Shear-Induced Isostructural Phase Transition and Metallization of

Feb 16, 2016 - Pressure-induced structural and electronic transformations of tungsten disulfide (WS2) have been studied to 60 GPa, in both hydrostatic...
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Shear-Induced Isostructural Phase Transition and Metallization of Layered Tungsten Disulfide under Nonhydrostatic Compression Sakun Duwal and Choong-Shik Yoo*

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Department of Chemistry and Institute of Shock Physics, Washington State University, Pullman, Washington 99164, United States ABSTRACT: Pressure-induced structural and electronic transformations of tungsten disulfide (WS2) have been studied to 60 GPa, in both hydrostatic and nonhydrostatic conditions, using four-probe electrical resistance measurements, micro-Raman spectroscopy, and synchrotron X-ray diffraction. The results show the evidence for an isostructural phase transition from hexagonal 2Hc phase to hexagonal 2Ha phase, which accompanies the metallization at ∼37 GPa. This isostructural transition occurs displacively over a large pressure range between 15 and 45 GPa and is driven by the presence of strong shear stress developed in the layer structure of WS2 under nonhydrostatic compression. Interestingly, this transition is absent in hydrostatic conditions using He pressure medium, underscoring its strong dependence on the state of stress. We attribute the absence to the incorporation of He atoms between the layers, mitigating the development of shear stress. We also conjecture a possibility of magnetic ordering in WS2 that may occur at low temperature near the metallization.

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INTRODUCTION Transition metal dichalcogenides (TMDs or MX2 where M = Mo, W and X = S, Se) are semiconductors with indirect band gaps ranging from 1.0 to 1.4 eV for bulk forms. These indirect gaps are located slightly under direct gaps ranging from 1.6 to 2.5 eV.1 Because of the presence of tunable band gaps in TMDs, they are widely used in mechanical and electronic applications,2 and are often subjected to high-pressure investigations. TMD materials crystallize into layered structures made of a varying degree of covalency and ionicity in M−X bonds; these layered structures control the electronic band gaps of bulk and thin films. Yet, these layers are held together only by weak van der Waals forces, giving rise to interesting mechanical properties such as extremely low friction shears and high compressibilities. Combining the two, strong covalent bonds within layers and weak van der Waals interactions between layers results in highly anisotropic crystal structures and intriguing optical and electronic properties, significant to various applications for catalysis, solar cells, batteries, and optoelectronic devices.3,4 MoS2, for example, is predicted to be an indirect band gap semiconductor in a bulk crystal, but a direct band gap semiconductor in its monolayer form.3 MoS2 has also shown great promise as an electrocatalyst for hydrogen evolution reactions.5 For similar reasons, various TMDs in bulk and multilayers have been subjected to extensive studies at both ambient and high pressures.3,6−9 The application of external pressure is an excellent way to tune mechanical, chemical, and electronic properties of TMDs and investigate the nature of chemical bonding, crystal structure, and phase transformation. Interestingly, greatly diverse properties are often observed at high pressures among various TMD materials, despite their periodic and structural © 2016 American Chemical Society

similarities. This divergence, on the other hand, underscores the significance of the strongly anisotropic crystal structure and chemical bonding, which can change greatly depending on the state of stress, structural distortion, and topological packing. Multilayer MoS2, for example, undergoes a semiconductor-tometal transition at 19 GPa,9 whereas bulk MoS2 undergoes an isostructural transition followed by a semiconductor-to-metal transition above 28 GPa.10 Moreover, MoS2 becomes a superconductor at 5 K and 90 GPa.11 The structure of MoSe2 continuously evolves from anisotropic two-dimensional (2D) layers to an isotropic three-dimensional network, as pressure increases above 30 GPa.12 MoSe2 is also found to transform into a metallic phase between 20 and 35 GPa.12 Despite the extensive studies on MoS2 and MoSe2,6−12 there have been only a few studies on WS2 at high pressures. Because both Mo and W belong to the same periodic group, it is often assumed that the corresponding dichalcogenides possess similar properties. Contrary to this assumption, Bandaru et al.13 reported that bulk WS2 exhibits no structural phase transition up to 52 GPa in hydrostatic conditions using helium as a pressure medium. Nayak et al.14 reported that multilayered WS2 undergoes an isostructural semiconductor-to-metal transition at 22 GPa at 280 K. While these studies have furthered the understanding of the high-pressure behavior on WS2, the results seem to show diverse behaviors of WS2 depending on the state of stress. To address this issue, we have investigated WS2 in both hydrostatic and nonhydrostatic conditions. The present results indeed indicate a strong stress dependence on the transition in WS2. WS2 undergoes an isostructural phase Received: November 3, 2015 Revised: January 22, 2016 Published: February 16, 2016 5101

