High-Pressure Phases of a S-Based Compound: Dimethyl Sulfide

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High-Pressure Phases of a S-Based Compound: Dimethyl Sulfide Qingmei Zhang, Zhenxing Qin, and Xiaozhi Zhan J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 25 Jul 2017 Downloaded from http://pubs.acs.org on July 25, 2017

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The Journal of Physical Chemistry

High-Pressure Phases of a S-based Compound: Dimethyl Sulfide Zhenxing Qin1, Xiaozhi Zhan2,and Qingmei Zhang1† 1

Department of Physics, Taiyuan University of Science and Technology, Taiyuan 030024, P. R. China 2

Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, P. R. China

Abstract:The high-pressure behavior of dimethyl sulfide was investigated at room temperature by Raman scattering measurements with pressures up to 30.1 GPa. Phase transitions at 1.3, 3.6-5.8, and 17.2 GPa were found and evidenced by the frequency shifts, pressure coefficients and changes in FWHM of related modes. These phase transitions were suggested to result from the changes in the inter- and intra-molecular bonding of the material. Interestingly, the CH3 groups was compelled to be frozen in positions at a relatively low pressure, suggested by the disappearing of the relative modes softening. In addition, the appearing of lattice mode can also be found at a modest pressure, which makes it possible to gain a superiority for this compound to further investigate the superconductivity with high transition temperatures of bulk hydrogen.

†

Corresponding author. E-mail: [email protected]

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I.

Introduction The study of hydrogen-rich compounds has been mostly motivated by their

potential high-temperature superconducivities induced by pressure1-9 since Ashcroft firstly suggested in 2004 that hydrogen-dominant hydrides could be high-temperature superconductors in monatomic and molecular phases under pressure.10 By this argument, such covalent hydrides could exhibit metallization at significantly reduced pressures compared with pure hydrogen due to the “chemical precompression” and therefore provid an alternative way to achieve metallic hydrogen. Group IVa hydrides were specifically invigorated as potential candidates for this material and many experimental and theoretical efforts were underway in past years to investigate this prediction, such as SiH4,1-3,5,11-16 GeH4,6,17-20 SnH4,21-23 and PbH4.24 However, experiments reported a possible decomposition of SiH4 under irradiation from x-ray and lasers,25,26 which has become an obstacle for further investigating the metallization of bulk hydrogen in these compounds. Fortunately, an exception was found in tetramethylsilane (Si(CH3)4), which is also one of the Group IVa hydrides yet shows no decomposion at pressures up to 142 GPa.27 Although it remains unkown whether Si(CH3)4 could be metalized or not, our previous work by Raman spectroscopy revealed that new Raman modes, which appeared at around 100 GPa,27 underwent a dramatic softening and characterized Si(CH3)4 as semi-metallic state possibly as in the case of hydrogen at high pressures.28,29 Even more interestingly, the CH3 groups within such meterials were proved to be a motivating factor both for maintaining stability under compression and for the softening behaviors of those modes.27,30-32 Moreover, the CH3 group also shows various interesting behaviors at high pressures, including the restriction in the rotation of the CH3 group in some compounds (such as CH3HgM (M = Cl, Br, I)33 and (CH3)2XM (X = Sn or Tl)34,35) and the different rotational angles in cubic Si(CH3)4 at 0.58 GPa36 etc. Additionally, the recent breakthrough in the discovery of superconductivity above recorded high 190 K in H-S compound37 indicates the possibility of high-temperature superconductivities in hydrogen-dominant hydrides. Obviously, hydrides containing sulfur element have an advantage over the Group IVa hydrides so far in achieving metallization and


