Confirmation of the Structural Phase Transitions in XeF2 under High

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Confirmation of the Structural Phase Transitions in XeF Under High Pressure Gang Wu, Xiaoli Huang, Yanping Huang, Lingyun Pan, Fangfei Li, Xin Li, Mingkun Liu, Bingbing Liu, and Tian Cui J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11558 • Publication Date (Web): 03 Mar 2017 Downloaded from http://pubs.acs.org on March 6, 2017

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The Journal of Physical Chemistry

Confirmation of the Structural Phase Transitions in XeF2 under High Pressure Gang Wu, Xiaoli Huang, Yanping Huang, Lingyun Pan, Fangfei Li, Xin Li, Mingkun Liu, Bingbing Liu and Tian Cui* State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, P.R. China

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Abstract Unique structures and properties will be introduced upon compression. To figure out the high pressure crystal structure of XeF2, in-situ Synchrotron X-ray diffraction, Raman spectra, UV-VIS absorption spectra and theoretical calculations have been performed up to 86 GPa. The structural dispute between reported experimental and theoretical results is settled by this study. The experimental and theoretical Raman spectra results indicated that the ambient structure of XeF2 (I4/mmm) transformed into a Immm structure at 28 GPa. Then this Immm structure transformed into Pnma structure at 59 GPa. The Rietveld refinement of the XRD results was in accordance with our Raman study. The optical absorption spectra revealed a reduction in the band gap as pressure increases. The reduction in the band gap decreases to 1.83 eV at 82 GPa while the color of the sample is getting dark. Our results provide a new insight into the high pressure behavior of noble gas compounds with the example of XeF2.

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Introduction Noble gas compounds always have interesting bonding properties for its specific closed-shell electron structure.1-3. For example, XePtF6 is constituted by XeFPtF5 and XeFPt2F11 units which contain special bonds.4 However, the high ionization potentials bring a poor stability simultaneously. In xenon fluorides, xenon has longer atomic radius, leading to weaker attraction for outer sphere. And fluorine has the largest electronegativity. For this reason, xenon fluorides, as the simplest noble gas compounds, possess a better stability. Besides, because of the strong oxidation ability, xenon fluorides are served as a prominent fluorinating agent, which are widely used in chemical synthesis. On this account, there has been wide investigation on xenon fluoride.5-7 As the most stable stoichiometric ratio of the xenon fluorides, XeF2 attracts considerable interest particularly.8-10 At ambient condition, XeF2 crystallizes as a body-centered tetragonal structure (space group I4/mmm, Z=2). According to the VSEPR theory, XeF2 unit has a linear geometry structure with two equal distance Xe-F bonds.11 Its great stability has been interpreted by the Rundle–Pimentel model combining with charge-shift bonding, and the resonance valence bond model.12-16 High pressure is considered to bring materials with some unique properties, for example, solid hydrogen is predicted to become a metallic phase under ultra-high pressure, which is considered to be room temperature superconductor.17, 18 Moreover, the crystal structure of XeF2 under pressure also attracts attentions, but both crystal structure and phase transition sequence remain controversial under high pressure.19-21 Recently, XeF2 was reported to become metallic under high pressure. By means of X-ray diffraction (XRD), four high pressure phases were proposed by M. Kim, M. Debessai and C.-S. Yoo,.19 They proposed that the ambient structure (I4/mmm) transformed into phase II (space group Immm), phase III (space group Pnnm-1) and phase IV (Pnnm-2) at 7 GPa, 13 GPa and 23 GPa, respectively. A fluorite like metallic structure phase V (Fmmm) was observed finally. But subsequently, D.

Kurzydzowski et al. indicated that the structures suggested from experiment19 had 3



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either much higher enthalpies than the I4/mmm structure or converged to that structure upon geometry optimization.20 A high pressure Pnma phase with bent F-Xe-F molecules and unequal Xe-F bond length was presented above 105 GPa via theoretical predication. The very recent published theoretical calculation research by F. Peng et al. indicated that this Pnma structure was energy favorable refer to the ambient structure at 110 GPa, while XeF2 tended to decompose into Xe2F and XeF4 at 81 GPa.21 Up to now, as the key to understand the intriguing high pressure behavior of XeF2, the crystal structures have remained elusive and urgently needed to figure out. To solve the controversy on high pressure structures of XeF2, we have performed a systematic high pressure study on XeF2 by Raman spectra and synchrotron XRD measurements up to 80 GPa. The optical absorption spectra up to 82 GPa are measured to probe into the variation of band gap under compression. Meanwhile, Raman spectra of candidate structures were simulated via first-principle theory. The experimental and theoretical Raman spectra results indicate that the ambient structure of XeF2 (I4/mmm) transforms into a Immm structure at 28 GPa. Then another high pressure phase (space group Pnma) appears at 59 GPa. The Rietveld refinement of the XRD result is in accordance with our Raman study.

