Layer reconstruction, collapse and metallization of van der Waals bonded ZrS2 under high pressure

doi:10.21203/rs.3.rs-3590821/v1

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Layer reconstruction, collapse and metallization of van der Waals bonded ZrS2 under high pressure

https://doi.org/10.21203/rs.3.rs-3590821/v1

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In contrast to two-dimensional (2D) monolayer materials, van der Waals layered transition metal dichalcogenides exhibit rich polymorphism, making them promising candidates for novel superconductors, topological insulators, and high-performance electrochemical catalysts. Here, we combine Raman scattering, electrical conductivity, and synchrotron X-ray diffraction measurements to reveal a series of phase transitions in van der Waals layered ZrS2, driven by the formation of a distorted metastable structure under pressure. Unlike layered sliding observed in archetypal MoS2, ZrS2 undergoes a dramatic structural reconstruction, rearranging the original ZrS6 octahedra into ZrS8 cuboids at 5.5 GPa, leading to an abrupt 8.8% volume reduction. The unique cuboids coordination of Zr atoms in the single-layer is thermodynamically metastable and collapses to a partially disordered phase at 17.4 GPa, and ultimately metallize above 30.0 GPa. Decompressing metallic ZrS2 restores its semiconductor properties. These complex structural transitions present the highly tunable electronic properties of compressed ZrS2 with possible implications for optoelectronic devices.

Physical sciences/Chemistry/Inorganic chemistry/Solid-state chemistry

Physical sciences/Materials science/Condensed-matter physics/Phase transitions and critical phenomena

The discovery of graphene has ignited a fervent pursuit of novel two-dimension (2D) materials with exotic optical and electronic properties. Among the myriad candidates, transition metal dichalcogenides (TMDs) have garnered substantial attention due to their remarkable polymorphism and highly tunable band-gap structures. Their electronic structures were reported to span the entire range of solid states, encompassing Mott insulating¹, semiconducting², semimetallic to metallic³, and even superconducting states^4,5. Such versatility of electronic property stems from their unique atomic arrangements, characterized by the stacking of sandwiched X-M-X monolayer (X: chalcogen, M: transition metal) interacted through weak van der Waals (vdW) forces^6,7. To date, our investigations summarize three fundamental crystal symmetries that govern the interlayer stacking: 1T (trigonal), 2H (hexagonal), and 3R (rhombohedral), where the numbers of 1, 2, 3 denote the number of layers per unit cell. The interplay between the strong ionic-bonded interlayered structure and the weak vdW interaction give rise to the complex polymorphism under varying thermodynamical conditions.

Phase transitions of TMDs can be induced by various external stimuli, such as mechanical exfoliation⁸, transverse electric field⁹ and intercalation¹⁰. Among these methods, applying pressure is a powerful and clean approach to modulate the electronic and crystal structures of TMDs without introducing impurities^11–13. Taking the prototypal 2H phase (e.g. MoS₂, MoSe₂, MoTe₂, WS₂ and WSe₂) as an example^14–18, phase transitions under pressure are widely reported to be triggered by layer sliding or interlayer compression. A case in point is 2H-MoS₂, which undergoes an isostructural phase transition from 2Hc to 2Ha via the lateral displacement of adjacent atom layers, accompanied by metallization driven by the reduction of interlayer spacing under compression¹⁹. Such an interlayer phase transition does not alter the arrangement of intralayer atoms, their coordination number, or the unit cell volume. Since the vdW force is typically associated with the softest vibrational modes and collapse prior to strong interlayer bonding, lattice reconstruction within the vdW layer is rarely observed in pressurized systems. However, polymorphism within low dimensions has emerged as the origin of novel superconducting and topological states^20,21, potentially useful for making optics and optoelectronic devices.

Unlike the 2H polytype, the 1T phase has even more intricate structure polymorphism and exhibits distinct physical behaviors under pressure. This work focuses on the semiconducting 1T phase ZrS₂, which comprises six-fold ZrS₆ under ambient conditions. Theoretical predictions suggest that the ZrS₆ octahedron is inherently unstable and transforms to ZrS₁₀ bicapped cuboids under pressure²². The abrupt coordination change in 1T-ZrS₂ is an anomaly among conventional AB₂-type TMDs and implies the existence of metastable structures with unique structural motifs.

