Pressure-induced Td to 1 T ′ structural phase transition in WTe 2

WTe2 is provoking immense interest owing to its extraordinary properties, such as large positive magnetoresistance, pressure-driven superconductivity and possible type-II Weyl semimetal state. Here we report results of high-pressure synchrotron X-ray diffraction (XRD), Raman and electrical transport measurements on WTe2. Both the XRD and Raman results reveal a structural transition upon compression, starting at 6.0 GPa and completing above 15.5 GPa. We have determined that the high-pressure lattice symmetry is monoclinic 1T′ with space group of P21/m. This transition is related to a lateral sliding of adjacent Te-W-Te layers and results in a collapse of the unit cell volume by ∼20.5%. The structural transition also casts a pressure range with the broadened superconducting transition, where the zero resistance disappears.

(Received 24 February 2016; accepted 6 July 2016; published online 13 July 2016) WTe 2 is provoking immense interest owing to its extraordinary properties, such as large positive magnetoresistance, pressure-driven superconductivity and possible type-II Weyl semimetal state.Here we report results of high-pressure synchrotron X-ray diffraction (XRD), Raman and electrical transport measurements on WTe 2 .Both the XRD and Raman results reveal a structural transition upon compression, starting at 6.0 GPa and completing above 15.5 GPa.We have determined that the high-pressure lattice symmetry is monoclinic 1T ′ with space group of P2 1 /m.This transition is related to a lateral sliding of adjacent Te-W-Te layers and results in a collapse of the unit cell volume by ∼20.5%.The structural transition also casts a pressure range with the broadened superconducting transition, where the zero resistance disappears.C 2016 Author(s).All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).[http://dx.doi.org/10.1063/1.4959026]2][3] With typical layered structure resembling graphite, the MX 2 compounds can crystallize into various phases including 2H, 1T, 1T ′ and Td.Accordingly, their physical properties are not only related to the sample thickness or dimensions but also determined by their crystalline structures.The well-studied hexagonal 2H phase exhibits a transition from an indirect band gap to a direct one with thickness reduction down to monolayer, which can be applied in optoelectronic devices.The two-dimensional (2D) monoclinic (distorted octahedral or 1T ′ ) phase has been proposed to be a class of large-gap Quantum Spin Hall (QSH) insulators, which hosts the low-dissipation edge transport. 4Moreover, the 2D MX 2 materials possess the degrees of freedom of layer pseudospin and valley pseudospin, thus appealing the potential applications in spintronics and valleytronics devices. 3mong the TMD family, WTe 2 has triggered great interest due to the recent experimental discovery of non-saturating extremely large positive magnetoresistance (XMR). 5At ambient conditions, it forms the Td (orthorhombic) phase with perfectly balanced electron-hole populations. 68][9] Substantially different Weyl nodes from TaAs family are simulated, on which the recent progress has been achieved with the observation of the topological Fermi arc. 10 In addition, superconductivity appears in WTe 2 when high pressure is applied, which is accompanied by the dramatically suppressed MR. 11,12 The ab initio calculations have suggested that the ambient orthorhombic (Td) phase will transform into the hexagonal (2H) phase at around 10 GPa, 13 while in situ high-pressure synchrotron X-ray diffraction measurements have shown no structural phase transition up to 20.1 GPa. 12 Since the XMR and topological properties in WTe 2 is strongly related with its unique Td structure, the clarification of pressure-dependent structural evolution will be crucial for understanding the exotic properties of WTe 2 system and may shed light on the relation between superconductivity and topological electronic states.
In this letter, we aim to address the above issues in WTe 2 by combining synchrotron X-ray diffraction and polarized Raman scattering measurements.We have obtained clear evidences for a pressure-driven Td to 1T ′ structural transition.The transition is related to a shifting of adjacent Te-W-Te layers.In addition, the superconducting temperature broadens violently and zero resistance disappears in the intermediate pressure region.The zero resistance conductivity recovers when the structural transition is completed.