Experimental and theoretical details A diamond anvil cell (DAC) was used to create the high pressure that reached up to 86 GPa in the XRD experiment, 82 GPa in the UV-VIS absorption spectra experiment and the Raman spectra measurement. The DAC used in all of the experiments were with a culet face of 200 µm in diameter. Re gasket pre-compressed to about 30 µm was placed between two diamond anvils. In the middle of the gasket, a hole was drilled with a diameter of 50 µm, which was served as a sample chamber to seal the XeF2 (Alfa Aesar, 99.5%) in the DAC. The moisture-sensitive sample was loaded into DAC in a nitrogen atmosphere glove box. No pressure transmitting medium was used. The pressure in the sample chamber was determined by the standard ruby fluorescence method and the first order Raman band of the 4


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diamond.22-24 To confirm the quasi-hydrostatic pressure condition upon compression, three ruby balls were placed in the sample chamber during one experimental Raman spectra run. High pressure Raman experiment was carried out on a JY-T64000 spectrometer equipped with a nitrogen cooled CCD detector, combined with a 532 nm laser excited by doubled solid-state Diode Nd:Yanadate laser (Coherent Company). The UV-VIS absorption spectra were measured by the optics of Ocean Optics QE65000, combined with a deuterium-halogen light with the wavelength range from 350 nm to 2000 nm. The synchrotron angle-resolved XRD data were collected on the 4W2 beamline of the Beijing Synchrotron Radiation Facility (BSRF, wavelength λ = 0.6199 Å) at room temperature. The two-dimensional diffraction image was integrated by FIT2D software25, yielding one-dimensional intensity versus diffraction angle 2θ patterns. The Rietveld refinement was carried out to solve the structures via Reflex modules in Material studio. 26 The generalized-gradient approximation (GGA) with Perdew- Burke- Ernzerh (PBE) functional was used to obtain accurate exchange and correlation energies for a given structural arrangement.27 Experimentally established and theoretically predicted structures were used as input data. The structural optimizations and Raman spectra calculations for selected structures were performed with CASTEP code.28 The norm-conserving pseudo-potentials for F and Xe were used. Van der Waals interactions were corrected by the TS method of DFT-D.29 The scheme parameters sR and d were set as 0.74 and 18 respectively. All calculations chose a plane-wave cutoff energy of 940 eV and a k-point spacing of 0.06 Å-1, which were obtained from a convergence test.

Result and discussion A. Raman and UV-VIS absorption experiments Fig. 1 shows the Raman spectra of XeF2 under different pressures. Below 28 GPa, XeF2 is stable with the tetragonal structure (space group I4/mmm). There are only two Raman active mode of this structure. The low frequency double degenerate 5



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mode (Eg mode) distributes from 200 to 300 cm-1, corresponding to a wiggle motion of the F atoms relative to the Xe atom in the middle of XeF2 linear unit. The higher frequency Raman mode (Ag) is related to the symmetric stretching of two F atoms in the linear unit. Notably, in the whole pressure region, we didn’t observe the extra peaks of ReF4 impurities, which ruled out the reaction between XeF2 and the Re gasket.

Fig. 1 The selected Raman spectra of XeF2 with increasing pressure up to 82 GPa. The Raman Spectra of I4/mmm, Immm and Pnma are colored with black, red and blue respectively. Inset: Multiple Gaussian peak fitting on spectra of (a) 28 GPa and (b) 59 GPa; the sample photo of (1) to (4) are taken at 11 GPa, 35 GPa, 52 GPa and 69 GPa, respectively. All of the Raman peaks move towards the high frequency under pressure, 6