High-quality ZrS₂ powder with purity of 99.99% was commercially acquired from Sunano company, Shanghai, China. The initial sample was characterized by an X’ Pert Pro X-ray powder diffractometer with the copper Kα radiation. As shown in Fig. 1, the X-ray diffraction pattern indicated the ZrS₂ powder belongs to the trigonal lattice (space group: P\(\stackrel{-}{3}\)m1) at ambient condition. MDI Jade 6.0 software was adopted to obtain lattice constants and unit cell volume by Rietveld refinement as follows: a = b = 3.660 (2) Å, c = 5.825 (4) Å, and V = 67.58 (9) Å³. High-pressure angular dispersive XRD measurements were carried out at the 13BM-C beamline in Advanced Phonon Source (APS), Argonne National Laboratory (ANL), USA. The size of the beam spot was about 15×15 µm, and the X-ray wavelength was 0.4340 Å at the time of experiment. LaB6 was used to calibrate the image plate orientation angles and sample-to-detector distances. The collected XRD diffraction data were integrated with the Dioptas program and later analyzed by the UnitCell software²³.

Raman scattering measurements were implemented in symmetrical diamond anvil cells (DACs) with culet size of 250 µm. The sample was loaded into a 100 µm hole in a Re gasket together with a small piece of ruby, which was used as the pressure calibration material²⁴. The mixed solution of methanol and ethanol was chosen to achieve quasi-hydrostatic conditions. Raman spectra were acquired through a micro-confocal Renishaw Raman spectrometer with a 532 nm excitation source, a spectra resolution of -1 cm^− 1 and a grating of 2400 gr/mm. We used a proper laser power of 20 mV to avoid destroying sample structure by the laser energy. The acquisition time is 120 s to ensure the quantity of spectra.

In situ electrical conductivity (EC) measurements were conducted in designed diamond anvil cells combined with a Solartron-1260 impedance analyzer. A Re gasket was initially pre-indented into a 25 µm thickness of pit, and then its center was drilled by the laser drilling device to form a 200 µm hole. As excellent insulating materials, a mixture powder of epoxy and cubic boron nitride was compressed into the hole, and finally another 100 µm hole was drilled to serve as the insulating sample chamber. Differing from the measurement of Raman spectra, we did not use any pressure medium in the sample chamber in EC experiments in order to the exclude the impurity influence. A ruby was placed on the surface of the electron to determine pressure. AC impedance were acquired in the frequency range of 10^‒1–10⁷ Hz. The noble metal platinum foil was selected as the electron material. The collected AC impedance data were analyzed by the Z-View software.

In situ synchrotron XRD experiments of compressed ZrS₂ were conducted up to 32.3 GPa at room temperature. Figure 2a displayed representative XRD patterns during the compression. The diffraction patterns at pressures below 5.5 GPa are readily indexed to the trigonal phase (\(P\stackrel{-}{3}m1\), phase I) as illustrated in the inset of Fig. 2b with Re gasket and minor amount of ruby. All of Bragg peaks for the trigonal ZrS₂ shifted to higher angles with increasing pressure, indicating conventional compression behavior. Pronounced changes in XRD pattern were observed when pressure was increased to 5.5 GPa, at which several new diffraction peaks appeared at 2θ angles of around 7.6°, 7.8°, 8.0°, 12.9°, 13.4° and 13.9°, suggesting the onset of structural phase transition (devoted as phase II). Further compression beyond 17.4 GPa results in the disappearance of several diffraction peaks, leaving only four broad peaks (as shown in Fig. S1 of the Supplemental Material). At this point, the crystalline of the sample began to diminish, and a degree of structural disordering (denoted by phase III) came into play. The limited number and severe broadening of diffraction peaks in phase III preclude determination of its lattice parameters.

We employed a Monte Carlo indexing algorithm²⁵ to identify the lattice associated with the newly appeared diffraction peaks of phase II and refined its space group using the Crysfire software package²⁶. The results pointed to an orthorhombic structure with the space group of Pmm2 with lattice parameters a = 3.297 (6) Å, b = 3.627 (7) Å, c = 9.509 (11) Å and V = 56.85 (34) Å³ at 5.5 GPa. Although the Pmm2 was not predicted as a stable phase by previous structural searching simulation²², it can be realized as a conjugate subgroup of the host tetragonal structure (I4/mmm), which was previously regarded as the ground state at above 25 GPa. Such lattice distortion was well documented for phase transitions under spontaneous strain²⁷. The indexed twelve XRD peaks for this high-pressure phase are listed in Table 1.

