Pressure-Induced Reversible Phase Transitions in a New Metastable

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C: Physical Processes in Nanomaterials and Nanostructures

Pressure-Induced Reversible Phase Transitions in a New Metastable Phase of Vanadium Dioxide Huafang Zhang, Quanjun Li, Fei Wang, Ran Liu, Yanli Mao, Zhenxian Liu, Xiaodong Li, Ke Yang, Tian Cui, and Bingbing Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11643 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 11, 2018

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The Journal of Physical Chemistry

Pressure-Induced Reversible Phase Transitions in A New Metastable Phase of Vanadium Dioxide Huafang Zhang,1,2 Quanjun Li,1* Fei Wang,1 Ran Liu,1 Yanli Mao,2 Zhenxian Liu,3 Xiaodong Li,4 Ke Yang,5 Tian Cui,1 Bingbing Liu1* 1

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China

2

School of Physics and Electronics, Henan University, Kaifeng 475004, China

3

U2A Beam line, Carnegie Institution of Washington, Upton, New York 11973

4

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of

Sciences, 100049, Beijing, China 5

Shanghai Institute of Applied Physics, Chinese Academy of Sciences, 201204, Shanghai, China

Corresponding Author Quanjun Li, E-mail: [email protected] Bingbing Liu, E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Exploring the structural and physical properties of new vanadium dioxide (VO2) allotropes have attracted considerable interest because of the structure diversity and unique physical properties of VO2. Here we demonstrate a reversible pressure-induced structural transition and metallization of the novel metastable polymorph VO2(Mx'), and a thermallydriven structural transition from VO2(Mx') to the monoclinic phase VO2(M1) at relative low temperature based on X-ray diffraction (XRD), Raman and infrared spectroscopy. It is shown that the metastable phase VO2(Mx') undergo the structural transitions of VO2(Mx')-(12 GPa)VO2(Mx'')-(30-80 GPa)VO2(X) upon compression, obviously different with the pressureinduced amorphization observed in other metastable phases VO2(A) and VO2(B). Moreover, the IR data demonstrated that the pressure-induced metallization (PIM) occurs in the VO2(Mx'') phase at about 40 Gpa, which is mainly associated with electron-electron correlations. Further analysis suggests that all the sample transforming into the same high pressure VO2(X) phase with the stable M1 phase could mainly results from the VO6 octahedra and empty spaces between VO6 octahedra in their intermediate high pressure phases VO2(Mx'') and VO2(M1') following similar variations under pressure. These findings present new insight into the differences of structural transitions and physical properties between the stable and metastable phases of transition metal oxides under pressure.


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1. Introduction Transition metal oxides that contain diverse polymorphs (in metastable or stable states) and show unique physical properties are fundamentally interesting to condensed matter physics and practical industrial applications.1-4 The physical properties of these materials depend sensitively on structure, and can be adjusted by changing temperature and stoichiometry.5-6 Another important facile and clean way for tuning the physical properties of materials in both metastable and stable states is via cold compression.7-8 This interest have promoted exploration of the novel high-pressure behaviors of substances. In recent years, pressure-induced structural transitions have been observed in transition metal oxides, such as TiO2, ZrO2 and MnO2, V2O3.9-20 It is interesting to note that the structures, in both stable and metastable states, transform into the same high-pressure phase under pressure for part of the transition metal oxides,9, 14-15, 18-19 but follow different transition sequences for the others.11-12 Although several scenarios have been proposed to explain the difference of the high pressure behaviors of the structures in stable and metastable states for different transition metal oxides, the mechanism remains unclear so far because none of the investigated samples have both the metastable phases that parts transform into the same high pressure structure with the corresponding stable phase and the others show unique phase transition sequence under pressure. Vanadium dioxide (VO2) is an typical transition metal oxides with diverse polymorphs at ambient conditions,21-24 such as the stable crystalline structure VO2 (M1) and the other two common metastable crystalline structures VO2 (A) and VO2 (B). These different polymorphs are composed of V-O octahedrons, each V atoms surrounded by six O atoms, arranged in different ways,24-26 providing an ideal model for investigating the behavior of stable and metastable phases at high pressure. Recently, great efforts have been focused on the structural evolution and