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indicating an enhanced interaction among atoms. The peak intensity of Eg mode decreases and its width broadens. The lattice mode can be fitted by two individual peaks (Inset a of Fig. 1). All these changes prove that XeF2 has changed into a new phase (denoted by HP-1 phase) at 28 GPa. Changes on the degenerate Eg mode signify the non-degeneration process, leading to the symmetry reduction. Upon further compression to 59 GPa, two additional peaks appear at 347 cm-1 and 700 cm-1 (Inset b of Fig. 1), respectively. This indicates the emergence of another high pressure phase (denoted by HP-2 phase). In addition, the original stretching mode still exists up to 82 GPa. The evolution of Raman peak frequency as pressure increasing is shown in Fig S1. Besides, the photos of the sample chamber (Inset 1- 4 of Fig. 1) are collected during the Raman experimental run. The sample becomes yellow at 52 GPa. At 69 GPa, the color turns into red. Importantly, high pressure UV-VIS absorption spectra are measured to probe into the variation of band gap under compression. As shown in Fig. 2, before 25 GPa, no absorption signal is detected among our experiment wavelength range (350 ~800 nm), which is in accordance with the electronic structure of XeF2 reported by previous report.30 The first allowed electronic excitation energy is 5.64 eV.30 Diamond also has strong absorption approximately from 350 nm to 420 nm. Absorption spectrum is also collected with a sapphire cell (Insert a of Fig. 2), which can avert the influence of diamond. The result is in accordance with the result obtained by DAC, indicating the absorption of diamond has been offset after subtracting background.

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Fig. 2 The optical absorption spectra of XeF2 under high pressure. Inset: (a) Absorption of XeF2 in a sapphire cell at 3 GPa. (b) The optic band gap as a function of pressure,. The red line is fitted result which indicates that the band gap closing pressure is estimated to be 152 GPa.

At 28 GPa, a strong absorption edge distributing from 350 nm to 400 nm appears abruptly. As pressure increasing, this absorption edge moves toward larger wavelength, corresponding to smaller excitation energy. The band gap of corresponding pressure is calculated as shown in the insert b of Fig. 2. From the trend of band gap varying with pressure, we can infer that the absorption edge occurring at 28 GPa is contributed to a symmetry-forbidden electronic transition in the I4/mmm structure, whose transition energy is 4.33 eV at ambient condition.30 In addition, the critical pressure at 28 GPa, is in good agreement with the first phase transition pressure observed in our Raman experiment. The appearance of symmetry-forbidden transition can be accounted for the symmetry reduction in the first phase transition. The fitted band gap varies from 3.22 eV (28 GPa) to 1.83 eV (82 GPa) under pressure. The band gap narrows as the pressure increasing, accompanying with the color 8


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changes of the sample. But a metallic phase has not been reached among the absorption experiment pressure region. From the trend of band gap varying with pressure, the metallic pressure is inferred to be 152 GPa.

B. Theoretical calculations Although the measured Raman spectra prove the appearance of phase transition, it is difficult to obtain the precise high pressure structures only through the experimental Raman spectra. Combining with theoretical calculations，it will be beneficial to clarify the crystal structures under high pressure. TS method of DFT-D is introduced to correct the Van der Waals interactions.28 The scheme parameters sR and d were set as 0.74 and 18, respectively. The lattice parameters obtained by geometry optimization are in good agreement with our XRD results (see Table S1.). Structures proposed in the former experimental study (Immm, Pnnm-1, Pnnm-2, Fmmm) and two theoritically predicted energy favorable structures (I4/mmm, Pnma) are taken into account as the candidate structures19, 20. All these structures are relaxed to corresponding pressure region as reported. The optimization results of all these candidate structures are listed in Table S2., The Immm and Pnnm-1 structures both collapse into I4/mmm structures after geometry optimization, which are same as previous theoretical study.20 Indeed there is only tiny difference on the lattice parameters among these two experimentally proposed structures and the ambient I4/mmm structure. The Pnnm-2 structure collapses into Immm structure after geometry optimization. The Fmmm changes into another I4/mmm structure which is denoted as I4/mmm-1 to differ from the ambient structure. The Pnma structure maintains its symmetry after optimization. Obviously, these experiment structures (Immm, Pnnm-1, Pnnm-2 and Fmmm) all transform to structures with higher symmetry after optimization. It should be noted that in both the previous research (ref.19) and our experiments, no pressure transmitting medium is used. It is considerable to take the non-hydrostatic condition into account when optimizing these structures. The pressure of different place in sample chamber has been measured by three ruby balls (Fig S2). The unequal axial pressure in the sample chamber can be confirmed by the pressure deviation among 9



