Symmetry-Controlled Structural Phase Transition Temperature in

Nov 15, 2016 - by chemical doping is becoming of fundamental and technological interest. Here ... effect of Cr-doping on the phase transition of VO2 i...
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Symmetry-Controlled Structural Phase Transition Temperature in Chromium-Doped Vanadium Dioxide Xiaogang Tan,†,‡,§ Wei Liu,†,§ Ran Long,‡ Xiaodong Zhang,‡ Tao Yao,*,† Qinghua Liu,† Zhihu Sun,† Yuanjie Cao,† and Shiqiang Wei*,† †

National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, China

‡

ABSTRACT: Understanding the key factor controlling phase transitions of correlated oxides by chemical doping is becoming of fundamental and technological interest. Here, we report vanadium chain symmetry as an effective means of mediating the structural phase transition (SPT) temperature in hydrothermal Cr-doped VO2. In-situ X-ray absorption fine structure spectroscopy unveils that high structural symmetry with equidistant 2.93 Å V−V zigzag chains, along with the dimerized V−V straight chain induced by Cr-doping, can stabilize VO2 at elevated temperature, thus raising the critical temperature of the VO2 phase transition. The Cr-doped VO2 system exhibited a SPT process involving lattice expansion, accompanied by V−V chain reconstruction into the rutile phase. These findings provide novel insights and guidance in chemically tailoring the phase transition of VO2.

1. INTRODUCTION Vanadium dioxide (VO2), a prototypical strongly correlated material, is known for its striking first-order metal−insulator transition (MIT) accompanied by a structural phase transition (SPT) from monoclinic (M1) to rutile (R).1−4 Thus, VO2 has been recognized as a popular candidate for ultrafast optical and electrical switching applications.5,6 Even though VO2 has received intense attention, experimental difficulties in identifying the MIT process as well as the involvement of multiple competing insulating phases have led to persistent controversy on its nature for decades.7−9 The transitional critical temperature (TC) of VO2 in bulk form is at about 341 K, and it can be varied across a wide range by the tensile stress or impurity doping.10−13 For example, donor impurity doping, such as with tungsten, yields a pronounced reduction of TC by ca. 20 K/ atom % W for the bulk, the picture of which is clearly described by the formation and propagations of the W-encampassed rutile-like nuclei.11,12 The second class of doping (acceptor impurity), of which the most studied member is chromium, could lead to a complex phase diagram with increment of TC (as roughly depicted in Figure 1a).14−16 However, in contrast to the intense studies on W-doping, the investigation on the effect of Cr-doping on the phase transition of VO2 is rather scarce in recent years. Insight into the rich physics of Cr-doped VO2 would largely deepen our understanding of the nature of the phase transition of VO2. It has been reported that the acceptor impurity of Cr has minor effects on conductivity but gives rise to significant structural modifications.15,16 It is known that Cr-doping would stabilize other monoclinic insulating phases, namely, M2, M3, and M4, mainly characterized by the different arrangements of the vanadium chains along the cR axis,15 the properties of which © XXXX American Chemical Society

are markedly different from those of the M1 phase of pure VO2. Particularly, the known insulating M2, in which only one set of chains dimerize (Figure 1b), is usually observed in the lowcontent Cr-doped (3%) shows marked variation and controversy. Moreover, compared with the Wdoping case, the conflicting structural determination, as well as the underlying mechanism correlated with the influence of Crdoping on the SPT of VO2, still remains to be resolved. In order to attain physical insights into Cr-induced TC increase, an accurate atomic structure determination, along with an in situ investigation of the atomic evolutions during the SPT process, would clarify the critical parameter that controls the VO2 phase transition. In this work, we report an effective means of mediating the SPT temperature in Cr-doped VO2, by gaining insights into the nature of the influence of Cr-doping on the VO2. We present the atomic kinetics across the SPT in Cr-doped VO2 with different doping concentration, by exploring an in situ temperature-dependent X-ray absorption fine structure (XAFS) technique that has demonstrated high sensitivity to detecting the structural signatures of VO2.23 In situ XAFS measurements were performed in transmission mode at the U7B and U7C stations of the National Synchrotron Radiation Received: August 25, 2016 Revised: October 27, 2016 Published: November 15, 2016 A

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Figure 1. (a) T−x(Cr) phase diagram of V1−xCrxO2 from refs 14 and 15 and our work. R, M1, M2, M3, and M4 indicate the rutile and four monoclinic phases. (b) Different arrangements of vanadium ions in the R, M1, and M2 phases. (c) XRD patterns of V1−xCrxO2 (x = 3.5% and 10%) samples, pure VO2, and relative standard references. (d) Twenty-times circle DSC measurement results of V1−xCrxO2 samples and pure VO2.