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physical properties of VO2 under pressure,2, 7, 27-34 and typical examples are the pressure-induced structural transitions of VO2(M1)→(13GPa)VO2(M1')→(26-59GPa) VO2(X) and the PIM in the VO2(M1')phase at about 43 GPa due to electron correlations.30, 33-34 While the metastable phase VO2 (A) are more stable in low pressure region and then becomes amorphous at about 32 GPa. The PIM has also been observed in the metastable phase VO2 (A) at about 28 GPa before amorphous,31 similarly, the metastable phase VO2 (B) being amorphous at about 17-20 GPa, but remains a semiconductor even after been amorphous.2 Existing results indicate that the structure transition sequence of the metastable phases VO2 obviously different with that of the stable phase under pressure. In recent high-pressure study on VO2, we have synthesized a new monoclinic VO2 polymorph (named VO2 (Mx')) by high-pressure treatment of the VO2 (M1) up to 63 GPa.32-33 And this new structure becomes metallic without structural transition with 11% W-doped, which is obviously different with the other polymorphs of VO2.32 These results indicate that the VO2 (Mx') has good structural stability, thus may present new phase transition behavior and physical properties under pressure and provide the chance for getting insight into the difference of the phase transition behaviors between the metastable and stable phase under pressure. Here, we performed high-pressure study on the structural and optical properties of VO2 (Mx'). We find a novel structural phase transition process of the metastable VO2 (Mx') under pressure, VO2(Mx')→(10 GPa)VO2(Mx'')→(31-81 GPa)VO2(X), for which the high pressure phase is the same with that of the stable phase VO2 (M1). And we demonstrate the VO2 (Mx') was a metastable phase by a thermal-driven structural transition from VO2 (Mx') into VO2 (M1). By comparing with the high pressure behavior of the stable phase M1,30, 33-34 it can be deduced that the variation of microstructure is the key factor in determining the structural phase transition


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process for the metastable phases under pressure. Moreover, using IR spectroscopy, we further unveil the evolutions of the electrical properties of the metastable VO2 (Mx') under pressure. 2. Methods 2.1 Sample preparation and characterizations. The new VO2(Mx') phase were synthesized by high-pressure treatment of VO2(M1) using diamond anvil cells (DACs) with 200 μm culet size. In a typical procedure, the rhenium gaskets were preindented to 40–50 μm and drilled with a center hole of 80 μm as sample chamber, a proper amount of VO2(M1) powders (purchased from Sigma-Alorich (99.9%)), with a 4:1 methanol-ethanol mixture as pressure-transmitting medium, were compressed up to 61.3 GPa by using DACs. The pressures were calibrated by ruby fluorescence technique. In the decompressing process, the new VO2(Mx') phase was obtained at about 20 GPa (Supplementary figure S1), and was successfully retained after released to ambient conditions. Structure refinements were performed with the GSAS software package. In addition, recovered sample was characterized using transmission electron microscopy (TEM) (200 KV, HITACHI, H-81001V), and high-resolution transmission electron microscopy (HRTEM) (JEOL JEM-3010). 2.2 In situ high-pressure measurements. High-pressure XRD and Raman spectra were measured by recompression obtained VO2 (Mx') with a 4:1 methanol-ethanol mixture as pressure-transmitting medium, the high-pressure IR measurements were carried out with pure VO2 (Mx') sample. The high-pressure XRD measurements were performed at the 4W2 beamline of Beijing Synchrotron Radiation Facility (BSRF) (λ = 0.6199 Å) and the BL15U beamline of Shanghai Synchrotron Radiation Facility (SSRF) (λ = 0.6199 Å). High-pressure Raman spectra were recorded on a LabRam HR Evolution equipped with 532 lasers. The IR reflection spectra


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(Rsd(ω)) were collected through a Bruker Vertex 80v FTIR spectrometer and a Hyperion 2000 IR microscope equipped with a liquid nitrogen cooled MCT detector based on the method reported in our early study. 2.3 Ex situ annealing experiments. The new VO2(Mx') were removed with the gaskets from DACs, and then annealed at different temperatures (200, 300, 400 ℃) within a vacuum furnace for 30 min, and the resulting samples were characterized by synchrotron XRD and Raman spectra (532 nm). 3. Results and Discussion