DOI: 10.1021/acs.jpcc.5b10759 J. Phys. Chem. C 2016, 120, 5101−5107

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EXPERIMENTAL METHODS WS2 powder (99.8% purity from Alfa Aesar) was used in the present study. The powder samples were loaded into a small hole (140 μm in diameter) drilled on a Re gasket (200 μm thick preindented to ∼40 μm) mounted between two diamond anvils (Type IA, 300 μm flats) of a membrane-driven diamond anvil cell (DAC) without a pressure medium for nonhydrostatic experiments. For hydrostatic experiments, high-pressure helium gas (∼2000 atm) was used as a pressure-transmitting medium (PTM), loaded using a custom-built high-pressure gas loader. A few small chips of ruby crystals were also loaded together with the sample for pressure determination based on the peak position of R1 luminescence.16 Raman spectra were collected using a home-built, confocal micro-Raman system that consists of a Nd:YAG laser (532 nm, Verdi 6W, Coherent); a 20× objective lens (infinitive corrected, Edmund); a confocal device (a pair of lenses and a 2D knifeedge slit); a 0.5 m spectrograph; a liquid N2-cooled, chargecoupled device (CCD) detector; and various optics such as a holographic diffraction beam splitter and a Raman notch filter (both from Kaiser Optics). Raman spectra were collected in a backscattering geometry with a spectral resolution of ∼0.3 cm−1. Electric resistance of the sample was measured using a standard four-probe technique for the DAC application. Alumina powder was used to insulate the gasket. A sample hole was drilled at the center of an insulated alumina layer densely packed in the gasket hole. Four 5 μm thick platinum electrodes were placed on the top of insulating alumina, and Cu wires were used as an extension to measure the resistance. A fifth Cu wire was soldered onto the gasket to check for a short circuit while taking the measurements. The sample resistance was measured using a 1 μA current power source (Lakeshore 120) and a voltmeter (Keithley 2000 DVM). For diffraction experiments, we used microfocused (20 μm × 20 μm) synchrotron X-rays at 16BMD/HPCAT (λ = 0.4066 Å) at the Advanced Photon Source. The X-ray diffraction intensities were recorded over a large range of 2θ between 3 and 40°, using a large 2D image plate detector (MAR CCD). We used the Fit2D software to convert the 2D diffraction image to the one-dimensional angle-resolved X-ray diffraction (ARXD) pattern and, then, employed various diffraction analysis software including XRDA, GSAS, and CRYSTAL MAKER to analyze the ARXD data.

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Figure 1. (a) Crystal structure of WS2 in P63/mmc (or 2Hc phase) at ambient conditions. (b) Schematics of vibrational Raman modes. The red dotted lines represent weak van der Waals bonds between the adjacent layers. (Reproduced with permission from ref 17. Copyright 2013 The Optical Society.)