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superconductivity in hydrogen-rich materials. Therefore, it seems worthwhile to investigate the high-temperature superconducivity of hydrogen-rich compounds where interactions between the CH3 groups and S element exist. Dimethyl sulfide (i.e. (CH3)2S, named DMS), a typical hydride with sulfur element, has spurred a tremendous interest in the spectroscopy since 1940s because such molecules with two internal tops can exhibit a fine structure that has its origin in the coupling between the angular momenta of internal and over-all rotation.38-45 In the molecular structure, the S element is coordinated between outspread methyl groups (-CH3), making the molecule follow the C2v point-group symmetry.39-41,45 Additionally, DMS is not only one of typical odorous volatile organic sulfur compounds from photoresist processes of photoelectric industry, anaerobic wastewater treatment plants, composting plants, and rendering plants,47 its production but also plays a key role in climate feedback.48 To our knowledge, however, little information can be referenced about the properties of DMS at pressures although high-pressure measurement has been a unique pathway to explore new materials and new structures in material science. In this paper we present the pressure-induced phase transitions of the DMS by Raman spectra. Since vibrational excitations are extremely sensitive to changes in crystal structure, vibrational spectroscopy is crucial for making direct measurements on the state of bonding of the material at high pressures. In the light of the information on assignment of the vibrational modes of these compounds, Raman spectroscopic technique has been a shortcut means for identifying phase transitions in such materials at high pressures.49,50 The fact that softening of the modes disappeared only at 3.6 GPa besides of several possible phase transitions identified at 1.3, 3.6-5.8, and 17.2 GPa. Lattice vibrational modes incidentally appearing at 9.9 GPa play a vital role to illustrate the interaction of crystal lattices. Our findings have great implications for achieving metallization under high pressure based on hydrogen-rich compounds including sulfur, which will be elaborated in Section III in detail. II. Experimental details DMS (m.p. -98 ℃, b.p. 38 ℃) as transparency liquid with ＞99% purity was purchased from TCI and used without further purification. The high-pressure



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experiments for DMS were performed using DACs with culets of 300 µm. A hole of ～100 µm in diameter drilled in a preindented steel gasket served as the sample chamber. To avoid volatilizing, the bottoms of the DACs were put on ice bag before loading the sample. Liquid DMS was loaded into the chamber of the DACs with a syringe. Because of the liquid nature of DMS, no pressure medium was used and ruby grains had been placed previously for pressure calibration. Raman spectra were obtained in a backscattering geometry with a spectrometer (with 1800 mm−1 grating) equipped with a di-monochromator and a charge coupled device detector, giving a resolution of 1–2 cm−1. Radiation of 532 nm from a solid-state laser (0.5 W CW) was used for the excitation of the Raman spectra and all spectra were measured at ambient temperature. The scattering light was captured with a single exposure of the CCD with a spectral resolution of 1 cm−1. III. Results and Discussion In a single molecule of DMS, the sulfur atom is outspreadly bonded to two methyl groups. Under ideally C2ν molecular symmetry, one S–C and C–H bond distance, one C–S–C angle, two S–C–H and H–C–H angles would fully describe the molecular geometry,42 which results in the vibrational representation of DMS arises as Γν =7a1+4a2+4b1+6b2, where the selection rules allow Raman activity for all modes. However, a third of the modes would be impossible to display in the spectra of DMS in view of the polarity,41 which makes it much simpler to assign the type of vibrations to each Raman signal of DMS at ambient conditions as shown in Table I. Moreover, several modes possess inherently weak and broad Raman signals at around the 1430 cm-1, which are obscured by the strong peak of diamond at 1332 cm−1. Therefore our following discussion will focus on the 10 distinct modes as listed in Table I. Vibrational spectroscopy is crucial for characterizing high-pressure phase transition of low-Z molecular materials. Figure 1 shows the selected Raman vibrational spectra of DMS as a function of pressure ranging from 0.2 to 30.1 GPa. Clearly, the Raman spectra could be divided into four regions based on the molecular nature of the complex: the torsion and C–S–C bend region (100–300 cm−1), the C–S stretch region (500–700 cm−1), the CH3 rock and CH2 rock/H-C-S stretch region