Figure 1. (a) Refinements result, (b) IR reflectivity Rsd(ω) and (c) transmittance spectrum T(ω) for the VO2 (Mx') phase at ambient pressure. Insets of (a): HRTEM images of the VO2 (Mx') phase. The Rsd(ω) and T(ω) spectra were measured in DACs. The data at about 1700-2700 cm-1 in Rsd(ω) and T(ω) were cut from the spectra due to diamond absorption. The structure and physical properties of the VO2 (Mx') at ambient conditions. Figure 1a shows the XRD pattern and the HRTEM images of the VO2(Mx') phase measured at ambient


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conditions. The two clear interplanar distances of ～0.34 nm and ～0.24 nm correspond to the diffraction peaks at 10.35°and 14.65°of the VO2(Mx') phase, respectively. These results suggest that the VO2(Mx') phase is stable at ambient conditions with good crystallinity. We fitted the XRD pattern with a monoclinic structure (P 21/c), and summarized the lattice parameters in table 1. These lattice parameters were close to that of the nanosized VO2(Mx') phase reported in our early study.32-33 We further use the obtained crystal information simulated the TEM/SAED patterns, and found that it matches well with the observed TEM/SAED pattern for the collected sample (Supplementary figure S2). These results indicated the rationality of indexing the structure of the VO2(Mx') to the monoclinic unite cell, whereas the exact structure of VO2(Mx') is still cannot be determined due to the limited precision of the diffraction data. In order to probe the optical properties of VO2(Mx'), we carried out Rsd(ω) and T(ω) spectra measurements on the obtained sample. Figure 1b shows the measured Rsd(ω) spectrum, the lowfrequency reflectivity is characterized by a steep rise at about 800 cm-1, which may related with phononic contribution as observed on the M1 phase VO27, on the high-frequency side, the reflectivity presents relative low values. Figure 1c shows the measured T(ω) spectrum, the bump at about 3500 cm-1 (labeled with black arrow) is due to a bad compensation of diamond absorption.27 The transmittance presents maxima at about 1000 cm-1, and there is no transmitted light in the frequency region above 5800 cm-1 (labeled with blue arrow in figure 1c). We calculated the bandgap value based on the T(ω) spectrum using the method illustrated in our early study.34 The estimated band gap value is about 0.67 eV, indicating that the VO2 (Mx') phase is a semiconductor. Table 1. Parameters of the VO2(Mx') phase at ambient conditions.


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a (Å)

b (Å)

c (Å)

β (o)

V (Å3)

Ref.

VO2(Mx') bulk

4.61(0)

5.25(2)

6.37(3)

102.10(2)

150.91(5)

This study

VO2(Mx') nano

4.631(7) 5.308(9) 6.394(7) 101.094(6) 151.820(5) Ref. 33

Phase

Size

Figure 2. (a) XRD patterns, (b) variation of relative d-spacings, (c) variation of lattice parameters and (d) pressure versus volume data of VO2 at various pressures collected upon compression. * indicate the diffraction of high pressure X phase, # indicate the diffraction of Re. Pressure-induced reversible structural phase transitions and metallization in VO2 (Mx'). In order to follow the structural and optical properties evolutions of the newfound VO2(Mx') phase under pressure, we carried out high pressure XRD, Raman and reflectance measurements.


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Figure 2(a) shows the XRD patterns of the VO2(Mx') phase as a function of pressure. With increasing pressure, all the diffraction peaks gradually shift to smaller d-spacing values due to lattice compression. Note that the diffraction peak at about 2.7 Å (labeled with black arrows in figure 2a) became weaker upon compression and finally disappeared at about 10 GPa. The variations of relative d-spacings for different diffraction peaks show different decreasing rate within 0-10 GPa, then noticeable change in slopes was observed, and the decreasing rates become more similar with further compression (shown in figure 2b). These obvious changes may be related to a structural phase transition. Despite these changes, the structure can still be described with the monoclinic symmetry (P 21/c) up to 31 GPa, we named as VO2(Mx''). When the pressure was increased above 31 GPa, new diffraction peaks at about 2.82 Å, 2.36 Å and 2.08 Å start to appear, which can be safely indexed to the diffraction peaks of the high pressure VO2(X) phase as observed in our early studies on the VO2(M1).30, 33-34 With further increasing pressure, the intensity of the diffraction peaks from the high pressure VO2(X) phase increases considerably. The VO2(Mx'') phase transforms into the high pressure VO2(X) phase completely at about 81 GPa, with the VO2(Mx'') and VO2(X) phases coexist over a pressure region of 50 GPa. By comparing the measured XRD patterns of the Mx', X phases and the calculated XRD patterns of VOx (x=6,7,8,9) polyhedron, we find that VO6 and VO8 polyhedron may coexist in the Mx' phase, and only the VO7 polyhedron exists in the high pressure X phase.35 Thus, it is most likely that the VO8 polyhedron transforms into VO7 polyhedron under pressure, and the redundant O atom together with the VO6 polyhedron form the VO7 polyhedron, realizing the structural transition from Mx' into the X phase. Upon decompression, the sample transforms into the original VO2(Mx'). These results demonstrate that different with the other two metastable A