Details about the sample preparation procedures and characterizations can be found elsewhere. 11High-pressure synchrotron X-ray diffraction (XRD) were performed at 16-BM-D, HP-CAT 14 of APS, Argonne National Lab using the angle-dispersive XRD mode (λ = 0.4246 Å).Raman spectra were collected using 532 nm solid-state laser, in both z(x x) z and z( y y) z configurations.Here, the z direction corresponds to the c axis perpendicular to the W-Te plane and the x and y directions align with the axes a and b, respectively.Resistance measurements were performed by employing the standard four-terminal method in a Be-Cu cell.Daphne 7373 was used as pressure-transmitting media in all the above experiments.Pressure was calibrated via the ruby fluorescence method at room temperature. 15t ambient conditions, WTe 2 crystallizes in a orthorhombic lattice with the space group of Pnm2 1 (No.31). 16Tungsten atoms are coordinated by tellurium atoms in an octahedral environment.The unit cell contains two Te-W-Te sheets, with one sheet rotating 180 • with respect to the other.This stacking sequence is referred to as Td-WTe 2 .In situ high-pressure synchrotron X-ray diffraction (XRD) measurements were performed with pressure up to 33.8 GPa and the XRD profiles are displayed in Fig. 1.Upon compression, all the peaks shift to higher angles, indicative of a shrinkage of the Td-WTe 2 lattice.When the pressure comes to 6.0 GPa, a new characteristic peak appears at ∼12.5 • .At 18.2 GPa, in addition to the complete vanishing of the peak at about 4.6 • , the relative intensities of some peaks have been changed dramatically (red down arrows).With continuous compression to 33.8 GPa, the diffraction patterns remain unchanged except for a progressive right shift of all the peaks.We have carried out the standard Rietveld refinement using the GSAS program 17 and found that both the low-and high-pressure patterns, indeed, can be described by the single Td and 1T ′ (Space group of P2 1 /m, No. 11) phases, respectively.In the pressure range of 6.0-15.5 GPa, however, the XRD profiles contained mixture of the low-and high-pressure phases.When decompressing back to 0.41 GPa, we observed that the XRD pattern came back to the starting structure, evidencing that the pressure-driven structural transition is reversible.
The detailed lattice parameters as a function of pressure are presented in Fig. 2(a).In the Td phase, while the parameter a decreases rapidly, the lattice parameters b and c evolve almost linearly with increasing pressure.These behaviors reflect an anisotropic characteristic of the Td phase, which can be related to the weak inter-layer van der Waals force and strong intra-layer chemical bonding.In the 1T ′ phase, the pressure-dependent lattice parameters seem to be similar to the behavior of the low-pressure phase.The volume-pressure relationship in both phases are fitted by the third-order Birch-Murnaghan equation of states 18 as shown in Fig. 2(b).The fitting results give the ambient-pressure bulk modulus of B 0 = 56(4) GPa with its first pressure derivative B 0 ′ = 6.5 for the low-pressure phase, and B 0 = 83(9) GPa with B 0 ′ = 6.2 for the high-pressure one.The structural transition from Td to 1T ′ leads to a contraction of the unit cell volume of about 20.5% at 11.0 GPa, owing to the WTe 2 layer sliding as illustrated in the inset of Fig. 2 phenomena have also been observed in the zinc sulphide with the volume decrease by 21% and zinc selenide by 28% over the pressure-driven structural transitions. 19here are two points should be mentioned.First, Kang et al. conducted a pressure-dependent synchrotron XRD in WTe 2 at a wavelength of 0.6199 Å and to a pressure of 20.1 GPa and claimed no structural transition. 12Here, by using the synchrotron XRD with a smaller wavelength of 0.4246 Å and therefore in an enlarged 2θ range as well as extending highest pressure up to 33.8 GPa, we observed a pressure-induced Td to 1T ′ structural transition in WTe 2 .We found that the high-pressure 1T ′ phase sets in initially at 6.0 GPa and completes above 18.2 GPa.The structural transition involves only a slight layer sliding, resulting in an extreme similarity in XRD patterns between the two phases and thus is hardly distinguished experimentally coupled with peak broadening effect due to pressure gradient.These factors may explain why the structural transition was not perceived in the previous report.Second, a recent theoretical structural study in WTe 2 was conducted and predicted a Td to 2H structural transition at about 10 GPa. 13 However, our high-pressure XRD patterns cannot be fitted by the suggested 2H structure at all.Moreover, our high-pressure Raman data does not agree with the calculated one of the 2H phase.As a result, the possibility of 2H phase for WTe 2 at pressures up to 33.8 GPa can be ruled out.