where the TC is nearly independent of the x at low doping level and increases with x at heavy doping levels.15 The initial crystalline structures of Cr-doped VO2 samples were determined by the high-resolution X-ray diffraction (XRD) measurements at room temperature (Figure 1d). It can be found that the peak patterns for both Cr-doped VO2 samples are quite different from that of pure monoclinic M1 VO2 or that of pure rutile VO2, indicating the formation of a new structural symmetry induced by Cr-doping. A deep inspection of the diffraction peaks at 27.4°, 28.5°, 54.6°, and 56.0° (in the dotted frame) reveals that the XRD patterns of Cr0.035V0.965O2 and Cr0.1V0.9O2 samples resemble that of standard patterns (No. 24-0317). No extra diffraction peaks from any Cr-related secondary phases or impurities can be detected in the XRD patterns, suggesting the substitutional doping of Cr into the VO2 lattice. The XRD results indicate that the Cr0.035V0.965O2 and Cr0.1V0.9O2 are highly crystalline and belong to the orthorhombic lattice,24,25 instead of the monoclinic (M2) symmetry commonly reported for Cr−VO2 system previously. Upon increasing the Cr-doped content, the diffraction peaks shifted slightly toward the low-angle side, implying the structural expansion. Indeed, the newly formed orthorhombic lattice is close to the M2 structure with similar vanadium local structure characteristics in the (101) plane but different lattice symmetry. Moreover, this structural change is

Laboratory (NSRL) in China. A structural model with high long-range orthorhombic symmetry, but differing from M1, M2, and R phase structures, has been put forwarded. From the point of structure, the new V−V chain arrangement with high symmetry stabilizes the VO2 at elevated temperature, thus leading to an increase of TC in the Cr−VO2 system.

2. RESULTS AND DISCUSSION Highly crystalline Cr-doped VO2 was prepared by a hydrothermal reaction followed by an annealing method. A properly proportioned mixture of vanadyl acetylacetonate [VO(acac)2] and chromic chloride (CrCl3) was dissolved in an aqueous solution of ethylene glycol and was heated at 200 °C for 24 h. The resulting product was then calcined at 500 °C for 12 h under high-purity Ar to obtain the Cr-doped VO2 samples. Element analysis by ICP determined the Cr-doped concentration of two samples as about 3.5% and 10%. The TC values for the Cr1−xVxO2 (x = 3.5% and 10%) were determined by DSC measurements. As shown in Figure 1c, both of the asprepared Cr-doped VO2 samples display a distinct temperaturedriven phase transition. Increasing Cr content from 3.5% to 10% significantly increases TC from 80.8 to 111.5 °C, respectively, implying an approximate temperature rise rate of 4.7 K/atom % Cr. This is consistent with the previous reports, B

DOI: 10.1021/acs.jpcc.6b08586 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. (a) The V K-edge EXAFS oscillations χ(k) for Cr0.035V0.965O2. Fourier transforms (FTs) for Cr0.035V0.965O2 (b) and Cr0.1V0.9O2 (c). (d) Evolution of magnified FTs of V−V1 and V−V2 coordination peaks for Cr0.035V0.965O2.

agreement with the orthorhombic structure observed in XRD results. However, this highly symmetric structure is also different from the rutile phase, which can be reflected by the lowered and high-R-shifted V−V2 peak, as well as the low-Rshifted V−V1 peak for the FT curves of two samples below the TC. Typically, at about 3 °C around TC, the χ(k) function within the k range of 4−6 and 8−10 Å−1 for Cr0.035V0.965O2 exhibits a gradual change into that of the rutile phase (Figure 2a). Correlated evolutions can be identified in FTs profiles (Figure 2b,c). By increasing the temperature across the TC, the position of the first V−O peak remains almost unchanged, while the intensity of the V−O and V−V2 coordination peaks increases during the phase transition process, confirming the increased symmetry of the rutile phase with temperature. More importantly, taking the Cr0.035V0.965O2 sample as an example, both V−V1 and V−V2 coordination peaks display an evolution of high-R shift below 77 °C, followed by low-R shift above 79 °C, as shown in Figure 2d. This demonstrates that the Crdoped VO2 undergoes a continuous lattice expansion near TC, followed by a contraction after the phase transition.