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and B phases, the VO2(Mx') transforms into the same high pressure phase with that of the stable M1 phase under pressure.2, 31 Figure 2c shows the pressure-dependent of the lattice parameters (a/a0, b/b0 and c/c0) obtained by Rietveld refinements of XRD patters of the VO2(Mx') and VO2(Mx'') phases under pressure. Within the 0-10 GPa, the c axis is more compressible than a and b axes, indicating an anisotropic compression of the VO2(Mx') phase. When the applied pressure increased above 10 GPa, a noticeable change in slope was observed, and the compressibility of the c axis becomes close to the another two axes. Figure 2d shows the pressure versus volume data in the pressure region of 0-30 GPa. The volume decreases with increasing pressure, and illustrates an abrupt change at about 10 GPa, which is related to the phase transition from VO2(Mx') to the VO2(Mx'') phase. The pressure-volume data before and after the phase transition were fitted to the thirdorder Birch-Murnaghan equation,

[( )

( ) ]

{

(

) [( )

]}

where B0 and B1 are the bulk modulus and its first pressure derivative, P is the applied pressure, V0 and V are the volume at ambient conditions and under pressure P. B1 is fixed to 4. As shown in Table 2, the obtained bulk modulus for the VO2(Mx') and intermediate phase VO2(Mx'') are 187 (7) and 161 (5) GPa, respectively. The bulk modulus for the VO2(M1), VO2(M1'), VO2(A) and VO2(B) phases were list in Table 2 for comparison. It is worth noting that the bulk modulus for the VO2(Mx') phase 187(3) is more close to that for the stable VO2(M1) phase 213(2) GPa, than the metastable A and B phases, 151(3) GPa and 113(2) GPa. Moreover, the bulk modulus for the VO2(Mx'') phase is lower than that of the phase VO2(Mx'), suggesting that the VO2(Mx')


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phase transforms into a more compressible phase under pressure, which agrees well with that observed in the VO2(M1) phase.33-34 Table 2. Bulk modulus B0 for different phases in VO2. Phase

B0 (GPa)

Ref.

VO2(Mx')

187(7)

This study

VO2(Mx'')

161 (5)

This study

VO2(M1)

213(2)

Ref. 30

VO2(M1')

167(4)

Ref. 30

VO2(A)

151(3)

Fit Ref. 31

VO2(B)

113(2)

Fit Ref. 2

Figure 3. (a) Raman spectra and (b) pressure dependencies of phonon frequencies of VO2 upon compression. Raman spectroscopy is a powerful tool for exploring the short-range features of crystal structure, thus, we carried out high pressure Raman measurements to follow the local structural evolution of the VO2(Mx') phase under pressure. Figure 3a shows the measured Raman spectra


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collected upon compression. The intensity of the typical Raman vibration peak at about 170 cm-1 for the VO2(Mx') phase decreases considerably under pressure and disappears above 10.2 GPa (labeled with arrows in figure 3a), then a new Raman peak appears at 413 cm-1 as pressure increased to 15.9 GPa (labeled with star in figure 3a). These obvious changes are related to the structural phase transition from the VO2(Mx') phase into the VO2(Mx'') phase as observed in XRD study. With further increasing pressure, all the Raman peaks gradually broaden and weaken and disappeared above 39.5 GPa. No Raman peaks are observed up to 81.3 GPa. When released to ambient pressure, the sample transforms into the original VO2(Mx') (the Raman spectra collected in decompressing process are shown in Supplementary figure S3). Figure 3b shows the pressure dependence of selective Raman peaks. According to early reports,27 the Raman peak at about 639 cm-1 (3.57 GPa) was associated with the V-O mode in VO6 octahedrons that mainly involves the oxygen ions connecting the different V chains along the c axis. The Raman peaks at about 192 cm-1 and 225 cm-1 (3.57 GPa) were associated with the V-V dimers paring and tilting motion, which are consist of the nearest two V atoms at the center of adjacent VO6 octahedrons along the c axis.27,

