Structural and Transport Properties of 1T-VSe2 Single Crystal Under High Pressures

Two-dimensional transition metal dichalcogenide 1T-VSe₂ exhibits a unique three-dimensional charge density wave (CDW) order below ∼110 K at ambient pressure, which shows unusual evolution under pressure. Here we report on the high-pressure structural and transport properties of 1T-VSe₂ by extending the pressure up to 57.8 GPa, through electrical transport, synchrotron X-ray diffraction (XRD) and Raman scattering measurements, which unravel two critical pressure points. The CDW transition is found to be enhanced under compression at a rate of 16.5 K/GPa up to the first critical pressure P_C1 ∼ 12 GPa, at which a structural phase transition from hexagonal P-3m1 to monoclinic C2/m phase takes place. The second critical pressure P_C2 ∼ 33 GPa corresponds to another structural transition from monoclinic C2/m to P2₁/m phase. These findings extend the phase diagram of pressurized 1T-VSe₂ and may help to understand pressure tuning of structures in transition metal dichalcogenides.

Introduction

Two-dimensional transition metal dichalcogenides (TMDs) exhibit rich physical properties, which have been a hot research field in condensed matter physics for both fundamental interests and potential applications in electronics and optoelectronics (Sipos et al., 2008; Choi et al., 2013; Nayak et al., 2014; Novoselov et al., 2016). Among them, 1T-VSe₂ has attracted extensive interests due to its unique three-dimensional (3D) charge density wave (CDW) order (Tsutsumi, 1982; Terashima et al., 2003; Strocov et al., 2012; Jolie et al., 2019), intrinsic photocatalytic (He et al., 2017) and photoluminescence properties (Ghobadi et al., 2018). Bulk 1T-VSe₂ has a hexagonal layered structure, namely 1T (P-3m1) phase. The adjacent Se-V-Se sandwiches are held together by van der Waals interactions, and each V atom is surrounding by the nearest six Se atoms, constituting VSe₆ octahedron Figure 1A (van Bruggen and Haas, 1976). It exhibits an incommensurate CDW with 4a × 4a × 3.18c periodic lattice distortion upon cooling to around 110 K, followed by a second transition to a commensurate CDW state at ∼ 80 K (Thompson and Silbernagel, 1979; Tsutsumi, 1982; Eaglesham et al., 1986; Terashima et al., 2003; Pandey and Soni, 2020). The mechanism of the 3D CDW was initially explained by Fermi-surface nesting resulting from the warped 3D electron pocket centered around the M point at the edge of the Brillouin zone (Terashima et al., 2003; Sato et al., 2004; Strocov et al., 2012). Recently, electron-phonon coupling was also found to contribute to the formation of the CDW (Pandey and Soni, 2020; Si et al., 2020). Previous studies showed that the valence band structure of 1T-VSe₂ can be transformed into two-dimensional (2D) character by alkali metal intercalation, which was attributed to charge transfer and decoupling of the layers induced by the intercalated alkali ions (Starnberg et al., 1993; Brauer et al., 1998). In addition, the CDW ordering temperature can also be tuned effectively by the reduction of sample thickness owing to reduced interlayer coupling and enhanced quantum confinement (Yang et al., 2014; Pásztor et al., 2017). At the monolayer limit, many interesting phenomena have been reported, such as ferromagnetism (Ma et al., 2012; Bonilla et al., 2018), distinct CDW order (Zhang et al., 2017; Feng et al., 2018; Duvjir et al., 2018) as well as Mott/Peierls insulating state (Duvjir et al., 2018; Umemoto et al., 2018), depending on preparation conditions.

FIGURE 1

FIGURE 1. (A) Crystal structure of 1T-VSe₂. (B) Room temperature powder X-ray diffraction pattern. Inset: Single-crystal XRD pattern. (C) Energy dispersive spectrum for single crystalline 1T-VSe₂. (D) Resistance for a single crystal as a function of temperature. Inset: The first derivative of the resistance dR/dT.