E1u), three translational acoustic modes (A2u and E1u), and five inactive modes (2B2g, B1u, and E2u). Vibrations of the Raman active modes are represented in Figure 1b. The 2-fold degenerate E2g (E12g + E22g) modes represent in-plane vibrations of tungsten and sulfur atoms (shear modes),18 whereas the A1g mode corresponds to out-of-plane sulfur vibrations (a surface mode) along the c-axis. The IR active A2u and E1u modes are then understood in terms of the asymmetric counterparts of the surface A1g and shear E1g modes, where tungsten and sulfur atoms move in the opposite directions. Figure 2 shows Raman spectra of WS2 consisting of the A1g and E12g modes at several pressures in (a) nonhydrostatic and (b) hydrostatic conditions. At ambient pressure, the A1g mode is observed at ∼421 cm−1. The E12g mode is observed at ∼353 cm−1 as shown in Figure 2. Even though the E1g mode is Raman active, this mode only appears as a very weak feature in the present backscattering setup.1 This is because the E1g phonon is forbidden in a backscattering geometry on a surface perpendicular to the c-axis and, at high pressures, the layered structure develops a highly preferable orientation along the same [001] direction. The peak at 266 cm−1 corresponds to a higher order resonant Raman peak, previously assigned based on the neutron inelastic scattering data.19 Other weak Raman modes have also been assigned in Table 1 of ref 1: the peak at ∼300 cm−1 to the resonant phonon mode of 2LA − 2E22g, the ∼520 cm−1 peak to the E12g + LA mode, and the ∼585 cm−1 peak to the A1g + LA mode (LA, longitudinal acoustic).1 The other E22g mode is known to appear at a very low wavenumber (∼27 cm−1), which is outside the spectral window of our instrumentation. The A1g mode is observed until 62 GPa, whereas the E12g mode disappears around 37 GPa, as shown in Figure 2a when no pressure-transmitting medium (PTM) was used. The higher intensity of the A1g mode, compared to E12g at 0.2 GPa, can be understood in terms of strong coupling of A1g phonons with excited dz2 states, as the dz2 orbital points perpendicular to the basal plane. Hence, the displacement of atoms along the cdirection has a greater effect on polarizability, resulting in higher intensity for the A1g mode. On the other hand, the atomic displacement in the E12g vibration is parallel to the plane, which gives a smaller change in polarizability. Therefore, the E12g mode is expected to weaken as pressure increases. However, at pressures below 20 GPa, the relatively stronger intensity of these two modes keeps fluctuating. This is likely because E12g is buried in the 2LA peak at 352 cm−1.18,20 At 0.4 GPa the intensity of the two peaks switches, and a shoulder on A1g starts

RESULTS

Figure 1a shows the crystal structure of double-layered hexagonal (2H) WS2, which belongs to the space group D46h (P63/mmc). In this structure, tungsten ions are coordinated with six sulfur atoms forming edge-shared trigonal prismatic layers. These layers are then stacked up along the c-axis (or the [001] direction) bounded by weak van der Waals forces. A unit cell of WS2 is then composed of two metal atoms occupying sites with the point-group symmetry D3h. This structure with two WS2 per unit cell allows a total of 18 modes of lattice vibrations: Γvib = A1g + 2A2u + 2B2g + B1u + E1g + 2E1u + 2E2g + E2u. These modes are then composed of seven Raman active modes (A1g, E1g, and 2E2g), three IR active modes (A2u and 5102

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Figure 2. (a) Raman Spectra of WS2 at room temperature in nonhydrostatic condition, showing appearance of a shoulder peak in A1g mode and weakening of the Raman peaks above 25 GPa. (b) Raman Spectra of WS2 at room temperature using helium as a pressure transmitting medium (PTM).

to appear. Following the same interpretation by Zhao et al.,20 we interpret this new feature on the A1g shoulder as a Ramaninactive B1u mode. The B1u mode is a part of the Davydov doublet with the A1g mode, arising as a result of resonant effect.21 The appearance of the B1u mode can then be an indication of enhanced disorder in the layered structure introduced by shear frictions. Importantly, the intensity of Raman spectra rapidly decreases as pressure increases above 15 GPa. At 44 GPa nearly all Raman features disappear except a small remnant of the A1g mode. Therefore, this result appears to indicate a structural deformation occurring over a broad pressure range, starting from ∼15 GPa and finishing above ∼44 GPa, where WS2 becomes a metallic solid. Raman spectra of WS2 collected in hydrostatic conditions using He as a PTM (Figure 2b) is consistent with those previously reported by Bandaru et al.13 As pressure increases, the A1g mode shifts much faster at the rate of 2.27 (±0.06) cm−1/GPa than the E12g mode at 1.33 (±0.05) cm−1/GPa (see Figure 3). This makes sense because the A1g mode corresponds to interlayer sulfur−sulfur vibrations, which are bound by weak van der Waals interactions. The E12g mode corresponds to tungsten−sulfur vibrations that are linked via strong covalent bonds. Furthermore, the present peak positions of WS2 agree well with those of Bandaru et al.,13 but are slightly higher than those of Nayak et al.14 for multilayered WS2, as compared in Figure 3. These spectral changes are reversible upon pressure unloading in both hydrostatic and nonhydrostatic conditions. The present Raman data clearly shows that high-pressure behaviors of WS2 depend strongly on the state of stress in the sample. It also reconciles the different observations between Bandaru et al.13 reporting the absence of phase transition in hydrostatic conditions and Nayak et al.14 reporting a continuously occurring isostructural transition in nonhydrostatic conditions. It is believed that the presence of small helium atoms limits the slip motion of the layers and thereby inhibits the occurrence of shear-induced structural transformation. In fact, helium is known to resist the shear stress.22 In this respect, the presence of the A1g shoulder peak in nonhydrostatic