30

Thus, the pressure dependences of these

Raman peaks reflect the changes of the VO6 octahedrons and variations between VO6 octahedrons under pressure. The pressure dependences of these Raman mode frequencies (dν/dp) were obtained by linear fit the data in the pressure range of 0-10 GPa and 10-30 GPa. As shown in table 3, in the low pressure region for the VO2(Mx') phase the pressure coefficient (CP) for the V-V dimers paring and tilting motion are 0.32 and 0.42 cm−1/GPa, respectively, and for V-O mode is 2.92 cm−1/GPa. In high pressure region for the VO2(Mx'') phase, the corresponding CPs are 0.20, 0.63, 2.12 cm−1/GPa, respectively, which are 0.83, 0.94, 0.97 times (CPMx''/ CPM1') of that for the M1' phase (0.24, 0.67, 2.19 cm−1/GPa).30 We note that the CPMx''/ CPM1' values of


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these Raman peaks are quite closed, suggesting the VO6 octahedron and empty spaces in VO2(Mx'') and VO2(M1') phases show the similar variations under pressure, which could make these two intermediate phases transforming into the same high pressure phase VO2(X). In addition, the relative low CP values in VO2(Mx'') phase reveal that both the VO6 octahedron and empty spaces between the VO6 octahedrons for the VO2(Mx'') phase are less compressible than that in the VO2(M1') phase under pressure, thus, resulting in the more sluggish phase transition process from VO2(Mx'') into the VO2(X) phase.30 Table 3. The pressure dependences (dν/dp) for the Raman peaks of V-V dimers paring and tilting motion, and V-O mode.

Phase

V-V paring

V-V tilting

V-O modes

Ref

(cm−1/GPa)

(cm−1/GPa)

cm−1/GPa

VO2(Mx')

0.32

0.42

2.92

This study

VO2(Mx'')

0.20

0.63

2.12

This study

VO2(M1)

0.19

0.37

3.56

Fit Ref. 30

VO2(M1')

0.24

0.67

2.19

Fit Ref. 30


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Figure 4. IR reflectivity spectra of VO2 upon (a) compression and (b) decompression. Inset: the pressure-reflectivity diagram of the wave numbers 1300 cm-1 under pressure. VO2 phases as typical strongly correlated materials always show interesting optic and electric properties. To explore these properties of VO2(Mx') under pressure, in situ high pressure IR reflectivity spectra were carried out. Figure 4a and 4b show the evolution of the Rsd(ω) spectra of VO2(Mx') within the 600-7500 cm-1 frequency range upon compression and decompression, and insets show the IR reflectivity at 1300 cm-1 as functions of pressure. With increasing pressure, Rsd(ω) increased quickly within 0-11 GPa. Then Rsd(ω) started to increase slowly above 11 GPa and been more subtle above 28 GPa (shown in the figure 4a and the inset). We note that the pressures of these noticeable changes in slope nearly the same with the structural phase transition pressures of the VO2(Mx')-VO2(Mx'') at 10 GPa and the VO2(Mx'')- VO2(X) at about 31 GPa, observed in both our XRD and Raman studies, suggesting that the changes in reflectivity could mainly related to the structural phase transitions. With further increasing pressure, Rsd(ω) almost show no change within 40-48 GPa, but then started to decrease above 48 GPa, and finally became nearly independent of the applied pressure above 66 GPa. According to early reports on MnO, MnTe and VO2, the pressure independence of reflectivity suggest that the sample is metallic.7,

36

Thus, these results demonstrate that VO2 been metallic at about 40 GPa. The

metallicity decreases with further compressing, then the sample transforms into a bad metal at about 66 GPa. Figure 5 shows the relative intensity of the strongest diffraction peaks for the VO2(Mx'') phase at about 3.09 Å (PMx'') and for the X phase at 2.83 Å (PX) (30 GPa) within 30.9-81.1 GPa. We note that the relative intensity of the PMx'' (and PX) is gradually decreasing (and increasing) with increasing pressure, but the suddenly decreases (increases) to 0.2 (and 0.9) within 50-64 GPa