Pressure has been considered as a clean and effective method to tune lattice degrees of freedom, and thereby the physical properties of materials (Mao et al., 2018). In 1T-VSe₂, a pressure-induced structural phase transition from 1T phase to monoclinic C2/m phase with a novel superstructure around 15 GPa were reported via theoretical calculations, Raman spectroscopy and synchrotron X-ray diffraction (XRD) (Sereika et al., 2020; Feng et al., 2020). As for the transport properties of 1T-VSe₂ under pressure, controversial results have been reported. Specifically, Sahoo et al. reported that the CDW transition temperature (T_CDW) first increases marginally up to 5 GPa and then increases rapidly, and accompanying by non-hydrostatic pressure-induced suppression of CDW, superconductivity emerges at ∼15 GPa (Sahoo et al., 2020). However, soon after, Feng et al. found that the CDW transition temperature was enhanced linearly by increasing pressure, reaching 358 K at 14.6 GPa and no superconducting transition is observed under quasi-hydrostatic pressure up to 29.6 GPa (Feng et al., 2020). Obviously, the detailed properties of 1T-VSe₂ under pressure remain far from being well explored. In addition, in these reports, the pressures applied in the XRD, Raman, and resistance measurements are below 30–35 GPa (Friend et al., 1978; Sereika et al., 2020; Feng et al., 2020; Sahoo et al., 2020). For pressures above 35 GPa, the evolution of structural and transport properties are still unclear.

In this paper, we synthesize high-quality 1T-VSe₂ single crystals and investigate the evolution of structural and transport properties under pressure up to 57.8 GPa. Resistance measurements show that the T_CDW shifts gradually to higher temperature with increasing pressure up to P_C1 ∼ 12 GPa, at which the hexagonal P-3m1 structure undergoes a transition to monoclinic C2/m phase. And no sign of superconductivity is observed in the pressurized 1T-VSe₂ up to 42 GPa. Moreover, our synchrotron XRD and Raman experiments reveal a second structural transition from the monoclinic C2/m to P2₁/m phase with further increasing pressure to the second critical pressure P_C2 ∼ 33 GPa.

Materials and Methods

Single crystals of 1T-VSe₂ were synthesized by chemical vapor transport method using selenium (about 2 mg/cm³) as transport agent (van Bruggen and Haas, 1976). The mixture of Vanadium (3N) and Selenium (5N) powders was sealed in an evacuated quartz tube. Then the quartz tube was heated for 1 week in a two-zone furnace with a temperature field of ΔT = (850–780)°C. The structure of the single-crystalline samples was analyzed by XRD experiments with Cu-Kα radiation (λ = 1.5418 Å). The atomic ratio was characterized by the energy dispersive X-ray spectrometry. The temperature-dependent resistance was measured using a standard four-point-probe method.

High-pressure resistance measurements were performed in a nonmagnetic Be-Cu diamond anvil cell (Zhou et al., 2019). Diamond anvils of 300 μm culets and a T301 stainless-steel gasket covered with a mixture of epoxy and fine cubic boron nitride (c-BN) powder were used. A hole with diameter of 150 mm was drilled at the center of the c-BN-covered pit, and then filled with pressure-transmitting medium. Daphne 7373 and NaCl were used as pressure-transmitting medium in Run 1 and Run 2, respectively. A rectangular single-crystal sample was loaded into the hole, together with some ruby powder as the pressure marker. Pt foils with a thickness of 7.5 μm were used as the electrode leads. The cell was then put into an in-house multifunctional physical properties measurement system. The resistance R was collected using the standard four-probe method via sweeping temperature.

Synchrotron XRD and Raman scattering measurements under pressure were performed in a Mao-Bell cell at room temperature. Daphne 7373 was used as pressure-transmitting medium for both experiments. The XRD was performed on powder crushed from single crystals at the beamline BL15U1 of the Shanghai Synchrotron Radiation Facility (SSRF). The wavelength of the focused monochromatic X-ray beam is 0.6199 Å. The DIOPTAS program (Clemens and Vitali, 2015) was used for image integrations. The XRD patterns were fitted by using the RIETICA program (Hunter, 1998) with the Le Bail method. Raman spectra were measured with a freshly cleaved 1T-VSe₂ single crystal using the 532-nm solid-state laser. The pressure was calibrated by using the ruby fluorescence shift at room temperature for all of the high-pressure experiments above (Mao et al., 1986).