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these three ruby. Another run of geometry optimization with non-hydrostatic condition are carried out. For I4/mmm, Immm, Pnnm-1 and Pnnm-2 structures, the axial pressure in x, y, z direction was set to 30, 31, 32 GPa respectively, consistent with the real status in sample chamber. Analogously, for Fmmm structure, axial pressure is set as 60, 62, 64 GPa in x, y, z direction. The I4/mmm，Immm, Pnnm-1 and Pnnm-2 structures all change into the Immm polymorph with a diversity of 0.05 Å between lattice parameter a and b after optimization. It can be seen that the structural stability of I4/mmm structure is easily shaken in the experimental condition. In the ambient I4/mmm structure, the lattice parameter a and b are equal, resulting a double degenerate Eg mode, while the slight deviation between the lattice parameter a and b giving rise to the non-degenerate Eg peak in the Immm structure. This is in accordance with the HP-1 phase observed in our Raman experiment with the splitting of the Eg mode at 28 GPa. The similar phenomenon has been reported in the study on iodine31 that non-hydrostatic effect leads to a decrease in lattice symmetry of iodine. In the case of Fmmm structure, it changes into a C2/m structure after non-hydrostatic pressure optimization. Since the difference in the XRD pattern of these candidate structure is ambiguous, to further distinguish these candidate structures, we have performed a Raman spectra calculation with CASTEP code in the Material Studio. The calculated Raman spectra are compared with our experimental results. The calculated Raman spectrum of I4/mmm structure at 9 GPa shows two peaks at 194 cm-1 and 528 cm-1 respectively. Correspondingly, two Raman peaks are observed at 207 cm-1 and 522 cm-1, which is within the tolerance of diversity between theoretical simulation and experimental data (Fig. 3 a). I4/mmm, Immm, Pnnm-1 and Pnnm-2 structures all transform into the Immm symmetry after optimization with unequal axial pressure. Raman spectrum of the Immm structure is calculated. The simulated peak position of the Immm structure (two peaks around 285 cm-1 and the rest one at 590 cm-1) is also in good agreement with the observed spectra (two peaks around 315 cm-1 and the rest one at 570 cm-1) (Fig. 3 b). What’s more, in Immm structure, the distance between Xe and the nearest F atom 10


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is similar to the I4/mmm structure. This can interpret the linear blue-shift of the Ag peak in our Raman experiment. The Pnnm-2 polymorph changes sharply into the Immm structure after optimization, which is different from the resemblance among I4/mmm, Pnnm-1 and Immm structure. Therefore, Raman of Pnnm-2 structure is calculated in addition. Raman calculations based on the Pnnm-2 structure (ref.19) at 25 GPa indicate two Raman peaks at 570 cm-1 and 1150 cm-1, which are in contradiction with the observed Raman peak at 293 cm-1 and 558 cm-1( Inset 1 of Fig. 3). Fmmm polymorph transforms into another I4/mmm structure (denoted as I4/mmm-1) after optimization procedure, which is different from the ambient structure. When applying the unequal axial pressure, it changes into a C2/m structure. The Raman spectra of these three structures are all simulated. The spectra of the Fmmm structure (ref.19) at 64 GPa shows two peaks at 288 cm-1 and 290 cm-1 ( Inset 2 of Fig. 3). Same as the low frequencies in Fmmm structure, the I4/mmm-1 indicates two Raman peaks at 309 cm-1 and 310cm-1( Inset 3 of Fig. 3). The Raman calculations based on the C2/m structure show one strong peak at 670cm-1 and two faint ones at 303 cm-1 and 485 cm-1. Raman peaks observed at 349 cm-1,386 cm-1, 407 cm-1 and 705 cm-1 are absent in the simulated Raman spectrum of C2/m structure ( Fig. 3c). These four structures (Pnnm-2, Fmmm, I4/mmm-1 and C2/m) can be ruled out for their abrupt Raman peak positions. Further, the calculated Raman spectra of Pnma structure (ref.20) shows good agreement with the experimental spectra after 59 GPa (Fig. 3d). Obviously, the Raman peaks observed in experiment (386cm-1, 622cm-1 and 705cm-1) appear in the calculate results (364cm-1, 623cm-1 and 678cm-1). It should be noted that the calculate results also show two weak Raman peak in 338cm-1 and 415cm-1 (as marked by asterisk in Fig. 3d), which is corresponding with two Raman peaks observed in 349cm-1 and 407cm-1. The Raman calculations via DFT theory are performed at 0K, while our experimental spectra are measured at 300K. This may result in the discrepancy of peak intensity. The Pnma structure consists of the bending F-Xe-F unit, in which the two F atoms are not equal to the Xe atom. This different geometrical 11



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form can bring new Raman peaks in stretching region and lattice region, which has been observed in experiment above 59 GPa. Though there are some additional tiny peaks in the simulated Raman spectrum (230cm-1, 504cm-1 and 557cm-1), but the slight discrepancy between the calculated and experimental spectra might be caused by both the limitation of the experimental resolution and the weak signal of Raman scattering under high pressure.