distinctly different from the case of W-doping, which induces the local structural transition into tetragonal-like structure around the dopants.11 Hence, the Cr dopants are substituted into the VO2 lattice and introduce a new long-range-ordered structure. In order to unravel the influence of Cr-doping on the local structure of VO2 below the TC as well as the structural evolution during the SPT process, we performed in situ variable-temperature XAFS measurements on V atom. XAFS data were collected in transmission mode with a fixed-exit Si(111) flat double crystals as a monochromator at U7B and U7C stations of NSRL. The XAFS oscillation χ(k) functions and their Fourier transforms (FTs) are shown in Figure 2. It can be found that two samples below the TC exhibited similar FT features in terms of a single first nearest V−O at about 1.50 Å, a V−V1 peak at ca. 2.37 Å, and a V−V2 peak at ca. 3.07 Å. These are different from the spectrum of pure M1 phase, which displays the distinct V−O splitting peaks at 1.20 and 1.63 Å (Figure 2b,c) but is approaching the spectrum of pure R phase, indicating that the Cr-doping leads to a higher structural symmetry in relation to the M1 phase for the host VO2, in C

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Figure 3. Bond lengths as a function of temperature as obtained by the EXAFS fitting results of (a) pure VO2, (b) Cr0.035V0.965O2, and (c) Cr0.1V0.9O2.

The quantitative structural information is obtained from EXAFS data fitting using the ARTEMIS module of IFEFFIT.26 Considering the similarity between M2 and orthorhombic VO2, the M2 (C2/m) and R (P42/mnm) phase structures were used as the models for fitting the EXAFS data of sample at the temperatures below and above the TC, respectively. Here, the fitted bond lengths (R) of Cr0.035V0.965O2 and Cr0.1V0.9O2 as functions of temperature are summarized in Figure 3. We can find that the V−V1 coordination along the V atom chain is contributed by three types of V−V pairs, including the equidistant V−V bonds in the zigzag chain [noted as V− V1(zz)] and the long and short V−V bonds in the linear chain [noted as V−V1(sl) and V−V1(ss), respectively]. Upon increasing the Cr-doping concentration from 3.5% to 10%, the lattice structure displays expansion, which can be confirmed by the increased average V−V bond lengths. In order to rule out the effect of the thermal expansion, V−V and V−O bond lengths vs temperature for pure VO2 are also shown in Figure 3a for comparison. It can be found that the neighbor coordination displays contraction upon increasing the temperature, which is due to the structural evolution from M to R phase, as reflected by the decreased V−V and V−O bond lengths. With an increase in the temperature, these three V−V1 bonds gradually merged into one V−V1 bond of 2.87 Å in the R phase. Meanwhile, the lattice expansion near TC followed by the contraction can be reflected by the evolution of the V−V bond lengths during the SPT process. Combining the above structural analysis, we reproduced a scenario where the Cr0.035V0.965O2 was gradually changed from the low-temperature orthorhombic phase, to the intermediate state near TC, and finally to the high-temperature R-phase, as shown in Figure 4. It is reported that the acceptor impurity Cr enters as Cr3+ ions, which could stabilize the V−V dimer pair and strengthen the electron localization due to the higher electronegativity of the Cr3+ ion.15,20 This leads to the formation of the short V−V dimer pair [V−V(ss)], where V 3d electrons were localized within the dimer pair. On the other hand, for charge compensation, electron loss of V ions to Cr3+

Figure 4. Models of the structural phase transition of Cr0.035V0.965O2 during the heating process. Two kinds of V−V chains along with their bond lengths are shown.

ions would reduce the pairing ability of the adjacent V−V chain and lead to the formation of an equidistant zigzag chain [V− V(zz)]. As the temperature increased near TC, the V−V pairs on the central V−V straight chain gradually expand, as reflected by the increase of the average V−V bond lengths. D