Results and Discussion

Figure 1B displays the XRD pattern for powdered 1T-VSe₂ single crystals. All peaks can be well indexed by the P-3m1 space group, and the fitting yields the cell parameters a = b = 3.36 Å, c = 6.09 Å, and V = 59.54 ų, in excellent accordance with the previous reports (van Bruggen and Haas, 1976; Pandey and Soni, 2020). A typical single-crystal diffraction pattern for 1T-VSe₂ is shown in the inset of Figure 1B. Only (00l) reflections are observed, indicating a c-axis orientation of the cleavage plane. The energy-dispersive X-ray spectrum gives the ratio of V: Se as 1:2.05 (Figure 1C). The in-plane resistance R as a function of temperature at ambient pressure is given in Figure 1D. The R-T curve shows an overall metallic behavior but with a clear hump feature at around 105 K related to the CDW transition, in agreement with the previous studies (van Bruggen and Haas, 1976; Pandey and Soni, 2020). The CDW transition temperature T_CDW is defined as the temperature at which the dR/dT is minimum (inset of Figure 1D). The characterizations above indicate high quality of our samples.

To check on the pressure evolution of CDW and pressure-induced superconductivity in 1T-VSe₂, we have performed resistance measurements under pressures using different pressure-transmitting mediums. In Run 1 (Figure 2), we used Daphne 7373 as the pressure medium with pressures up to 28.5 GPa. As one can see in Figure 2A, at p = 0.7 GPa, a resistance hump relating to the CDW transition is observed at around 110.7 K, which is higher than that at ambient pressure (Figure 1D). With further increasing pressure, the temperature of the resistance hump rises rapidly and reaches a maximum of 289.3 K at p = 10.4 GPa. This suggests an enhancement of the CDW under pressure, in agreement with the previous reports (Friend et al., 1978; Feng et al., 2020; Sahoo et al., 2020). For pressures higher than 10.4 GPa (Figure 2E), the resistance hump is indiscernible, namely, no sign of CDW is observed in the temperature range of 2–300 K, and the R(T) curve is monotonically shifted upwards.

FIGURE 2

FIGURE 2. High-pressure resistance measurements were performed using Daphne 7373 as the pressure medium (Run 1). (A)–(E) Temperature dependence of resistance of 1T-VSe₂ at various pressures. Insets of (A)–(E): Temperature dependence of dR/dT.

For comparison, we further carried out R(T) measurements using solid pressure medium (NaCl) up to 42.0 GPa in Run 2 (Figure 3), which is referred as non-hydrostatic pressure similar to that in Ref. (Sahoo et al., 2020). With increasing pressure from 0.4 to 10.9 GPa, the resistance hump moves toward higher temperatures, in conformity with that of Run 1, confirming that compression can increase T_CDW in 1T-VSe₂. The evident enhancement of T_CDW could be related to the pressure-enhanced CDW gap and improvement of out-of-plane Fermi surface nesting due to increase of the dimensionality (Feng et al., 2020). Nevertheless, either quasi-hydrostatic or non-hydrostatic compression, no sign of superconductivity is observed from 2 to 300 K measured up to 42.0 GPa in our measurements.

FIGURE 3

FIGURE 3. High-pressure resistance measurements were performed using NaCl as the pressure medium (Run 2). (A)–(G) Temperature dependence of resistance of 1T-VSe₂ at various pressures. Insets of (A)–(F): Temperature dependence of dR/dT.

Next, pressure evolution of crystal structure of 1T-VSe₂ was studied by synchrotron XRD measurements on powdered samples. Typical XRD patterns are shown in Figure 4A and three representative fittings for pressures p = 0.3, 16.9, and 36.2 GPa are presented in Figures 4B–D. Starting at 0.3 GPa, the XRD pattern can be well indexed by hexagonal crystal structure with space group P-3m1 (No. 164) (Figure 4D), which is the same as that of 1T-VSe₂ at ambient pressure. Upon compression, all peaks shift to higher angles due to shrinkage of the lattice. At P_C1∼13.2 GPa, some new diffraction peaks begin to occur at the 2θ angles of 12.6, 14.2, 15.9, and 17.0°, as denoted by arrows in Figure 4A, which indicates a structural transition. Phase II (red) can be fitted with the monoclinic C2/m structure (No. 12) as shown in Figure 4C, which is in accord with the results of Refs. (Feng et al., 2020; Sahoo et al., 2020). The C2/m structure can be considered as a distorted 1T phase, which involves the formation of trimers associated with tiny displacement of V atoms, leading to the formation of a novel 3 × 1 × 1 superstructure (Sereika et al., 2020; Feng et al., 2020). Moreover, when the pressure is increased to P_C2 ∼ 33.7 GPa, an additional peak appears at 15.8°(see the star in Figure 4A); and its intensity increases with pressure increasing, which indicates occurrence of another structural transition. It is well-known that depending on the various stacking sequences of the two-dimension layers and the coordination of transition metal atoms, two-dimensional transition metal dichalcogenides MX₂ can crystallize into different types, such as 1T (P-3m1), 1T′ (C2/m and P2₁/m), 2H (P63/mmc), 3R (R3m), Td (Pmn2) etc (Sofer et al., 2017). Excluding the ambient pressure P-3m1 and the first high-pressure C2/m phases, there are four remaining possible structures. We tried fitting our high-pressure XRD data by these candidate structures and found that the high-pressure XRD patterns above ∼33.7 GPa can be well fitted by the P2₁/m space group (Figure 4B), suggesting that the newly emergent phase III (blue) is a monoclinic structure with space group P2₁/m (No. 11), similar with that in pressurized TaTe₂ (Guo et al., 2017). When the pressure is released to 3.5 GPa (denoted by d, top of Figure 4A), the XRD pattern evolves back to the starting structure of 1T-VSe₂, implying that these two structural transitions are reversible.