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(1150–1250 cm−1), and the CH3 stretch region (2800–3200 cm−1). With increasing pressure, all the measured peaks shift to higher frequencies except for the ones in the region of 1150–1250 cm−1. In addition, all peaks become weak at pressure up to 30.1 GPa, especially in the 1150–1250 cm−1 region where the Raman signals almost vanish. These observations suggest that several pressure-induced phase transformations have taken place in DMS. The torsion and C–S–C bend region. As shown in figure 1a, there are three Raman signals appeared in the C–S–C bend and torsion region at 0.2 GPa and the corresponding modes are labled sequently as ν1, ν2 and ν3 in table 1. This is totally different from other reports on liquid DMS where only a very broad signal was observed in ref. 38, 41, 43. One reasonable explanation is that these signals submerge themselves in the high spectrum background in the low frequency region. On the other hand, according to previous calculations on Hamiltonian, there should be three Raman active modes proposed to exist in this region, and it was also reported that such signals appear in polycrystalline DMS.42 Therefore, another reasonable explanation for our observations is the possible existence of partial crystallization in DMS due to the pressure gradient inside the hole. At the onset of compression, all of the Raman signals in this region keep stable until the pressure reaches 3.6 GPa where the

peak

of

the

mode

ν1

disappears.

Then

the

mode

ν3

also

slowly merges into background at a pressure of 5.8 GPa, whilst the mode ν2 stays firm and even becomes more acute with increasing pressures. Continuing compression to 9.9 GPa, a Raman signal of lattice mode arises unexpectedly at a frequency of 109 cm-1, indicating DMS has been forced to perform the interactions between crystal lattices. With increasing pressures, the mode ν2 stays strong with a shoulder peak appearing on the left at 17.2 GPa. Compressed gradually to 30.1 GPa, the peak of lattice mode moves slowly to higher frequency, yet the mode ν2 shrivels quickly and intertwines with the shoulder peak into a very broad peak. The C–S stretch region. As shown in figure 1b, the Raman spectra of DMS are collected beginning from 0.2 GPa in the spectral region of 500–700 cm−1. Obviously, there are only two Raman signals appeared in this region at 0.2 GPa and the



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corresponding modes are labled as ν4 and ν5 in table 1 respectively. With increasing pressures, the drastic spectra changes indicate substantial changes in the crystal and/or molecular structures. A shoulder peak is observed on the left of the mode ν5 upon compression to 1.3 GPa, while the mode ν4 remains stable. With further increasing pressures, the mode ν4 seems to become “fat” quickly, suggesting that DMS starts to solidify by compression.51 Simultaneously, the intensities of ν5 and its shoulder peak exhibit a reversal change, which is evidence for the exchange of the symmetry assignment of mode ν5 and its shoulder peak as a result of Fermi resonance52 and loading to be separated each other. This phenomenon proceeds until 17.2 GPa, where the mode ν5 is squeezed out by broadening shoulder peak. Moreover mode ν4 also stops broadening at this pressure. Therefore, only a broadened ν4 and shoulder peak of mode ν5 in this region continue moving towards high Raman shift until 30.1 GPa. The CH3 rock and CH2 rock/H-C-S stretch region. It is not easy to assign vibrational type to the three Raman signals in this region as shown in figure 1c because the spectrum obtained for our sample is totally different from previous reports on DMS 41-43. According to these reports, two vibrational types of DMS can be found in this region: I. the CH3 rocking vibrations. It is of interest to note that only one Raman mode at 1032 cm-1 was observed in this region. However, this result is not consistent with theoretical computations on syrmnetry properties for the normal vibrations of (CH3)2X (X=O, S, Se and Te)41 that there should be four modes appeared in this region; II. the CH2 rock/H-C-S stretch vibrations, which is intended to include 910 cm-1, 953 cm-1, and 1032cm-1 on polycrystalline DMS combining with calculational results of 986 cm-1.42 Obviously, Raman signals in our spectrum exhibit three peaks located tightly at the Raman shift of 1175, 1180 and 1188cm-1 respectively,

which

are

labeled

as

ν6,

ν7

and

ν8

in

table

1.