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pressure region. These results demonstrate that the high pressure VO2(Mx'') phase is still the dominant phase at the pressure when the PIM occurs (40 GPa), and then most of the VO2(Mx'') phase transformed into the VO2(X) phase between 50-64 GPa, accompanied with the decrease in reflectivity (shown in figure 4a). Thus, it is reasonable to deduce that the VO2(Mx'') phase being metallic at about 40 GPa, and the high pressure VO2(X) phase is a bad metal.

Figure 5. The relative intensity of the diffraction peaks at about 3.2 Å for the VO2(Mx'') phase and at 2.83 Å for the VO2(X) phase as functions of pressure. IPX and IPMx'' are the intensity of the diffraction peaks of VO2(X) and VO2(Mx'') phase under pressure. Pxm and PMx''m are diffraction peaks of VO2(X) and VO2(Mx'') phase at 81.3 and 30 GPa, respectively, as shown in inset. Upon decompression, the reflectivity is independent of the applied pressure down to 14.3 GPa, and then decrease remarkably with further decompressing (figure 4b), which is related to the structural phase transition from VO2(X) to VO2(Mx') as observed in our Raman experiments (shown in Supplementary figure 2). After releasing to ambient conditions, the reflectivity of the quenched VO2(Mx') phase is nearly the same as the original VO2(Mx') phase.


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Figure 6. (a) XRD patterns and (b) Raman spectra of VO2 (Mx') upon annealing at different temperatures in a vacuum for 30 min, (c) Density-energy scheme of the VO2 system and the reversible structure switching. Thermal-driven structural phase transitions and the phase diagram. Temperature, the important tuning parameter, has great influences on the structure of substance. Therefore, investigating the structure evolutions of the VO2 (Mx') phase after treated under different temperature will provide insight into the physical property of the VO2(Mx') phase. Fig. 6a shows the XRD patterns of the VO2(Mx') phase before and after annealing at about 200 ℃, 300 ℃ and 400 ℃ for 30 min. After annealing at about 200 ℃, the intensity of the peaks is apparently strengthened without modification of the overall diffraction pattern, suggesting the increased crystallinity of the VO2(Mx') phase. After annealing at higher temperature, surprisingly, the diffraction peaks at bout 3.4, 2.8 and 2.7 Å are greatly weakened at about 300 ℃ and finally disappear up to 400 ℃, meanwhile, the intensity of all the rest diffraction peaks are further strengthened and a new peak appear at about 2.68 Å. The diffraction peak at about 3.2 Å is asymmetric and can be fitted with two peaks as shown in the inset of fig. 6a. We noticed that the diffraction pattern of the sample heat treated at about 400 ℃ presents the same characteristic


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peaks as the VO2(M1) phase, thus, a structure transition from the VO2(Mx') into the VO2(M1) phase in long-range order can be deduced. Fig. 6b show the Raman spectra of the VO2(Mx') phase after annealing under different temperatures. After annealing at 200 ℃, the intensity of all the Raman peaks is greatly increased. The typical Raman peak at about 170 cm-1 for the VO2(Mx') phase suddenly decreases to a small value after annealing at 300 ℃, and finally disappeared at about 400 ℃. The observed Raman spectrum also agree well with that of the VO2(M1) phase. These results indicate that the local structural also undergo the structural phase transition from the VO2(Mx') into the VO2(M1) phase within 300-400 ℃. The pressure and temperature-induced phase transformations among VO2 (M1), VO2 (Mx) and VO2 (Mx') are summarized in figure 6c. The VO2(Mx') phase does not transform into the VO2(M1) upon compression at ambient temperature but occurs after annealing at about 400 ℃, indicating that the VO2(Mx') phase is one of the metastable phase of VO2 and there is a higher kinetic barrier (Ebarrier) of atmic diffusion between these two phase. The phase transition temperature of VO2(Mx')-VO2(M1) is lower than the phase transition temperature from the metastable phase A and B into the VO2(M1), 450 ℃ and 500 ℃, reveling that the Ebarrier1 between VO2(Mx') and VO2(M1) phases is lower than the Ebarrier1 between VO2(Mx') and the metastable A and B phases. 4. Conclusions In summary, we reported the unique reversible pressure-induced structural phase transition and metalization of the metastable phase VO2 (Mx'). Upon compression, the VO2 (Mx') transform into an intermediate semiconducting phase VO2 (Mx'') at about 10GPa, then starts to transform into the metallic high-pressure VO2(X) phase at about 31 GPa. This structural transition process