Figure 3. Pressure-induced spectral shifts of Raman modes observed in WS2. Solid and open circles correspond to the present data taken with no PTM and He PTM, respectively. Triangles correspond to the previous data by Bandaru et al.,13 and short dashes correspond to the previous data by Nayak et al.14 for multilayered WS2.

conditions seems to support the occurrence of shear-driven structural change. At ambient pressure, WS2 is a semiconductor with an indirect band gap of 1.35 eV.23 We found that upon compression it transforms to a metallic state as shown in Figure 4. The resistance of WS2 at ambient condition is ∼104 Ω (or the estimated resistivity of ∼2 Ω m)the value after the initial compaction of powder samples and also after the pressure unloading. Upon compression, this resistance value decreases rapidly, at least by 3 orders of magnitude, to a steady value of less than ∼10 Ω at ∼37 GPa, indicating a semiconductor-tometal transition. For the current sample configuration in a 5103

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DAC, the residual resistance of 10 Ω is typical for that of metal or semimetal. In fact, the temperature-dependent resistance change (shown in Figure 4, inset) supports the metallic behavior above 37 GPa. Interestingly, the resistance curve at 37 GPa also shows an anomalous small bump at about 100 K, which could be an indication of structural changes and/or magnetic ordering at this temperature. The sample was further compressed to 110 GPa; however, no evidence for superconductivity was observed. In order to understand the structural change in nonhydrostatic conditions, we have obtained the X-ray diffraction data of WS2 to 60 GPa. The results are plotted in Figure 5: (a) the measured ARXD data to 57 GPa and (b) the d-spacings of observed (hkl) lattice planes as a function of pressure. The calculated peak positions in Figure 5a are based on a hexagonal structure in P63/mmc using the lattice parameters of a = 3.151 Å and c = 12.055 Å. The systematics of observed diffraction patterns are well matched to the calculated ones for the same structure, indicating no significant structure change under pressure. However, it is important to note that there are several subtle yet apparent changes in the diffraction patterns. For example, the (004) peak shifts rapidly and merges into the doublet of (100) and (101) peaks at 39 GPa. The doublet itself merges together, forming a single broad feature at high pressures. The (105) peak (marked by arrows) disappears at 45 GPa, while a new peak (marked by asterisks) emerges as a left shoulder of the (103) peak above 27 GPa. Importantly, these changes, the emergence of the new feature and the disappearance of the (105) peak were not observed in the hydrostatic condition reported in ref 13. The faster pressure shifts of both (002) and (004) indicate that the c-axis is much more compressible than the a-axis (see Figures 5b and 6a). This is clearly due to relatively weak van der Waals interactions along the c-axis. Note that there is a small difference in the aaxis observed in between hydrostatic (open symbols) and nonhydrostatic (closed symbols) conditions. This means that

Figure 4. Pressure-induced electric resistance change of WS2 showing a semiconductor-to-metal transition. The upper-right inset shows the temperature dependence of resistance at 37 and 82 GPa, showing a typical metallic behavior. The bottom-left inset is a microphotograph of WS2 sample with four Pt electrodes.

Figure 5. (a) Powder X-ray diffraction pattern of WS2 showing a structural phase transition over a broad pressure range of 15−42 GPa. The structural transition is evident by disappearance of the (105) peak at ∼45 GPa and an emergence of a small shoulder marked by asterisks above 15 GPa. Short vertical bars at the bottom are the calculated positions of (hkl) planes labeled on the diffraction pattern at 5 GPa. (b) Pressure dependence of d-spacings of WS2 showing disappearance of (004), (101), and (105) peaks. The symbols represent the experimental data, and the lines represent the polynomial fit. 5104

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Figure 6. (a) Pressure dependence of lattice parameters (a, c/2, and c/a in the top panel) and linear compressibilities (c/c0 and a/a0 at the bottom), showing a highly anisotropic layer structure of WS2. The solid symbols represent data from the present study, and the open symbols represent data from Bandaru et al.13 The lines represent the quadratic fits to the data. (b) Pressure−volume compression curve of WS2. The red line is the fit with Birch−Murnaghan equation of state. The blue line is the polynomial fit drawn at the phase transition region. The insets show crystal structures of 2Hc and 2Ha phases, showing the top view down the c-axis of a 3 × 2 × 1 supercell.