In view of the above-mentioned fact that a small part of DMS under pressure is likely to begin crystallizing at 0.2 GPa, elusive Raman signals here could possibly form owing to interactions between the liquid and crystalline DMS under pressure. With increasing pressures, the intensities of the mode ν6 and ν7 exhibit a reversal change only at 0.4 GPa suggesting again intense interactions between the liquid and solid


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molecules of DMS. With continuing compression, the spectrum remains stable until a new mode is extruded in the middle of the ν7 and ν8 modes at 5.1 GPa. The peaks of the mode ν6 and ν8 disappear at pressure up to 6.8 GPa, while the mode ν7 seems to be strengthened simultaneously. Therefore, it is suggested that DMS certainly undergoes a possible phase transition at pressures taking account of such rich phenomenons and the above-mentioned changes occurred in the torsion and C–S–C bend region at the same pressures. With further compression to 30.1 GPa, no more significant change is observed in this region except for the broadening and weakening of the mode ν7 and new mode which is likely to disappear at last. The CH3 stretch region. Sequentially, the spectra changes in the CH3 deformation region (i.e., in the frequency range of 1300-1500 cm-1) should be illustrated first. Unfortunately we are unable to collect these Raman signals due to the presence of an extremely strong peak of diamond at 1332 cm−1. Therefore, only the spectra in the CH3 stretch region will be discussed in the following. As shown in figure 1d, the Raman spectra of DMS are also collected from a pressure of 0.2 GPa in a frequency range of 2800-3200 cm−1. It is found that crystallizing DMS possesses a great deal of peaks in this region, yet most of which are weak and crowded against other three strong peaks at the Raman shift of 2916 cm-1, 2964 cm-1, and 2981 cm-1,42 which is nearly consistent with the strongly polarized Raman peak of 2910 cm-1 and the depolarized Raman peaks of 2966 cm-1 and 2982 cm-1 in liquid DMS.41 In view of the full width half maximum of 24 cm-1 achieved by Lorentz fitting, the peak at higher fenquency of this region should be a combination of two depolarized Raman peaks of 2966 and 2982 cm-1, which is labeled as mode ν10. Obviously, another peak here is mode ν9. Compressed to 3.6 GPa, a Raman signal emerges at the high frequency region of mode ν10. Then another new signal is extruded at the left shoulder of the mode ν10 with pressure up to 4.5 GPa. Interestingly with the same amounts of pressures, the spectra undergo substantial changes in other two regions as shown above. This provides further evidence for the existence of a phase transition at these pressures. With further compression to 17.2 GPa, a new peak is extruded at the right shoulder of mode ν9, which suggests that DMS undergoes