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Fig. 3 The calculated Raman spectra (red) of I4/mmm (a), Immm (b), C2/m (c) and Pnma (d) structure are compared with the experimental Raman spectrum (blue). The asterisks in (d) indicate two weak peaks in calculated Raman of Pnma structure. Inset: The calculated Raman frequencies of (1) Pnnm-2 structure, (2) Fmmm structure and (3) I4/mmm-1 structure are compared with corresponding observed spectrum. The red 13



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arrows at the bottom indicate the frequencies of calculated Raman peaks. The blue lines are the experimentally observed spectra. So we infer that XeF2 stabilizes as the I4/mmm structure below 28 GPa, above which it transforms into the Immm structure caused by the non-hydrostatic environment. The lattice parameter a and b become unequal in the Immm structure, but the atom position don’t have an abrupt change, according to the continuous blue shift of the Ag mode. Above 59 GPa, XeF2 turns into the Pnma structure with bent F-Xe-F molecules and unequal Xe-F bond length, accompanying with the extra Raman peaks.

C. XRD confirmation To further confirm the structural changes of XeF2 under high pressure, we have performed synchrotron XRD experiments up to 86 GPa. As shown in Fig. 4, all diffraction peaks move towards high angle with pressure increasing. This tendency indicates that all the lattice planes become closer under compression. The width of diffraction peaks become broaden dramatically arising from the decrease in the scale of crystal grain. The sample chamber shrinks during the pressure loading procedure, and diffraction peaks of Re gasket are also collected together with the XeF2 sample (denoted by the dot line in Fig. 4).

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Fig. 4 The selected XRD of XeF2 upon compression up to 86 GPa. From the Raman results, the patterns are separated into three phases signed by different color. The blue dash line arrow shows the increasing on peak intensity after 60 GPa. The diffraction peaks of the gasket Re are marked by black dot lines. In accordance with the Raman results, the diffraction patterns are assigned to three phases, as shown in Fig. 4. In our theoretical simulation, there was only a small deviation (about 0.06 Å) between the lattice parameter a and b in the Immm structure at 31GPa. This deviation could lead to a broadening width in the peak corresponding with (101) crystal plane. But this tiny change is submerged by the broadening caused by crystal grain decreasing. The difference between the I4/mmm and Immm is too small to distinguish from XRD pattern. On the other hand, the consistency of 15



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diffraction pattern around 26 GPa indicates that the position of F atoms in the XeF2 remains unchanged, which is consistent with the Raman results. Upon compression to 60 GPa, the intensity of peaks near 2 theta value 11°increases abruptly, which is assigned by dash line arrow in Fig. 4. This partial change suggests that an alteration of the atomic position happens in crystal structure of XeF2. This alteration is similar to the Immm to Pnma structural phase transition proposed in our Raman experiment. Moreover, the transition pressure is very adjacent. Though the previous theoretical research predicted that XeF2 decomposed into Xe2F and XeF4 at 81 GPa21, there is not any decomposition indication observed in both Raman and XRD experiments. The simulated XRD patterns of decomposition products (Xe2F and XeF4) are in contrast with the observed pattern at 84 GPa. As shown in Fig. 5 the XRD peak positions of Xe2F and XeF4 are different from observed ones obviously. The simulated XRD of Xe2F and XeF4 don’t show any diffraction peaks at 2 theta value 12.5°, 16°, 20.3° and 22.4°, at which diffraction peaks are observed at 84 GPa. Both the strong peak in XeF4 at 12° and Xe2F at 14.2° neither are absent in the observed XRD patterns at 84GPa. The predicted decomposition can be ruled out in our experiment pressure region.21

Fig. 5 The XRD pattern collected at 84 GPa is compared with the simulated XRD patterns of XeF4 (I4/m) and Xe2F (I4/mcm)21. Diffraction peaks of Re gasket are denoted by the black rhombus. To confirm this deduction, a Rietveld refinement is performed. The theoretical 16