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DISCUSSION The present spectral and diffraction results suggest that WS2 undergoes an isostructural phase transition in nonhydrostatic conditions. The evidence for the structural transition includes the emergence of new Raman and diffraction peaks, as well as the deviation of specific volumes from the BM-EOS fit at high pressures. The absence of the same transition in hydrostatic conditions, on the other hand, seems to indicate that the transition is kinetically controlled by the presence of strong shear stress in this layered structure of WS2. We conjecture that the presence of helium atoms between the layers blocks the movement of the slip planes (i.e., the (110) layers). The sluggish nature of the transition supports this mechanism, which occurs over a broad pressure range from ∼15 to ∼45 GPa. The EOS plot shows a discontinuity, which is evidence for a first-order phase-transition. Furthermore, the cell parameters versus the pressure plot show (Figure 6a) some discontinuity in the decreasing trend along the a-axis, but not along the c-axis. This illustrates that the pressure-induced structure change is likely related to the a-axis rather than the c-axis. This is consistent with the observed difference in the a-axis between nonhydrostatic and hydrostatic conditions (Figure 6a). Each layer of S−W−S can slide over another under high pressure.24 This kind of sliding can result in an isostructural phase transition from 2Hc (hexagonal, P63/mmc) to 2Ha (P63/mmc). In the 2Hc structure, the metal atoms occupy the positions at 2c (1/3 2/3 1/4), and the sulfur atoms occupy the positions at 4f (1/3 2/3 z).25 In the 2Ha structure, the metal atoms are at 2b (0 0 1/4) and the sulfur atoms are at 4f (1/3 2/3 z).25 This transition involves change in the interlayer stacking from (CaCAcA) in 2Hc to (AbACbC) in 2Ha.25 The z values used in this study are taken from ref 26. A similar transition has also been observed in MoS2.9,10,26 Structurally, the 2Ha structure is less anisotropic than the initial 2Hc. It has a smaller interlayer spacing and larger in-plane interatomic spacing in comparison to the initial 2Hc phase.27 Furthermore, ref 27 mentions that

the c/a ratio in nonhydrostatic conditions gradually decreases over the entire pressure range, whereas that in hydrostatic conditions initially decreases at low pressures below 10−15 GPa and then stays unchanged at higher pressures. Therefore, we attribute the observed diffraction changes to a subtle structural transition occurring over a broad pressure range between ∼20 and 45 GPa. The sluggish nature of the transition and the large collapse of the c-axis are in support of a sheardriven, displacive phase transition. The pressure-dependent structure change is illustrated in Figure 6 in terms of (a) lattice parameters and (b) the pressure−volume compression curve. The cell parameters from the present study are in agreement with those of ref 13. Note that the c-axis is substantially more compressible than the a- or b-axis. This anisotropic compression (also shown in the lower panel of Figure 6a), in turn, results in a rapid decrease in interlayer S···S distances with respect to intralayer W−S and S− S distances and thereby a rapid increase of interlayer S···S repulsion with pressure. Therefore, we attribute this anisotropic compression to be responsible for an isostructural phase transition from hexagonal 2Hc phase to another hexagonal 2Ha phase, as observed previously in MoS2.9,10,26 In the inset of Figure 6b, the 2Hc and 2Ha structures are presented in the abplane. Note that, in the 2Ha structure, sulfur atoms in two nearby trigonal prismatic sheets are staggered along the c-axis, minimizing the interlayer S···S repulsion. This is in contrast to sulfur atoms facing the trigonal face centers in the 2Hc structure. Evidence for a subtle structure change in WS2 can also be found in the pressure−volume compression curve in Figure 6b. The calculated volumes (red line) are based on the Birch− Murnaghan equation of state (BM-EOS) fit to the measured ones. The BM-EOS fit gives B0 = 69 ± 2.0 GPa (B0′ = 6.5 ± 0.1), which is almost comparable to the values obtained by Bandaru et al. of B0 = 63 ± 1 GPa (B0′ = 6.5 ± 0.1)13 and Selvi et al. of B0 = 61 ± 1 GPa (B0′ = 9.0 ± 0.3).2 It is important to note that the measured volumes above ∼42 GPa deviate from the calculated EOS fit, suggesting a subtle structure change. 5105