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another possible phase transition at this pressure. Compressed continually to 30.1 GPa, the new peak becomes more and more distinct, while the Raman signals that intertwine originally with the mode ν10 are getting much severer, which makes it difficult to distinguish these modes with increasing pressure to 30.1 GPa. Vibrational frequencies provide information on the high-pressure behaviors of DMS. To show the possible phase transitions on compression, the pressure dependences of Raman modes are depicted in figure 2 and can be quantitatively illustated in table 1 by fitting pressure coefficients (dν/dP (cm − 1/GPa)) of the monitored modes obtained by linear regression. From above we have found evidences for three phase transitions at approximately 1.3, 3.6-5.8, and 17.2 GPa over the pressure range studied, which are labeled as liquid phase (below 1.3 GPa), phase I (1.3–3.6 GPa), phase II (5.8–17.2 GPa), and phase III (above 17.2 GPa) in figure 2. For the liquid phase of DMS below 1.3 GPa, nearly all the modes exhibit the blue-shift with increasing pressure, whereas the ν6 and ν7 modes show a softening behavior, the negative pressure coefficients of both modes as listed in table 1, which is regarded as a typical character of rotational motions of the CH3 group.33–35 This phenomenon has also been observed in pressure-induced investigation on X(CH3)4 (X = Si, Ge and Sn)27,30,31, where equivalent CH3 groups would not remain stable and rotate by certain angles due to pressure-induce increase of intra- and inter-molecular interaction. Additionally, it is found that each mode has very different pressure coefficient as shown in table 1 and CH3 stretch modes exhibit the most intensive pressure effects in liquid phase. Besides the above-mentioned evidence for phase transition from liquid phase to phase I (i.e., the appearing of a new mode (labled as ν5' in fig. 2) in the C–S stretch region at 1.3 GPa), more evidences can be found in figure 2 and table 1. Firstly, although the ν6 and ν7 modes keep softening with increasing pressure, their pressure coefficients show a drastic reduction compared with the case in liquid phase as shown in table 1. Secondly, the pressure coefficients for most of the modes slightly decrease compared to that in the liquid phase, which possibly results from the lack of enough moveable space for the groups of DMS with increasing pressure. Note that the pressure coefficients of the CH3 stretch modes suffer such a


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rapid decrease that other modes (such as C-S stretch and in-phase torsion modes) follow a similar pressure effect, which is totally different from the case for liquid phase. However, it is of interest to note that the pressure coefficients of torsion modes increase obviously upon compression, which including the pressure coefficient of out-of-phase torsion almost doubles from 10.29 in liquid phase to 17.70 in phase I. In view of the situation here that DMS has been crystallized in phase I, such strong pressure effects of torsion modes should lead to the changes on crystal structure with increasing pressure. At last, the pressure coefficient of the mode ν8 also slightly increases although it is beyond our ability to assign vibrational type to this mode, suggesting a unique interaction of molecules into DMS at such low pressures. Sequentially, a transitional phase is marked at the pressure ranges of 3.6-5.8 GPa because several modes appear (labled as ν7', ν10', and ν10'' in fig. 2) or disappear (labled as ν1, ν3, ν6 and ν8 in fig. 2) singly with increasing pressures. To our surprise, the pressure coefficients of ν6 and ν7 modes change from negative to positive at 3.6 GPa, indicating the rotational motion of the CH3 groups was compelled to be frozen in positions.52 In phase II as shown in figure 2, nearly all the modes exhibit the blue-shift with further increasing pressure, and their pressure coefficients decreas slightly compared with the former phase. In addition, the appearing of lattice mode at 9.9 GPa is the most striking feature of phase II, which suggests the appearance of interactions of crystal lattices. This is mainly caused by the fact that the strong pressures shall lead to closer-packing atomic layer(s) into the molecules of DMS and further lock the atoms/groups in certain positions. In phase III, all vibrational modes remain stable and the pressure coefficients of all the modes stay unchanged. Besides, two new modes appear and are labled as ν2' and ν9' respectively, while the ν5 and ν5' modes happen to mergence at 17.2 GPa. Therefore, the number of the Raman modes increases by one moder in this phase, which possibly indicates a new phase with a lower symmetry. The phase transitions can be further confirmed by the pressure dependence of full width half maximum (FWHM) of vibrational modes as shown in figure 3. The FWHM of the mode ν2 reaches the inflection points at around 1.3 and 3.6-5.8 GPa,



