four Br−Sn bond lengths (intramolecular distances) in the tetrahedral molecule and two intermolecular atomic distances calculated at different pressure. The two dash-dotted lines indicate the transformation regions. The inset picture shows the molecular configuration corresponding to the separate region.

that the SnBr4 crystal transforms into the metallic and nonmolecular phase under high pressure (Figure 5b). 8385



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Figure 8. (a) Experimental and calculated XRD pattern of SnBr4 at ∼21.5 GPa, and the fit is good with Rwp = 1.89%, Rp = 2.67%. (b) Comparison between experimental and calculated Raman spectra at ∼21.2 GPa.

indicate a possible semiconductor−metal transition at ∼40.3 GPa.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental and calculated images. This material is available free of charge via the Internet at http://pubs.acs.org.

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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 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. 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), and project 20121035 supported by Graduate Innovation Fund of Jilin University.

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Structural and Electronic Changes of SnBr4 under High Pressure

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