Table 1

**The indexed XRD peaks for Phase II at 5.5 GPa**.
Phase II (5.5 GPa)	h	k	l	2θ (degree)	2θ (degree)	Δ 2θ
Phase II (5.5 GPa)	h	k	l	experiment	refined	Δ 2θ
Orthorhombic (Pmm2) a = 3.297 (6) Å b = 3.627 (7) Å c = 9.509 (11) Å V = 56.85 (34) Å³	1	0	0	7.599	7.6048	-0.0058
	0	0	3	7.818	7.8478	-0.0298
	1	0	1	8.012	8.0428	-0.0308
	1	0	2	9.226	9.2337	-0.0077
	1	0	3	10.975	10.9363	0.0387
	1	1	3	12.885	12.9028	-0.0178
	0	2	1	13.919	13.9264	-0.0074
	0	1	5	14.733	14.7837	-0.0507
	2	0	1	15.443	15.4685	-0.0255
	0	2	4	17.275	17.2546	0.0204
	2	1	4	19.809	19.7664	0.0426
	1	0	7	19.935	19.9125	0.0225

The variation of unit cell volume with pressure of ZrS₂ is plotted in Fig. 3a. The unit cell volume is abruptly decreased by 8.8% after the transformation from P\(\stackrel{-}{3}\)m1 to Pmm2 phase, thus attributes to first-order transition. The pressure-volume data for different phases were fitted using a third-order Birch-Murnaghan equation of state in EosFit7 program²⁸. The derived bulk modulus K₀ and zero-pressure unit cell volume V₀ for phase I were 32.7 (88) GPa and 68.6 (10) Å³, respectively. As for phase II, we acquired the bulk modulus K₀ = 66.1 (17) GPa, and V₀ = 61.3 (1) Å³. The higher K₀ value for phase II indicated that the eight-coordinated structure is less compressible. The pressure dependence of lattice parameter ratios is illustrated in Fig. 3b and 3c. For the ZrS₂ phase I, the c axis is more compressible than the a axis due to the weak vdW forces existed in the c axial orientation. In contrast, the high-pressure phase II exhibited much weak anisotropy in comparison with the P\(\stackrel{-}{3}\)m1 phase.

The substantial volume collapse and severe lattice distortion associated with phase II makes it impracticable for the precise determination of atomic coordinates. However, considering Phase II as a distorted tetragonal phase with two formula units in a conventional unit cell, and assuming the atomic coordinates from the undistorted tetragonal phase²², each Zr atom would be coordinated by 8 neighboring S atoms, forming ZrS₈ cuboids due to the elongation of b axis. This distortion maintains a substantial interlayer distance (4.75 Å at 5.5 GPa), Suggesting that the phase II may retains a layered structure. In particular, the interlayer structural transition in ZrS₂ can be compared with other AB₂-type TMDs with similar lattice structure, such as MoS₂, WS₂, and WSe₂^15,29,30. All of such layered binary compounds were reported to undergo pressure-induced phase transition through layer sliding, which is associated with the lateral shift of adjacent atom layers, and thus those are isostructural phase transition and still remain the layered nature²⁹. However, the phase transition of ZrS₂ reconstructed the interlayer lattice to form an orthorhombic structure and the coordination number of Zr changed from six to eight. The partially disordered phase III may eventually collapse into a 3D structure by layer sliding. The postponed layer sliding in ZrS₂ is possibly resulted from the relatively weak in-plane bonding in ZrS₂.The absence of layer sliding in ZrS₂ is possibly resulted from the relatively weak in-plane bonding in ZrS₂.

We also conducted Raman spectroscopy with focus on the interlayer structure. Theoretical group analysis of lattice vibrations of 1T-ZrS₂ at the Γ-point predicted two Raman-active modes represented as Γ = A_1g + E_g + 2A_2u + 2E_u ³¹. Figure 4a depicts Raman spectra collected at the frequency range from 100 cm^− 1 to 600 cm^− 1 in DAC up to 40.7 GPa at room temperature. At ambient conditions, the in-plane (E_g) and out-of-plane (A_1g) Raman active modes of ZrS₂ were clearly captured at positions of 248.8 cm^− 1 and 332.7 cm^− 1, respectively. A broadening peak near the A_1g band appeared at 312.7 cm^− 1, which can be explained by the non-harmonic effect induced by acoustic phonon despite coincidence with infrared-active A_2u mode, as was concerned in some literature³¹. Moreover, we also detected another two peaks at 140.5 cm^− 1 and 191.0 cm^− 1, which are associated with the density of two phonon states of ZrS₂³². All of these peaks obtained from this study are in good accordance with previous Raman data³³.

