Pressure-induced iso-structural phase transition and metallization in WSe2

We present in situ high-pressure synchrotron X-ray diffraction (XRD) and Raman spectroscopy study, and electrical transport measurement of single crystal WSe2 in diamond anvil cells with pressures up to 54.0–62.8 GPa. The XRD and Raman results show that the phase undergoes a pressure-induced iso-structural transition via layer sliding, beginning at 28.5 GPa and not being completed up to around 60 GPa. The Raman data also reveals a dominant role of the in-plane strain over the out-of plane compression in helping achieve the transition. Consistently, the electrical transport experiments down to 1.8 K reveals a pressure-induced metallization for WSe2 through a broad pressure range of 28.2–61.7 GPa, where a mixed semiconducting and metallic feature is observed due to the coexisting low- and high-pressure structures.

The pressure-induced metallization may not be always accompanied by a structural transition. In MoSe 2 , a pressure-induced metallization is observed, without any sign of structural change. The absence of structural transition in MoSe 2 is empirically related to the relatively delocalized 4p orbitals of Se 2− than the 3p orbitals of S 2− , which results in stronger interlayer interactions to prevent the sliding process and thus the structural transition 20 .
To date, the high-pressure properties of WSe 2 have not been well understood due to the lack of sufficient experimental data. Liu et al. found a slope change of electrical resistivity at ~38.1 GPa for WSe 2 , but the data was taken only in a temperature range of 290-420 K 21 . Based on theoretical calculations, this resistivity anomaly is related to a semiconductor to semimetal transition 21 . An early XRD study showed that the pristine 2H c structure of WSe 2 was stable up to 35.8 GPa 22 . Later, a Raman investigation confirmed no structural anomaly up to 20 GPa 23 . Recently, a first-principles study revealed an anomaly at 40 GPa consistently in the bulk modulus, Young's modulus and Poisson's ratio of WSe 2 , which may imply a possible structural transition 21,24 . More experimental efforts are therefore necessary to gain comprehensive knowledge of the high-pressure behavior of WSe 2 .
In the present study, we employ combined in situ synchrotron XRD and Raman spectroscopy, and electrical transport measurements to study the structural and electronic properties of single crystal WSe 2 . Both the XRD and the Raman data document an iso-structural transition. The resistivity measurements down to 1.8 K indicate that a pressure-induced metallization occurs in a broad pressure range, due to the coexistence of the low-pressure and high-pressure structural phases.

