Strong Electron–Phonon Coupling in the Excitonic Insulator Ta2NiSe5

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Cite This: Inorg. Chem. 2019, 58, 9036−9042

Strong Electron−Phonon Coupling in the Excitonic Insulator Ta2NiSe5 Jian Yan,†,‡,# Ruichun Xiao,†,‡,# Xuan Luo,*,† Hongyan Lv,† Ranran Zhang,§ Yan Sun,∥ Peng Tong,† Wenjian Lu,† Wenhai Song,† Xuebin Zhu,† and Yuping Sun*,†,§,⊥

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†

Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China ‡ University of Science and Technology of China, Hefei 230026, People’s Republic of China § High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China ∥ Institute of Physical Science and Information Technology, Anhui University, Hefei 230601, People’s Republic of China ⊥ Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China S Supporting Information *

ABSTRACT: An excitonic insulating (EI) state is a fantastic correlated electron phase in condensed matter physics, driven by screened electron−hole interaction. Ta2NiSe5 is an excitonic insulator with a critical temperature (TC) of 328 K. In the current study, temperature-dependent Raman spectroscopy is used to investigate the phonon vibrations in Ta2NiSe5. The following observations were made: (1) an abnormal blue shift around TC is observed, which originates from the monoclinic to orthorhombic structural phase transition; (2) the splitting of a mode and two new Raman modes at 147 and 235 cm−1 have been observed with the formation of an EI state. With the help of first-principles calculations and temperature-dependent X-ray diffraction (XRD) experiments, it is found that the TaSe6 octahedra are “frozen” and the NiSe4 tetrahedra are greatly distorted below TC. Thus, it seems that the distortion of NiSe4 tetrahedra plays an important role in the strong electron− phonon coupling (EPC) in Ta2NiSe5, while the strong EPC, coupled with electron−hole interaction, opens the energy gap to form the EI state in Ta2NiSe5.

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INTRODUCTION Although it was first discovered half a century ago, the excitonic insulating (EI) state is still a hot topic in the condensed matter physics due to its intriguing correlated phenomenon.1−4 In comparison to the pairing of electrons resulting in the superconducting state, electron−hole bound pairs, i.e., excitons, generated by weakly screened Coulomb interactions between conduction-band electrons and valenceband holes are condensed in small band overlap semimetals or small band gap semiconductors.5 In semiconductors, a Bose− Einstein condensation (BEC) of excitons can arise when the binding energy surpasses the bandgap, while it will undergo a Bardeen−Cooper−Schrieffer (BCS) transition in semimetals.5 Although several exotic properties, for example, superfluidity6,7 and the BCS-BEC crossover,8 were predicted, to determine an EI state in experiments is still a difficult puzzle, because the excitonic instability always coexists with instabilities of other ground states such as charge density waves (CDWs) or staggered orbital orders in real materials.9−11 In recent years, there has been a growing number of promising candidates for the EI state, raising researchers’ interest to study the EI transition further.7,12,13 For example, in the transition-metal chalcogenide (TMD) semimetallic 1TTiSe2, the flattening of the valence band observed by angleresolved photoemission spectroscopy (ARPES) has been © 2019 American Chemical Society

determined as a BCS-like EI transition with a CDW transition.13 Due to the combination of CDW and EI state in 1T-TiSe2, it is difficult to explore the origin of the EI state in that material.14,15 Much attention has been paid to exploring the EI without another quantum phase transition. Ta2NiSe5 is a typical example; in comparison with 1T-TiSe2, it is less complex and more interesting because the EI phase transition temperature (TC = 328 K) is near room temperature and the EI state is not accompanied by the formation of any other quantum phase transition. In short, Ta2NiSe5 offers an excellent platform to directly investigate electronic states involved in the exciton formation. Some experimental and theoretical works have been done on the EI state of Ta2NiSe5.16−24 On the experimental side, ARPES showed that the flattening of valence band, the feature of the EI transition, occurs below TC = 328 K.16,17 Spectroscopic ellipsometry showed that there is an overlapping exciton complex in Ta2NiSe5, which is consistent with the possibility of the EI state.18,19 On a theoretical level, first-principles calculations indicated that the excitonic BEC induces the instability of the phonon mode, resulting in the structural phase transition.23 Meanwhile, the structural transition and Received: February 14, 2019 Published: June 27, 2019 9036