As shown in Fig. 4a, the ambient Raman spectra pattern of 1T-ZrS₂ remained almost the same up to 3.3 GPa, above which some distinct variations were observed in the scattering profile. Several new peaks at 112.6, 134.4, 263.2 and 285.6 cm^− 1 started to emerge along with the vanishment of two original bands at 140.5 and 191.0 cm^− 1. The occurrences of new vibrational modes hint the structural rearrangement in 1T-ZrS₂, from a trigonal phase (P\(\stackrel{-}{3}\)m1) to an orthorhombic phase (Pmm2) as confirmed by our XRD results. In particular, over the pressure range of 0–17.4 GPa, we observed an interesting phenomenon that the intensity of the acoustic phonon at 312.7 cm^− 1 gradually increased with pressure, whereas the intensity of the A_1g mode at 332.7 cm^− 1 nearby the acoustic phonon decreased and finally vanished above 17.4 GPa. This indicates that the non-harmonic effect of out-of-plane (A_1g) mode would be greatly enhanced by pressure engineering. Further compression up to 17.4 GPa, multiple new bands along with the disappearance of original peaks were observed corresponding to the II-III transition, reaching agreement with synchrotron XRD data. Moreover, it was noteworthy that beyond 17.4 GPa the Raman spectra profile was extremely lumpy with attenuated Raman peaks and higher frequency uncertainties, which reflected the introduction of structural disordering in phase III. Following decompression experiment found the recovered peaks matches those of metastable phase II. This suggests that the trigonal-to-orthorhombic phase transition is irreversible but we are able to preserve the vdW phase II after removing pressure.

The pressure dependence of Raman modes was plotted in Fig. 4b, and a liner equation was used to fit these mode frequencies. On the basis of the discontinuous changes in dω/dp, two obvious inflection points were located at 5.4 GPa and 17.4 GPa as marked by dashed lines, corresponding to two phase transitions in 1T-ZrS₂ under high pressure. The frequencies of phonon modes and their pressure coefficient dω/dp in each phase of ZrS₂ is described in the Supplemental material. For the initial trigonal phase (0–5.4 GPa), all the phonon modes shifted to higher wave numbers with increasing pressure, but the dω/dp value of A_1g mode was relatively larger than that of the E_g mode. The A_1g mode reflects the weak vdW interaction along c axis, while the E_g mode is related to the atomic vibration in the basic X-M-X monolayer along ab plane. As usual, the stretching force constant between the interlayer distances was greatly improved by pressure³⁴, and thus the out-of-plane A_1g mode is more sensitive to compression than the in-plane E_g mode. In fact, the divergence in the sensitivity of A and E modes to volume compression has also been reported by MoSe₂ and InSe^35,36. For high-pressure phases, most phonon modes exhibited positive pressure dependence except the one at 112.6 cm^− 1 in phase II and mode of 157.8 cm^− 1 in phase III, which were featured by mild phonon softening with negative pressure coefficient of -0.228 cm^− 1/GPa and − 0.166 cm^− 1/GPa, respectively. This phonon softening is likely interrelated with structural phase transitions in ZrS₂. Some other TMDs with similar structure, like ReS₂³⁷, were also found to exhibit phonon softening under high pressure. In addition, we also calculated mode Grüneisen parameters of A_1g and E_g mode using the equation γ = K₀/ω₀(dω/dP), where K₀ and ω₀ are the zero-pressure bulk modulus and phonon frequency of Raman modes, respectively. The mode Grüneisen parameter of A_1g mode is 0.95, which is larger than that of E_g mode (γ = 0.64). This implies a stronger anharmonicity in crystal structure in the vicinity of A_1g vibration³⁸ where acoustic phonon is likely to occur.

Structural phase transition has a profound effect on their electronic properties and we then investigated the electronic properties of ZrS₂. The sample was connected by two Pt probes to measure the in situ AC impedance under high pressure, which has been widely applied in the measurement of electrical conductivity of materials^39–42. Figure 5a–5c showed the typical impedance spectrum of ZrS₂. These collected impedance spectrums were fitted by the equivalent circuit method to obtain the resistance of sample, which was then used to calculate the electrical conductivity with the equation σ = L/RS. It should be mentioned that we only selected the semicircular arcs at low frequency ranges to obtain the grain interior conduction. Figure 5d plotted the pressure dependence of EC during compression and decompression. Close to 1 atm, the electrical conductivity of ZrS₂ was about 2.68 (13)×10^− 4 S/cm, which is the typical value of semiconductor. The EC value slightly decreased upon pressurization 3.8 GPa, above which the EC jumped due to promoted charge carrier mobility in the orthorhombic phase II. Another electrical conductivity dump was observed at 17.6 GPa, at which the EC increased by roughly four orders of magnitude, indicating the occurrence of the partially disordered phase III. After 26.9 GPa, The EC remained a high value of 0.9 S/cm to the highest pressure studied, corresponding to a metal. The weak pressure dependence provided another evidence to support this metallization. As usual, the increase rate of carrier concentration with pressure would be substantially reduced with the progression of pressure-induced metallization.