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while the FWHM of vibrational mode ν4 encounters the inflection points not only at around 1.3 and 3.6-5.8 GPa but also at 17.2 GPa. However, because of weak Raman signals or midway disappearing of other vibrational modes, we cannot be sure whether or not there are changes in the inflection points with increasing pressures. Further theoretical computations of the possible high-pressure phases of DMS and eventually the structural investigations using x-ray or preferably neutron diffraction measurements, will be very useful to elucidate the pressure-induced phase transitions acquired by our Raman observations. In general, most of the Raman modes will exhibit pressure-induced blue shifts due to increased interactions between atoms with the application of pressure. The modes of ν6 and ν7 show softening behavior at our low pressure range, which contradicts with the general rule that an increase in pressure should increase the A–H bond frequencies of weak and medium-strength hydrogen bonds.53,54 This anomalous phenomenon also exists in other molecular compounds containing CH3 or NH3 groups where order-disorder transitions take place with temperature or pressure. For example, the pressure-induced restricted rotation of NH3/CH3 groups and the locked NH3/CH3 positions in the compounds of dihydrogen bonding molecule of NH3BH3,52 CH3HgM, (M = Cl, Br, I)33 and (CH3)2XM, (X = Sn or Tl)34,35 can be reflected by the softening of NH3/CH3 vibrational mode in the Raman spectra. This anomalous phenomenon could also be found in the high-pressure experiments of the hydrogen-rich compounds, X(CH3)4 (X = Si, Ge and Sn)27,30,31. For the Si(CH3)4, this softening behavior existed with the pressure up to 9.0 GPa, while it is 1.4 GPa and 0.9 Gpa for Ge(CH3)4 and Sn(CH3)4 respectively. Compared with these results, DMS is very easy to undergo phase transition via active rotation of the CH3 groups to freezing in a position only at 3.6 GPa in view of such small molecule relatively. To our knowledge, it is the first time that softening behavior has been found in such a sulfenyl hydrogen-rich compound. This may suggest that C–H bonds in solid S(CH3)2 are not isolated. Upon application of high external pressures, repulsion or attraction from surrounding neighbors will arise and possibly lead to new chemical bondings including C–H bonds, which provide a possibility for DMS to organize crystal structure with unique


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hydrogen such as graphenelike layers, elongated H2 dimers and so on55. On the other hand, the appearing of pressure-induced lattice mode has also been observed in other high-pressure Raman spectra of compounds such as H2S37, 56, 57 and S-H2 complex.58 This is a typical phenomenon to characterize the new conformations of corresponding materials due to the interaction of crystal lattices. Recently, H-S systems have attracted a great deal of excitement and stimulated a number of experimental and theoretical studies37,56-66 because of the high-temperature superconductivity at temperatures close to 200 K in H2S at pressure. The new compounds in stoichiometry, especially H3S, was considered as a key to obtain the high-temperature superconductivity at pressures37. However, the lattice modes was not observed in the Raman spectra until 17 GPa in H3S, which is obtained via pressure-induced H2S compound by the chemical reaction of molecular hydrogen with sulfur at high pressures. 58 Meanwhile, the lattice mode of “H2S” was not found at least upon 27 GPa57. Obviously, the appearing of lattice mode of DMS at pressures is much earlier although we can not determine whether or not a new compound in stoichiometry is constructed in the current work. This unique feature observed in this S-H system suggests a range of previously inaccessible intermolecular interactions in CH3-bearing molecular systems and a possiblity as a potential new class of dense low-Z materials to achieve superconductivity with high transition temperatures, yet at lower pressures. IV. Conclusions We performed Raman measurements of DMS at room temperature and at pressures up to 30.1 GPa. Our results revealed the phase transitions at 1.3, 3.6-5.8, and 17.2 GPa from the mode frequency shifts with pressure. These transitions were suggested to result from the changes in the inter- and intramolecular bonding of this material. It was found that rock modes of CH3 groups exhibited softening at low pressures, and their pressure coefficients changed from negative to positive at 3.6 GPa. In addition, a lattice mode appeared unexpectedly at 9.9 GPa. Compared with other similar materials, these phenomena suggest that DMS has a potentiality to achieve the superconductivity with high transition temperatures, whereas the required pressure is