Results and Discussion
High-quality plate-like single crystals (inset of Fig. 1(b)) with typical dimensions of 2 mm × 1 mm × 0.2 mm were grown by chemical vapor transport method 25,26 . The crystal structure was characterized by single crystalline XRD ( Fig. 1(b)), which shows a (00l) preferable orientation (defined as c axis). At ambient conditions, WSe 2 crystallizes in a hexagonal lattice with space group of P6 3 /mmc 27 , referred to as 2H c -WSe 2 . To enrich the knowledge of the high-pressure structural symmetry than a previous study to 35.8 GPa 22 , we carried out in situ synchrotron XRD measurements by extending the pressure up to 62.8 GPa at room temperature. Selected experimental patterns are displayed in Fig. 1(c). At 1.8 GPa, the XRD profile is well reproduced by the 2H c structure (see the bottom of Fig. 1(c)). Upon compression, there seems no evident anomaly throughout the whole pressure range studied except for a consistent broadening of all peaks above 19.1 GPa, which is in good agreement with a previous report to 35.8 GPa 22 . Assuming that the pristine 2H c structure of WSe 2 exists up to 62.8 GPa, we calculated the normalized cell parameters a/a 0 and c/c 0 as well as the volume V as a function of pressure. The results are displayed in Fig. 2. For comparison, previous theoretical and experimental structural data 22,24 are also included. It is observed that, below 20 GPa (marked as P C1 ), all data agrees nicely with each other. Above P C1 , however, a deviation starts to appear. The deviation above P C1 between the present and previous data may be related to a structural transition since all of these XRD data was fitted only based on a 2H c structural model, without considering the possibility of a structural transition induced by pressure. In addition, here the broadening of all XRD peaks above 19.1 GPa may be a manifestation of this pressure-induced structural anomaly. We fitted the pressure-volume data below P C1 using the third-order Birch-Murnaghan equation of state 28 (as shown by a gray zone in Fig. 2(c)). The bulk modulus is estimated to be B 0 = 72(5) GPa and its first pressure derivative ′ B 0 = 4.6(5). To further check whether there is a possible structural transition, we plotted the axis ratio (c/a), the Se-W-Se bond angle (θ) and the W-Se bond length (d W − Se ) as a function of pressure (as illustrated in Fig. 1(a)). From Fig. 3(a), c/a shows a nonlinear decrease during the initial compression, indicating that the cell parameter c is much more compressible than a. This can be related to the weaker vdW interlayer forces relative to the stronger intralayer covalent chemical bonding 29 . Above P C1 , the change rate of c/a varies drastically, following a roughly linear decrease with pressure, which suggests that the cell parameters a and c have nearly-isotropic contractions. Further compression to about 40 GPa (marked as P C2 ), the change rate of c/a shows a small uprise and then decreases almost linearly up to the highest pressure. At 62.8 GPa, a and c are reduced by 7.5% ( Fig. 2(a)) and 13.4% ( Fig. 2(b)), respectively, which clearly evidences an anisotropic compression. Meanwhile, both the θ and d W − Se also display two anomalies with increasing pressure, namely, changes of slope at P C1 and P C2 (Fig. 3(b) and (c)). In addition to the above mentioned broadening of peaks ( Fig. 1(c)) and deviation behavior ( Fig. 2(c)), the consistent discontinuities in c/a, θ and d W − Se at P C1 and P C2 should imply structural changes. It is noted that similar discontinuities in c/a or volume were also observed in high pressure MoS 2 , which was related to an iso-structural transition from 2H c to 2H a 15 . The iso-structural transition involves only a lateral sliding of adjacent layers and the change is subtle, probably making it not easy to decipher the structural changes directly from the raw XRD data.
Raman spectroscopy is a sensitive and effective technique in detecting small changes in the structural transition. We carried out a detailed Raman study with pressures up to 57.2 GPa. Selected Raman spectra during compression and decompression are displayed in  is increasing, while the peak intensity of A 1g shows only a small variation, the peak intensity of the E g 2 1 mode increases significantly. Going up from 28.5 GPa, the peak intensity of E g 2 1 reduces rapidly and the E g 2 1 mode begins to split into two peaks. Subsequently, a splitting of A 1g mode starts to appear at 35.8 GPa. The splitting of the E g 2 1 and A 1g Raman modes for WSe 2 under pressure is also reproduced in another run (see supplementary materials for details). Both the change of peak intensity and splitting of peaks give strong evidences of a pressure-induced iso-structural transition like in MoS 2 16 . Further increasing the pressure up to the highest pressure of 57.2 GPa, the splittings of E g 2 1 and A 1g can still be clearly observed, indicating an incompleteness of the structural phase transition and coexistence of the low-and high-pressure structural phases up to this pressure. In addition, we noted that the intensity of both Raman peaks from the high-pressure phase (red triangles, Fig. 4(a)) becomes dominant over those from the low-pressure phase (blue solid dots, Fig. 4(a)) above around 40 GPa, in excellent agreement with the critical pressure P C2 observed in our above XRD results (Fig. 3). When the pressure is released, the Raman spectra first show a splitting in the intermediate pressure ( Fig. 4(b)) and then came back (0.2 GPa) to the starting positions at 0.3 GPa (Fig. 4(c)), manifesting the reversible structural transition.
The pressure dependence of the frequencies of the E g 2 1 and A 1g modes are shown in Fig. 5. Upon compression, both modes show almost linear behaviors in the 2H c phase, in roughly accordance with previous results to 20 GPa 23 . The slope of the E g 2 1 and A 1g mode is 0.72 cm −1 GPa −1 and 1.52 cm −1 GPa −1 , respectively. As is known, the symmetry of WSe 2 lattice can be described by the D h 6 4 space group, and the irreducible representation of phonon modes at the center of the Brillouin zone is Γ = A 1g + 2 A 2u + B 1u + 2B 2g + E 1g + 2E 1u + E 2u + 2E 2g , where the A and B modes are out-of-plane lattice vibrations, while the E modes are in-plane 32 . The E g 2 1 mode is from the vibrations involved in both W and Se atoms in the basal plane, with the opposite directions from each other. The A 1g mode refers only to the vibrations of the Se atoms along the c-axis (see the inset of Fig. 5). Consequently, the smaller slope of the E g 2 1 mode than the A 1g mode can be understood in terms of the anisotropic compressibility as the c-axis decreases much faster than the a-axis (see Fig. 3(a)). For the 2H a phase, both modes increase linearly again but the slope of the E g  Upon approaching to the structural transition, the peak intensity of the A 1g mode increases slightly as can be seen from Figs 4 and 5 (color scale). Meanwhile, the intensity of E g 2 1 increases violently and then decreases rapidly at P C1 above which the iso-structural transition occurs. Outside the transition region, the peak intensity of both modes varies gradually with pressure. In addition, the slope of A 1g variation remains constant before and after the structural transition but the slope of E g 2 1 variation becomes larger from the 2H c (0.72 cm −1 GPa −1 ) to 2H a (0.89 cm −1 GPa −1 ). These observations imply that, while the out-of-plane compression is involved in the iso-structural transition due to anisotropic compressibility, the in-plane compression (strain) plays a dominant role in triggering the transition via layer sliding. This conclusion may explain the relatively low transition pressure (P C1 ~ 28.5 GPa) here compared with the previous theoretical calculations by only taking free mutual sliding of layers into consideration (~40 GPa) 21,24 .
We also performed in situ high-pressure resistivity (ρ) measurements for WSe 2 in a diamond anvil cell up to 61.7 GPa. Standard four-point probe method was used. In order to get a more comprehensive understanding on the electrical properties than a previous investigation (290-420 K) 21 , we extended the temperature range down to 1.8 K. The results are displayed in Fig. 6(a). Roughly, two regimes can be identified upon compression: firstly, below 19.1 GPa, a typical semiconductor behavior (dρ/dT < 0) is observed; secondly, from 28.2 GPa to the highest pressure of 61.7 GPa investigated in this experiment, the resistivity first shows a plateau between 100 K and 200 K and then a metallic-like behavior (dρ/dT > 0), which becomes increasingly dominant. Figure 6(b)-(d) display the representative ρ-T curves for each phase. Our results clearly reveal a pressure-induced metallization for WSe 2 . In addition, the resistivity in the intermediate phase can be considered as a superposition of the low-pressure semiconducting and high-pressure metallic behaviors. These characteristics can be related to a coexistence of the low-pressure 2H c and high-pressure 2H a structures, in good agreement with the above XRD and Raman data.