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predicted Pnma structure and the XRD pattern at 60 GPa are used as initial data. Diffraction peaks of ruby and gasket material Re, caused by the shrinkage of sample chamber under high pressure, are subtracted. The results of refinement are shown in Fig. 6. In addition, the Rietveld refinements on both I4/mmm and Immm structures show good agreement with corresponding XRD patterns. The detailed results of the refinement are listed in Table S3. The lattice parameters of XeF2 at different pressure are obtained and shown in Fig S3. It should be noted that the Pnnm-2 and Fmmm structures proposed previously (ref.19) can also reach a good agreement with our XRD data after refinement (as shown in Table S3). It seems that it’s difficult to get the proper structure information by the XRD data solely. Raman spectrum is an efficient method to analyze the molecular vibration. Changes on the molecular geometrical form can be inferred from the measured Raman spectra, such as the new Raman peaks above 59 GPa corresponding with the bent F-Xe-F molecules and unequal Xe-F bond length. Our conjecture can be verified by comparing with the DFT calculated Raman with measured results. XRD Rietveld refinement can further verify the candidate structures. UV-VIS absorption spectra is used for probing the optical Band gap under pressure. The results obtained by the combination of Raman, XRD and UV-VIS absorption spectra should be more convincing. Therefore, the high pressure structures of XeF2 are confirmed in this study. This will finally solve the existed controversy on its high pressure structures.

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Fig. 6 The results of Rietveld refinement on (a) I4/mmm, (b) Immm and (c) Pnma structure. Red open circle and black line represent the observed and simulated profile respectively. The blue bars show the position of diffraction peaks. The gray line at the bottom denotes the difference between observed and simulated patterns. The big ball represents Xe atom, and small ball represents F atoms. The nearest Xe-F distance changes from 2.005 Å at 13 GPa to 1.925 Å at 45 GPa, then 1.840 Å at 62 GPa. Though in the former theoretical study, XeF2 stabilizes as the I4/mmm structure until 105 GPa.20 Non-hydrostatic condition can significantly reduce the phase transition pressure and the lattice symmetry.31, 32 Furthermore, in consideration of the strong oxidation ability and easy subliming of XeF2, it’s hard to use pressure transmitting medium in high pressure experiment. The effect of non-hydrostatic pressure is inevitable in the experiment environment, which will arouse the different phase sequence. Therefore, to throw light on the behavior of compounds under high pressure,

more

experiments

and

theoretical

study

non-hydrostatic effect are needed.

Conclusion 18


in

consideration

with

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In summary, we have studied the structural phase transitions of XeF2 under pressure up to 86 GPa, by means of high pressure Raman spectra, UV-VIS absorption spectra, DFT calculations and synchrotron XRD. Two phase transitions are observed in Raman spectra at 28 GPa and 59 GPa, respectively. Theoretical calculation and XRD refinement show that these two high pressure phases are in good agreement with Immm and Pnma structures. The decomposition of XeF2 is ruled out by XRD data. A band narrowing happens as the pressure increasing indicated by the optical absorption study. The optical band gap of XeF2 at 82 GPa is 1.83 eV. The metallic pressure is estimated to be 152 GPa. The present study has settled the existed structural dispute between experimental and theoretical results of XeF2.

Associated Content Supporting Information Raman peak position as a function of pressure, pressure difference measured by three ruby balls in sample chamber, lattice parameter of XeF2 at different pressure, lattice parameters obtained by geometry optimization and XRD results at corresponding pressure, optimization results of candidate structures, Rietveld refinement results of candidate structures.

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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Acknowledgements The authors are grateful to Xiaodong Li and Yanchun Li for their help during the experimental research at BSRF. This work was supported by National Natural Science Foundation of China (Nos. 51572108, 51632002, 51025206, 11274137, 11474127, 11504127), Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1132), National Found for Fostering Talents of basic Science (No. J1103202), and China Postdoctoral Science Foundation (2015M570265).

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TOC Graphic

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Fig. 1 165x167mm (300 x 300 DPI)



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Fig. 2 82x63mm (300 x 300 DPI)


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Fig. 3 82x140mm (300 x 300 DPI)



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Fig. 4 82x87mm (300 x 300 DPI)


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Fig. 5 82x63mm (300 x 300 DPI)



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Fig. 6 165x131mm (300 x 300 DPI)


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TOC Graphic 67x44mm (300 x 300 DPI)


Confirmation of the Structural Phase Transitions in XeF2 under High

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