FIGURE 4

FIGURE 4. (A) High-pressure XRD patterns for powdered 1T-VSe₂ single crystals under pressures from 0.3 to 56.2 GPa, and then down to 3.5 GPa (“d” means decompression). The diffraction wavelength λ = 0.6199 Å. (B)–(D) Representative fittings of the XRD patterns at 36.2, 16.9, and 0.3 GPa. (E) Pressure dependence of the lattice parameters a, b, and c. (F) Pressure dependence of the lattice volume of a unit cell.

The extracted lattice parameters as a function of pressure are summarized in Figure 4E. The lattice parameters for P-3m1, C2/m, and P2₁/m phases decrease gradually upon compression. The pressure dependence of unit cell volume V/Z is plotted in Figure 4F. The isothermal equations of state (EoS) were fitted by the third-order Birch-Murnaghan formula (Birch, 1947), as indicated by the solid lines. Our fitting yields the zero-pressure volume $V_0$ = 59.5 Å³, bulk modulus $B_0$ = 42.3 GPa, and first-order derivative of the bulk modulus $B{\text{'}}_0$ = 9.3 for P-3m1 phase. With $B{\text{'}}_0$ fixed as 4, the calculated parameters are $V_0$ = 59.2 Å³, $B_0$ = 55.8 GPa for C2/m phase; and $V_0$ = 50.3 Å³, $B_0$ = 133.5 GPa for P2₁/m phase.

It is known that the properties of the CDW transition are closely related to the dimensionality of the material. In Supplementary Figure S1, we plotted the pressure-dependent T_CDW and the axial ratio c/a. It is obvious to see a direct correlation between the increase of T_CDW and decrease of c/a ratio. And the axial ratio c/a of 1T-VSe₂ decreases with pressure, that is, the dimensionality of the system increases (Feng et al., 2020). These results suggest that the enhancement of CDW is related to the inherent dimensionality of 1T-VSe₂.

Raman spectroscopy is also an effective and powerful tool in detecting structural transition. Bulk 1T-VSe₂ single-crystal displays a sharp A_1g mode ∼ 206 cm⁻¹ along with two additional modes associated with E_g ∼ 257 cm⁻¹ symmetry and two-phonon (2_ph) ∼ 332 cm⁻¹ interactions at room temperature and ambient pressure (Pandey and Soni, 2020). Figure 5A shows the room-temperature Raman spectra of 1T-VSe₂ single crystal at selected pressures. At a pressure of 0.9 GPa, the A_1g and E_g modes are detected at ∼ 212 cm⁻¹ and 255 cm⁻¹, respectively, in agreement with previous report (Pandey and Soni, 2020). Since the E_g mode cannot be identified accurately at the higher pressure, we only trace A_1g in this paper. With increasing pressure, the A_1g mode moves toward higher frequencies monotonically. At P_C1∼ 12.4 GPa, along with the structural transition to monoclinic C2/m structure, two new peaks are detected at 166, 202 cm⁻¹ (labeled as M1 and M2, respectively), which is consistent with the previous study (Feng et al., 2020). Considering the continuous evolution of the A_1g mode and the structural phase transition, we labeled the A_1g mode in the range of 12.4∼31.3 GPa as A_1g′mode. With increasing pressure up to 31.3 GPa, all A_1g′, M1, and M2 peaks shift toward high frequency. Upon further compression above P_C2 ∼ 33 GPa, the M2 mode disappears and another peak at 378 cm⁻¹ emerges, which can be attributed to the structural transition from C2/m to P2₁/m detected in the XRD measurements. Similarly, we labeled the M1 and A_1g′ modes above 31.3 GPa as the M1′ and A_1g″ modes, respectively. The remaining one is labeled as M3. The Raman frequencies of M1′, A_1g″, and M3 increase under further compression, and no additional peak is observed, suggesting a stable new high-pressure phase is formed up to 57.8 GPa. We note that the Raman peaks broaden obviously at high pressure, which is due to non-hydrostatic compression associated with the pressure-transmitting medium. With pressure released down to 0.7 GPa, the peaks in the Raman spectrum return to the initial position (denoted by d, at the top of Figure 5A), which means that these two structural transitions are reversible, in accordance with the results of XRD data.