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large pressure range between 15 and 45 GPa and is driven by the presence of strong shears developed in the layer structure of WS2 in nonhydrostatic conditions. We conjecture that the structural distortion to the 2Ha structure may give rise to a magnetic ordering transition at low temperatures near the metallization.

the demise of the 2Hc-MoS2 phase is signaled by the disappearance of (105),27 which is consistent with the present X-ray data for WS2. This further confirms that WS2 goes from 2Hc to 2Ha phase upon compression. In the P63/mmc structure, tungsten ions (d2) are coordinated with six sulfur atoms in a trigonal prismatic coordination (D3h), which splits the 5d-subshells of tungsten and causes a large energy gap between fully occupied dz2 and empty degenerated dxy and dx2−y2.28 Upon compression, this gap is expected to decrease as roughly proportional to the electron kinetic energy, ∼ρ2/3. Furthermore, upon anisotropic compression, the gap decreases even faster, as sulfur 3pz orbitals hybridize with tungsten dxy and dx2−y2 orbitals more effectively than the dz2. Therefore, it is conceivable that the observed structural distortion in the ab-plane to the 2Ha phase brings these states nearby or even pushes dxy and dx2−y2 below dz2, giving rise to a paramagnetic state. The anomalous resistivity change observed at ∼100 K at 37 GPa (Figure 3, inset) may then be understood in terms of a magnetic ordering transition. The excitation of electrons occurs from this filled dz2 orbital to the empty dxy or dx2−y2 orbitals. Theoretical band structure calculations29,30 suggest that the conduction band is determined primarily by the transition metal, and the valence band is determined by both the transition metal and the chalcogenide. Upon compression, the separation between this band decreases and the charge carriers of sulfur 3pz orbitals from the valence band increase. As a result, resistance decreases with increasing pressure. This transition is similar to the semiconductor-tosemimetal transition observed in WSe215 at 38.1 GPa and the semiconductor-to-metal transition in MoS2 at around 20 GPa.10 In the semimetallic transition for WSe2 (ref 15), the transition was attributed to W−Se covalent bonding in the layer rather than van der Waals bonding between the layers. This same mechanism can be held true for the transition in WS2 as well. The increased mixing of W 5d and S 3p can result in an enhanced orbital hybridization in the layer, leading to the observed metallic conductivity in WS2. The same transition to metallic MoS2 occurs at a considerably lower pressure compared to that of WS2. This can be a result of the lower ionization potential of Mo than that of W. The lower potential makes it easier for Mo to pass on charges, resulting in the lower transition pressure.9 Substantially reduced Raman intensities above 25 GPa in Figure 2a are likely due to the metallic nature of WS2. In the case of multilayered WS2, the semiconductor-to-metal transition was observed at 22 GPa with a 4 order of magnitude decrease in resistivity,14 which is similar to the resistance trend observed in the present study. This semiconductor-to-metal transition in multilayered WS2 seems to occur at a substantially lower pressure than what we observe in bulk WS2 (∼37 GPa). This difference could be due to the fact that the pressure at which metallization occurs decreases with an increase in the number of WS2 layers.14 In comparison, multilayer MoS2 metallizes at 19 GPa,9 whereas bulk MoS2 metallizes above 28 GPa.10 The lower metallization pressures in MoS2 than those in WS2 may be understood in terms of a larger crystal field effect in W than in Mo and a better hybridization of 3p orbitals of S with 4d orbitals of Mo than with 5d orbitals of W.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (509) 335-2712. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present study has been performed with the support of NSF-DMR (Grant 1203834) and DTRA (HDTRA1-12-010020). The X-ray work was performed at the HPCAT in support of CDAC. HPCAT operates under the support of DOE-NNSA (DE-NA0001974) and DOE-BES (DE-FG0299ER45775). We thank Dr. Minseob Kim for his assistance in synchrotron X-ray diffraction experiments at the APS.

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REFERENCES

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CONCLUSION We have presented the pressure-induced metallization in WS2 at ∼37 GPa, following an isostructural 2Hc to 2Ha phase transition. The isostructural transition occurs displacively over a 5106

DOI: 10.1021/acs.jpcc.5b10759 J. Phys. Chem. C 2016, 120, 5101−5107

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DOI: 10.1021/acs.jpcc.5b10759 J. Phys. Chem. C 2016, 120, 5101−5107