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modest for laboratory capability. Acknowledgments We are grateful to Jiang Zhang for his technical support during the Raman experiment in South China University of Technology. This work in China was supported by the National Natural Science Foundation of China (Grant No. 51502189), the Doctoral Project of Taiyuan University of Science and Technology (Grant No. 20132010), and the Shanxi Province Science Foundation for Youths (Grant No. 2015021019 and 201601D021019). 1. Feng, J.; Grochala, W.; Jaroń, T.; Hoffmamoto, R.; Bergara, A.; Ashcroft, N. W. Structures and Potential Superconductivity in SiH4 at High Pressure:En Route to ‘‘Metallic Hydrogen’’. Phys. Rev. Lett. 2006, 96, 017006. 2. Chen, X. J.; Wang, J. L.; Struzhkin, V. V.; Mao, H. K.; Hemley, R. J.; Lin, H. Q. Superconducting Behavior in Compressed Solid SiH4 with a Layered Structure. Phys. Rev. Lett. 2008, 101, 077002. 3. Pickard, C. J.; Needs, R. J. High-Pressure Phase of Silane. Phys. Rev. Lett. 2006, 97, 045504. 4. Goncharenko, I.; Eremets, M. I.; Hanlland, M.; Tse, J. S.; Amboage, M.; Yao, Y.; Trojan, I. A. Pressure-Induced Hydrogen-Dominant Metallic State in Aluminum Hydride. Phys. Rev. Lett. 2008, 100, 045504. 5. Eremets, M. I.; Trojan, I. A.; Medvedev, S. A.; Tse, J. S.; Yao, Y. Superconductivity in Hydrogen Dominant Materials: Silane. Science, 2008, 319, 1506-1509. 6. Gao, G.; Oganov, A. R.; Bergara, A.; Martinez-Canales, M.; Cui, T.; Iitaka, T.; Ma, Y.; Zou, G. Superconducting High Pressure Phase of Germane. Phys. Rev. lett. 2008, 101, 107002. 7. Tse, J. S.; Yao, Y.; Tanaka, K. Novel Superconductivity in Metallic SnH4 under High Pressure. Phys. Rev. lett. 2007,98, 117004. 8. Zurek, E.; Hoffmann, R.; Ashcroft, N. W.; Oganov, A. R.; Lyakhov, A. O. A Little Bit of Lithium Does a Lot for Hydrogen. Proc. Nat. Acad. Sci. USA, 2009, 106(42), 17640-17643.


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T.

Pressure-Induced

Metallization

of

Dense

(H2S)2H2

with

high-Tc Superconductivity. Sci. Rep. 2014, 4, 6968. 60. Bernstein, N.; Hellberg, C .S.; Johannes, M. D.; Mazin, I. I.; Mehl, M. J. What Superconducts in Sulfur Hydrides under Pressure and Why. Phys. Rev. B. 2015, 91, 060511(R). 61. Papaconstantopoulos, D. A.; Klein, B. M.; Mehl, M. J.; Pickett, W. E. Cubic H3S around 200 GPa: An Atomic Hydrogen Superconductor Stabilized by Sulfur. Phys. Rev. B. 2015, 91, 184511. 62. Li, Y.; Hao, J.; Liu, H.; Li, Y.; Ma, Y. The Metallization and Superconductivity of Dense Hydrogen Sulfide. J. Chem. Phys. 2014, 140, 174712. 63. Durajski, A. P. Quantitative Analysis of Nonadiabatic Effects in Dense H3S and PH3 Superconductors. Sci. Rep. 2016, 6, 38570. 64. Hirsch, J. E.; Marsiglio, F. Hole Superconductivity in H 2 S and Other Sulfides under High Pressure. Physica C. 2015, 511, 45-49. 65. Troyan, I.; Gavriliuk, A.; Rüffer, R.; Chumakov, A.; Mironovich, A.; Lyubutin, I.; Perekalin, D.; Drozdov, A. P.; Eremets, M. I. Observation of Superconductivity in



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Hydrogen Sulfide from Nuclear Resonant Scattering. Science. 2016, 351, 1303-1306. 66. Einaga, M.; Sakata, M.; Ishikawa, T.; Shimizu, K.; Eremets, M. I.; Drozdov, A. P.; Troyan, I. A.; Hirao, N.; Ohishi, Y. Crystal Structure of the Superconducting Phase of Sulfur Hydride. Nat. Phys. 2016, 12, 835-838.