FIGURE 5

FIGURE 5. (A) Room-temperature Raman spectra of 1T-VSe₂ single crystal under selected pressures. The Raman spectrum under decompression is labeled by “d”. The dashed lines are guides for the eye. (B) Pressure dependence of Raman frequencies of 1T-VSe₂. Solid lines are linear fits to data.

The extracted Raman frequencies as a function of pressure are plotted in Figure 5B. With increasing pressure, Raman frequencies increase linearly in each crystal structure, while the corresponding slope is visibly different. The pressure coefficients of Raman modes are fitted by linear lines using Eq. 1 as displayed in Figure 5B. The individual mode Grüneisen parameter ${\gamma }_i$ is calculated using Eq. 2, in which ${\omega }_i$ is ith phonon mode frequency and $B_0$ is the bulk modulus at zero pressure (Sherman, 1980). The corresponding results are listed in Table 1.

${\omega }_P={\omega }_0+\left(\frac{d_{\omega }}{d_p}\right)P(1)$

${\gamma }_i=\frac{B_0}{{\omega }_i}\left(\frac{d_{\omega }}{d_p}\right)(2)$

TABLE 1

TABLE 1. Pressure coefficient (dω/dP) and mode Grüneisen parameter (γ_i) for different Raman modes.

Combining the electrical transport, synchrotron XRD and Raman measurements under pressure, we established the phase diagram which describes the structural and transport properties of 1T-VSe₂ under pressure. As outlined in Figure 6, two critical pressure points can be discerned. With increasing pressure, T_CDW increases linearly at a rate of 16.5 K/GPa, suggesting that the CDW state gets strengthened with pressure in the P-3m1 phase, which is consistent with the previous report (Friend et al., 1978; Feng et al., 2020; Sahoo et al., 2020). At the first critical pressure P_C1 ∼ 12 GPa, the hexagonal P-3m1 structure undergoes a transition into monoclinic C2/m phase. The second critical pressure P_C2 ∼ 33 GPa corresponds to another structural transition from the monoclinic C2/m to P2₁/m phase. The pressure evolution of structure can also be traced by resistance measurements. One can see that the R_10K initially keeps a nearly constant value at low pressures. As the pressure increases up to P_C1, the R_10K suddenly goes up, while it abruptly drops around P_C2.

FIGURE 6

FIGURE 6. Pressure-temperature phase diagram combined with structural phase information for 1T-VSe₂. The left y-axis: The pressure-dependent T_CDW in Run 1 and Run 2, respectively. The right y-axis: Red (blue) circles represent the resistance values at 10 K extracted from the high-pressure resistance measurements of Run 1 (Run 2).

Conclusion

In conclusion, pressure evolutions of structure and resistance of single-crystalline 1T-VSe₂ were studied by combined high-pressure electrical transport, synchrotron XRD and Raman experiments. We demonstrate that the CDW transition in the hexagonal P-3m1 phase is enhanced remarkably by the application of pressure. And two structural phase transformation from the 1T to C2/m and from C2/m to P2₁/m is observed at P_C1 ∼ 12 and P_C2 ∼ 33 GPa, respectively. These findings will shed light on the understanding of structural and transport properties in pressurized transition metal dichalcogenides.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Author Contributions

MZ and XH synthesized the sample and carried out the experiments of sample characterization. DS carried out the high-pressure electrical transport measurement and high-pressure Raman scattering. YZ carried out the high-pressure X-ray diffraction measurement. XL and YZ reviewed and edited the manuscript. All authors have read and agreed to the submitted version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (NSFC) (Grant No. 11974016), Natural Science Foundation of Anhui Province (Grant No. 2008085QA40).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The X-ray diffraction experiment was performed at the beamline BL15U1, Shanghai Synchrotron Radiation Facility (SSRF).

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmats.2021.710849/full#supplementary-material

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