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is quite sluggish with the VO2(Mx'') and VO2(X) phases coexist over a pressure region of 50 GPa. The semiconducting VO2(Mx'') phase being metallic at about 40 GPa that mainly associated with electron–electron correlations. In addition, the metastable phase VO2 (Mx') releases excess energy and transforms into the stable phase VO2 (M1) upon annealing at about 400 ℃ for a short time. The structural phase transition process of the metastable VO2 (Mx') under pressure is quite different with that of the other two metastable phases VO2 (A) and VO2 (B) which being amorphous upon compression. And we found that the variation of VO6 octahedron and empty spaces between the VO6 octahedrons exhibit important roles in the structure transformations of the metastable VO2 phases under pressure. Our results present new insight into the difference of the pressure-induced structural transitions in the stable and metastable phases. ASSOCIATED CONTENT Supporting information The sample preparation, simulated the TEM/SAED patterns of VO2(Mx'), and the Raman spectra collected upon decompression. AUTHOR INFORMATION Corresponding Author Quanjun Li, E-mail: [email protected], Tel: +86 18043176111, Bingbing Liu, E-mail: [email protected], Tel: +86-431-85168256, Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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This work was financially supported by the NSFC (11374120, 11874172, 11634004, 51320105007, 11804079), the National Key R&D Program of China (No. 2018YFA0305900), the National Basic Research Program of China (2011CB808200), the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1132), the Cheung Kong Scholars Program of China, and Open Project of State Key Laboratory of Superhard Materials (Jilin University). REFERENCES 1. Chen, B. R.; Sun, W.; Kitchaev, D. A.; Mangum, J. S.; Thampy, V.; Garten, L. M.; Ginley, D. S.; Gorman, B. P.; Stone, K. H.; Ceder, G.; Toney, M. F.; Schelhas, L. T., Understanding crystallization pathways leading to manganese oxide polymorph formation.

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TOC Graphic


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Figure 1. (a) Refinements result, (b) IR reflectivity Rsd(ω) and (c) transmittance spectrum T(ω) for the Mx' phase at ambient pressure. Insets of (a): HRTEM images of the Mx' phase. The Rsd(ω) and T(ω) spectra were measured in DACs. The data at about 1700-2700 cm-1 in Rsd(ω) and T(ω) were cut from the spectra due to diamond absorption. 140x64mm (300 x 300 DPI)



Figure 2. (a) XRD patterns, (b) variation of relative d-spacings, (c) variation of lattice parameters and (d) pressure versus volume data of VO2 at various pressures collected upon compression. * indicate the diffraction of high pressure X phase, # indicate the diffraction of Re. 160x111mm (300 x 300 DPI)


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Figure 3. (a) Raman spectra and (b) pressure dependencies of phonon frequencies of VO2 upon compression. 285x121mm (300 x 300 DPI)



Figure 4. IR reflectivity spectra of VO2 upon (a) compression and (b) decompression. Inset: the pressurereflectivity diagram of the wave numbers 1300 cm-1 under pressure. 140x49mm (300 x 300 DPI)


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Figure 5. The relative intensity of the diffraction peaks at about 3.2 Ǻ for the Mx'' phase and at 2.83 Ǻ for the X phase as functions of pressure. IPX and IPMx'' are the intensity of the diffraction peaks of X and Mx'' phase under pressure. Pxm and PMx''m are diffraction peaks of X and Mx'' phase at 81.3 and 30 GPa, respectively, as shown in inset. 80x60mm (300 x 300 DPI)



Figure 6. (a) XRD patterns and (b) Raman spectra of VO2 (Mx') upon annealing at different temperatures in a vacuum for 30 min, (c) Density-energy scheme of the VO2 system and the reversible structure switching. 140x63mm (300 x 300 DPI)


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Pressure-Induced Reversible Phase Transitions in a New Metastable

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