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TABLE 1. Assignment of the observed Raman modes of DMS, changes of Raman modes with pressures, and the pressure coefficients of the corresponding frequencies of the Raman modes. Vibrational modes

Raman shift（（cm-1）[b]

dν /dP (cm−1/GPa) [c]

vibrational type[a] 0.2 GPa

1.3 GPa

3.6-5.8 GPa

lattice mode

9.9 GPa

17.2 GPa

109

122

ν1

in-phase torsion

136

145

/

ν2

C-S-C bend

174

183

204

212

ν3

out-of-phase torsion

203

210

269

/

ν4

C-S symmetrical stretch

515

liquid

I

I+II

II

III

1.28

0.74

1.30

7.24

9.43

/

7.49

6.51

3.31

2.26

10.29

17.7

11.3

/

207

ν5

C-S asymmetrical stretch

ν6 ν7

①CH3 rock

520

535

542

563

532

559

579

623

527

539

574

599

/

1175

1172

1170

/

1180.0

1179.5

1179.3

1180.4

1183.6

1186

1191

1201

1188

1190

1198

/

2924

2935

2973

3003

3024

3056

3134

3056

3077

3154

3065

3109

3171

②CH2 rock/H-C-S stretch ν8 ν9

CH3 symmetrical stretch

229

3046

1.33

5.52

4.08

2.77

2.36

2.25

6.51

5.56

5.1

3.27

9.15

9.16

6.81

5.16

/

-2.56

-1.02

0.18

/

-0.47

-0.04

0.02

0.34

0.6

0.65

1.40

1.27

1.63

1.74

1.96

/

10.61

9.21

8.11

6.59

6.29

9.38

5.45

11.35

9.18

6.59

12.33

6.86

6.91

3058

ν10

CH3 asymmetrical stretch

2999

3012

a

4.86

11.77

8.64

Reference 38-46. Observed at 0.2 GPa and room temperature in the liquid phase for all internal modes and appearing with compression in the proposed phases. c Obtained by linear fit of the Raman modes in four pressure regions, as indicated in Fig. 2. b


3.88


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Fig. 1 Representative Raman spectra of DMS in the full spectral regions at 0.2 GPa upon compression to 30.1 GPa. The red arrows (up and down arrows show appearance and disapperance of the new Raman signals with compression respectively) are to make the changes of part Raman modes clear to observe. The value of pressure is labeled only on black spectra to show clearly the possible phase transition.


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Fig. 2 Pressure dependence of the frequencies of DMS for the observed modes in all regions at room temperature. The marked area and vertical dashed lines at near 1.3, 3.6-5.8, and 17.2 GPa indicate the proposed phase boundaries.



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Fig. 3 Full width half maximum (FWHM) of vibrational modes ν2 (fig.3a) and ν4 (fig. 3b) of DMS as a function of pressure from 0.2 to 30.1 GPa. Error bars indicated for all pressures where the least squares fitting of the band profile carries smaller uncertainties than average. The solid lines crossing the solid symbols are for eye guidance. The marked area and vertical dashed lines demarcate the position of inflection point at 1.3 and 3.6-5.8 GPa for ν2 vibrational modes and 1.3, 3.6-5.8 and 17.2 GPa for ν4 vibrational modes, respectively.


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TOC Graphic:


High-Pressure Phases of a S-Based Compound: Dimethyl Sulfide

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