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reaction followed by an annealing method. In a typical synthesis, a mixture in proper proportion of vanadyl acetylacetonate [VO(acac)2] and chromic chloride (CrCl3) was dissolved and stirred in an aqueous solution of ethylene glycol. The resulting mixture was transferred into a Teflon-lined stainless steel autoclave. The autoclave was heated at 200 °C for 24 h. After cooling, the resulting product was washed, dried, and then calcined at 500 °C for 12 h under high-purity Ar. The obtained samples were pressed into a thin film with an optimum absorption thickness for XAFS measurement. The crystalline structure of the samples was characterized by XRD with a Japan Rigaku D/max rA X-ray diffractometer equipped with graphite-monochromatized high-intensity Cu Kα radiation (λ = 1.541 78 Å). In Situ XAFS and Analysis. Temperature-dependent in situ XAFS measurements were performed in transmission mode at U7B and U7C stations at NSRL. The electron-beam energy of NSRL was 0.8 GeV and the maximum stored current was about 200 mA. A fixed-exit Si(111) flat double crystal was used as a monochromator. The high-precision temperature controller enables us to control the temperature variation within ±0.2 °C. The XAFS signal of V K-edge was collected in transmission mode with ionization chambers during the variation of the temperature. The in situ EXAFS data were analyzed by using the ATHENA module implemented in the IFEFFIT package. The theoretical scattering amplitude and phase shift functions for all the paths were calculated by using the FEFF7 code. The fitting was done in the R range of 1.0−3.6 Å for all the data. The XAFS spectra below and above the TC can only be fitted well using the structural model of monoclinic VO2 (C2/m) and tetragonal VO2 (P42/mnm).

Correspondingly, the zigzag-shaped V−V chain tends to be straightened. Above TC, the paired V−V linear chain evolved into an identical V−V linear chain, and the average V−V bond lengths decreases from 2.92 Å of the intermediate state to 2.87 Å of the R phase (Figure 4), indicating the obvious contraction of the crystal lattice. This lattice expansion followed by contraction during SPT is consistent with the evolution of lattice volume vs temperature reported previously.16 Furthermore, we will discuss the reason for the Cr-dopinginduced increment of TC for VO2 from the perspective of the relationship between the physical phase and symmetry. In contrast to the case of W-doping, which drives the nearby symmetric monoclinic VO2 lattice toward the rutile phase, Crdoping induces a global SPT of the VO2 lattice. It should be noted that the structure of our Cr-doped VO2 system can be characterized by a symmetry-central point “C” seen from the {101} plane (Figure 4). Centering about such a point, we can find that the structure of Cr-doped VO2 is symmetric along the (100) and (001) directions. This symmetric center cannot be found in either the M1 or M2 phase. Therefore, the orthorhombic structure of Cr-doped VO2 exhibited higher symmetry than the M1 structure of pure VO2. Moreover, upon increasing the Cr-doping concentration from 3.5% to 10%, the V−V chain displays a slight expansion, which can be confirmed by the increased V−V bond length, in agreement with the XRD results and the previous report by Goodenough et al. The discrepancies among the V−V bond lengths for Cr0.1V0.9O2 are smaller in relation to those for Cr0.035V0.965O2. This suggests that Cr0.1V0.9O2 possesses a higher structural symmetry compared with Cr0.035V0.965O2. From a thermodynamic point of view, it is well-known that for two comparable systems, the isotropic structure (i.e., higher symmetry) is more stable than the anisotropic structure (i.e., lower symmetry) at high temperature. Hence, the Cr-doped VO2 is more stable at high temperature in relation to the M1 phase. As a result, large thermal energy is required to disassociate the tighter V−V1(ss) pairs and their surrounding zigzag symmetrical V−V pairs. Consequently, the Cr-doping can significantly increase the phase transition temperature of the VO2 system. Meanwhile, the high V−V chain symmetry induced by increasing the Crdoping concentration can lead to an increment of TC. Above TC, the Cr-doped VO2 transforms into the R phase structure due to the much higher symmetry and thermal stability of the R phase.

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AUTHOR INFORMATION

Corresponding Authors

*T.Y.: e-mail, [email protected]; tel, +86-551-63601997. *S.Q.W.: e-mail, [email protected]; tel, +86-551-63601997. Author Contributions §

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (2012CB825800), the National Natural Science Foundation of China (21533007, 11422547, 21471143, 11435012, U1532265), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (11321503), the Fundamental Research Funds for the Central Universities (WK2310000050 and KY2310000019), and Youth Innovation Promotion Association CAS (CX2310000054).

3. CONCLUSION In summary, by using in situ XAFS spectroscopy, in conjunction with the XRD measurement, we present experimentally that the high symmetry from the V−V chain’s perspective arising from Cr-doping improved the high-temperature stability of the Cr-doped VO2 system and increased the TC. The detailed atomic kinetics involving the lattice expansion followed by the contraction of V−V chain in Cr-doped VO2 across the SPT process has been figured out for the first time. Our findings open new ways to the symmetric control of the SPT in VO2 and inspire further theoretical predications on tailoring the V−V chain to trigger a phase transition in VO2 or other correlated materials.

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REFERENCES

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