Conclusions
In summary, we have investigated the structural and electrical properties on single crystal WSe 2 . In spite of the lack of evident changes in the XRD patterns, the pressure dependent c/a, θ and d W − Se consistently reveal two anomalies at P C1 ~ 20 GPa and P C2 ~ 40 GPa. The Raman active E g 2 1 and A 1g modes split into two peaks, beginning at 28.5 GPa and 35.8 GPa, respectively, and lasting to the highest pressure of 57.2 GPa. These results strongly evidence that there exists an iso-structural transition for high-pressure WSe 2 . The distinctly different trend of peak intensity between E g 2 1 and A 1g with pressure suggests that the in-plane strain plays a dominant role in triggering the transition over the out-of-plane compression. Consistently, the resistivity measurements show a pressure-induced metallization, through a broad pressure range of 28.2-61.7 GPa, where the resistivity feature can be treated as a superposition of the low-pressure semiconducting and high-pressure metallic behaviors.

Methods
WSe 2 single crystals were grown via chemical vapour transport technique. High purity powders of tungsten (99%, Alfa Aesar) and selenium (99.99%, Alfa Aesar) with chemical stoichiometry were ground and sealed in an evacuated quartz ampoule (< 10 −4 -torr vacuums). First, the components were made to react at 600 °C for 3 days. The loose sample was then re-ground and sealed in an evacuated quartz ampoule to get polycrystalline WSe 2 samples. Second, an ampoule containing some polycrystalline WSe 2 was placed in a two-zone furnace. The hot zone was maintained at 1060 °C and the cold zone was maintained at 1000 °C for 14 days. All weighing and mixing was carried out in a glove box. The crystal structure was characterized by room temperature XRD with Cu X-ray radiation (K α = 1.5418 Å).
In situ high pressure synchrotron radiation XRD experiments were performed at 16-BM-D, HPCAT 33 of Advanced Photon Source of Argonne National Laboratory using a Mao-Bell symmetric diamond anvil cell (DAC) with a culet of 300 μm diameter. The as-grown single crystals were ground into fine powder for XRD experiments. A focused monochromatic X-ray beam (4.3 μm (v) × 3.6 μm (h) in FWHM) with a wavelength of 0.4246 Å was used for the X-ray diffraction experiment. The T301 stainless-steel gasket was pre-indented from a thickness of 300 μm to ~35 μm, and a center hole at 140 μm in diameter was drilled to serve as the sample chamber. A pre-pressed powder sheet with a typical size of 30 μm × 30 μm × 5 μm was loaded into the chamber together with a ruby ball 34 and Daphne 7373 oil as pressure marker and pressure transmitting medium, respectively. The XRD patterns were collected with a Mar345 image plate detector. Refinements of the measured XRD patterns were performed by the GSAS software 35 . Raman scattering experiments were carried out at the Center for High Pressure Science and Technology Advanced Research (HPSTAR) in Shanghai. The Raman spectrum measurement was performed using a commercial Renishaw Raman spectroscopy system with a 532 nm laser excitation line. The Raman system was first calibrated by using single crystal silicon with the characteristic peak at 520 cm −1 and the uncertainty is within 0.5 cm −1 . The focused laser beam was about ~5 μm. The laser power was 50% and the collection time was 100 s. Mao-Bell type DAC was used to generate the pressure. The measurements were conducted on a freshly cleaved single crystal (dimensions of 30 μm × 25 μm × 10 μm). Daphne 7373 oil was used as the pressure medium. The diamond culet was 300 μm in diameter. A rhenium gasket was pre-indented to a thickness of 29 μm, and a center hole at 124 μm in diameter was drilled. The pressure was determined by the ruby fluorescence scale below 20 GPa 34 and the diamond Raman scale above 20 GPa 36 .
The pressure-dependent electrical transport experiments were performed by employing the standard four-probe method in a nonmagnetic Be-Cu cell. The diamond anvils is 300 μm culets. A T301 stainless-steel gasket covered with a mixture of epoxy and fine cubic boron nitride (c-BN) powder was used for high-pressure transport measurements. Pt-foil (5 μm) was used as the electrical lead. Standard four-probe method was applied in the ab plane of single crystals with typical dimensions of 90 × 40 × 10 μm 3 . NaCl powder was used as the pressure transmitting medium. Pressure was calibrated with the ruby fluorescence shift at room temperature 34 .