(b). Similar
Raman spectroscopy is an effective and powerful tool in detecting small changes over the structural transition.In the following, we present a detailed pressure related Raman study to support our conclusions from the above XRD data.Figure 3 shows representative Raman spectra of WTe 2 single crystal recorded at room temperature and pressures up to 24.6 GPa.1][22] The two peaks can be assigned to the A 5 1 and A 2 1 modes, respectively.The relative intensity of the two peaks differs for z(x x) z and z( y y) z configurations, which is attributed to the two-fold symmetry of the ab plane of this compound. 20Upon compression, we observed blue-shift of both modes due to the hydrostatic compression of the crystal lattice.However, both peaks display an abnormal red-shift in addition to a pronounced reduction in intensity above 6.0 GPa.Meanwhile, the peaks show an asymmetric feature and the A 2 1 mode even splits into two peaks above ∼7.1 GPa, indicative of a structural transition.
Figure 4 displays the pressure dependence of peak positions in both z(x x) z and z( y y) z configurations, which are fitted by one-or two-component Lorentz function.In the low-pressure Td phase region, the A 5 1 mode shows a linear pressure dependence while the A 2 1 mode manifests a nonlinear behavior.The A 5 1 mode is associated with vibrations along the a axis while the A 2 1 mode corresponds to complex vibrations in the bc plane (see the inset of Fig. 4).The anisotropic features of A 2 1 and A 5 1 under pressure are closely related to the evolution of lattice parameters c and a as shown in Fig. 2(a).Upon further compression, all modes develop almost linearly.It should be noted that if the high-pressure phase belonged to a theoretically predicted hexagonal (2H) structure, 13 then the characteristic Raman modes would be 135 and 210 cm −1 at 20 GPa.Obviously, the predicted mode frequencies do not agree with our experimental observations (209.4 and 242.9 cm −1 at 19.0 GPa).
In order to examine the effect of structural phase transition on the electrical properties, we further performed the high-pressure resistance measurements by using Daphne 7373 oil as the pressure-transmitting medium.As shown in Fig. 5, the superconductivity appears in WTe 2 at 4.0 GPa and zero resistance is observed around 3 K.With increasing pressure, the critical temperature (T c onset ) increases monotonically, however a long tail with non-zero resistance emerges upon cooling till 1.8 K at both 7.2 and 10.8 GPa.Zero resistance re-emerges when the pressure is further phases are superconducting while the 2H phase does not show any signature of superconductivity under pressure up to 40 GPa. 23n conclusion, by combining the high-pressure synchrotron XRD and Raman spectroscopy as well as the electrical transport investigations in WTe 2 , we found that a pressure-driven structural transition starts at around 6.0 GPa and completes above 15.5 GPa.The determined high-pressure structure was the so-called 1T ′ phase via the layer sliding of the low-pressure Td phase.On the other hand, the zero resistance residing outside the structural transition zone indicated that both the Td and 1T ′ phases are superconducting in WTe 2 .

FIG. 1 .
FIG. 1. Synchrotron X-ray diffraction patterns of WTe 2 during the compression (denoted by c) and decompression (by d) runs (λ = 0.4246 Å).Numbers represent pressures in unit of GPa.Arrows indicate anomalies which appear at the phase evolution.The patterns shown in blue correspond to the mixed phase region.The observed and Rietveld refined profiles at 0.9 and 28.8 GPa are shown for the Td (Pnm2 1 , No. 31) and 1T ′ (P2 1 /m, No. 11) phases.The open circles and red lines are the experimental and calculated data.The positions of the Bragg reflections are marked by vertical sticks.The refined values are R P = 2.00%, weighted profile R WP = 1.49% at 0.9 GPa and R P = 0.77%, weighted profile R WP = 0.97% at 28.8 GPa, respectively.

FIG. 2 .
FIG. 2. (a) Pressure dependences of the lattice parameters of WTe 2 .(b) Volume per formula unit as a function of pressure.The solid lines demonstrate the fitting data with respect to the third-order Birch-Murnaghan equation of states of Td (Pnm2 1 ) and 1T ′ (P2 1 /m) and the corresponding results are also displayed.The insets illustrate the atomic arrangement of the Td and 1T ′ structures (W, blue balls; Te, dark yellow balls).The structural transition due to the layer sliding results in a collapse of the unit cell per formula in about 20.5% at 11.0 GPa.

FIG. 3 .
FIG. 3. Room temperature Raman spectra of WTe 2 at different pressures in the (a) z(x x) z and (b) z(y y) z configurations, respectively.Here only the spectrum at 24.6 GPa is shown, which is fitted by two-component Lorentz function, as indicated by the color solid lines and arrows.

FIG. 4 .
FIG. 4. Pressure dependence of the frequencies of the Raman peaks in WTe 2 .Solid lines are guided to the eyes.