We further conducted temperature dependence of EC (Fig. 6) to diagnose the semiconductor-to-metal transition. For typical insulator and semiconductors, the EC exhibits a positive correlation with temperature. In our experiments, the slope of log10(σ)/T approach zero between 23.0 and 30.2 GPa, indicating the pressure range within which metallization occurs. Our observed temperature dependence confirmed the phase II is still a semiconductor and pressure-induced metallization occur in phase III. The four-order-of-magnitude increase in EC during compression makes ZrS₂ a promising material for designing optoelectronic switches controlled by pressure.

We finally decompressed the sample to examine the reversibility of this metallic phase. Initially, the EC remained high level, and rapidly dropped after 6.4 GPa. The hysteretic effect of transition pressure point between compression and decompression is a common phenomenon for structural phase transformation. After decompression to 1 atm, the EC value is 3.89 (21)×10^− 3 S/cm, which is higher than the original state at ambient condition. This unrecoverable EC value is consistent with our Raman results.

Fig .5. a)–c) Typical impedance spectra for ZrS₂ at high pressures in the frequency region of 10^‒1–10⁷ Hz during pressurization. The horizontal axe indicates the real part of the complex impedance, while vertical axe represents the imaginary part. d) The logarithm of electrical conductivity as a function of pressure in the process of compression and decompression.

Fig .6 a) The temperature dependence of the electrical conductivity (EC) of ZrS₂ at various pressure points. b) The metallization at 30.2 GPa and 45.8 GPa.

In summary, to elucidate the pressure-induced effect on 1T-ZrS₂, we conducted a comprehensive suite of high-pressure experiments in DACs up to 45.8 GPa. The XRD results unambiguously revealed a first-order structural phase transition in ZrS₂ at 5.5 GPa. In contrast to MoS₂, this phase transition is caused by the intralayer reconstruction rather than the interlayer sliding, highlighting the fundamental differences in their bonding mechanisms. This novel layered phase subsequently transforms to a partially disordered phase above 17.4 GPa. Our electrical conductivity measurements unveiled a semiconductor-to-metal electronic transformation within the disordered phase. The pressure-quenchable, vdW-interacted phase II expands the polymorphic complexity of TMDs and opens avenues for further exploration in the design Zr-based dichalcogenides for optoelectronic implications.

Acknowledgements

This study is supported by was supported by the National Key R&D Program of China (Grant No. 2022YFA1405500) and the National Science Foundation of China (Grant No. U1530402). We acknowledge the use of synchrotron X-ray diffraction at the 13BM-C of GSECARS, Advanced Photon Source, Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation- Earth Sciences (EAR-1634415) and the Department of Energy-GeoSciences (DE-FG02-94ER14466). 13BM-C is partially supported by COMPRES under NSF Cooperative Agreement EAR − 1606856. Q.H. was supported by the New Cornerstone Science Foundation through the Xplorer Prize (XPLORER-2020-1013).

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In contrast to two-dimensional (2D) monolayer materials, van der Waals layered transition metal dichalcogenides exhibit rich polymorphism, making them promising candidates for novel superconductors, topological insulators, and high-performance electrochemical catalysts. Here, we combine Raman scattering, electrical conductivity, and synchrotron X-ray diffraction measurements to reveal a series of phase transitions in van der Waals layered ZrS₂, driven by the formation of a distorted metastable structure under pressure. Unlike layered sliding observed in archetypal MoS₂, ZrS₂ undergoes a dramatic structural reconstruction, rearranging the original ZrS₆ octahedra into ZrS₈ cuboids at 5.5 GPa, leading to an abrupt 8.8% volume reduction. The unique cuboids coordination of Zr atoms in the single-layer is thermodynamically metastable and collapses to a partially disordered phase at 17.4 GPa, and ultimately metallize above 30.0 GPa. Decompressing metallic ZrS₂ restores its semiconductor properties. These complex structural transitions present the highly tunable electronic properties of compressed ZrS₂ with possible implications for optoelectronic devices.

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