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band flattening are attributed to the formation of excitons between Ta 5d electrons and hybridized Ni 3d-Se 4p holes.23 Furthermore, the excitionic fluctuation induced by band flattening should be taken into account.24 However, it is hard to understand the EI phase itself. According to a theoretical study, in addition to the electron−hole interaction, electron−phonon coupling (EPC) should be also considered to realize the EI phase in Ta2NiSe5.25 Due to the strong coupling between the freedom of charge and lattice, ellipsometry experiments with the pump probe technology revealed a fingerprint toward the existence of exciton−phonon complexes in Ta2NiSe5.20,26 Recently, a scanning tunneling microscope/spectroscopy (STM/STS) study revealed that the EPC always emerges and coexists with an electron−hole interaction to open up an energy gap. However, the STM/STS experiments only focused on the interband electron−hole interaction.27 Thus, exploring the phonon effects in Ta2NiSe5 may be another effective method to exploring the origin of the EI state in Ta2NiSe5. In this article, the temperature-dependent Raman spectra of Ta2NiSe5 single crystals are studied. The obviously localized distortion of NiSe4 tetrahedra is observed, and the TaSe6 octahedra are “frozen” below the TC. The obvious distortion of NiSe4 plays an important role in strong EPC in Ta2NiSe5, which is consistent with the result of firstprinciples calculations. Furthermore, the strong EPC, coupled with electron−hole interaction, opens the energy gap to form the EI state in Ta2NiSe5.27

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Article

RESULTS AND DISCUSSION The ternary chalcogenide Ta2NiSe5 crystallizes in the monoclinic structure with a van der Waals (vdW) gap.31,32

Figure 1. (a) Crystal structure of Ta2NiSe5 viewed along the [100] and [010] directions. (b) Crystal structure of orthorhombic and monoclinic phases in Ta2NiSe5. (c) XRD pattern of the crystal measured on the cleaved surface. The inset presents a typical X-ray rocking curve of the (020) Bragg peak and a photo of the studied Ta2NiSe5 single crystal.

EXPERIMENTAL AND CALCULATION DETAILS As displayed in Figure 1a, each layer is sandwich-like; the top and bottom sheets of the layer are Se atoms and the middle sheets of Ta and Ni atoms form TaSe6 and NiSe4 units, respectively, octahedrally and tetrahedrally coordinated by the top and bottom of Se atoms. Ta2NiSe5 shows a quasi-onedimensional structure where Ta double chains and Ni single change are arranged alternately along the c axis. For Ta2NiSe5, the EI transition is accompanied by an orthorhombic to monoclinic transition.23 Figure 1b shows the crystalline structure of orthorhombic and monoclinic phases in Ta2NiSe5. As shown in Figure 1c, the XRD pattern of Ta2NiSe5 presents the (0l0) peaks of the monoclinic space group C2/c. The inset of Figure 1c presents the full width at half-maximum (fwhm) of the (020) Bragg peak with 0.1°, which indicates the high quality of the Ta2NiSe5 single crystal. The observed (0l0) Bragg peaks demonstrate that the studied Ta2NiSe5 crystal presents a highly preferential orientation along the b axis. Figure 2a,b exhibits the typical Raman spectra acquired at 5 and 350 K. At T = 5 K, there are eight prominent peaks at the Raman shifts at 98, 122, 147, 180, 195, 218, 235, and 294 cm−1, all with Ag symmetry, which is consistent with previous report.22 These peaks have been marked as Agx (x = 1−8), respectively. Meanwhile, the calculated Raman modes are shown in Table 1. When T = 350 K, only six peaks are observed at the Raman shifts of 93, 119, 171, 185, 210, and 283 cm−1. There is a small impurity peak around 106 cm−1, which is not observed at every temperature. Moreover, because of the orthorhombic to monoclinic structure transition coupling with the change of symmetry, the Raman mode has different activity,33 resulting in the Ag2 mode splitting into two modes below TC = 328 K and Ag3 and Ag7 appearing below TC on the basis of the compatibility relations.17,24 Figure 2c−f shows the temperature dependence of some typical Raman

Sample Preparation. The chemical vapor transport (CVT) method was used to grow Ta2NiSe5 single crystals with I2 as a transport agent. Ta and Ni powders (99.99%, Alfa Aesar) and Se powder (99.99%, Alfa Aesar) in the mole ratio 2:1:5 were weighted and mixed with 0.25 g of I2, and the mixtures were placed into silicon quartz tubes. All of these procedures were done in an Ar-filled glovebox. The tubes were sealed under high vacuum. The sealed quartz tubes were put in a two-zone tube furnace. The hot side was 950 °C while the cold side was 850 °C, and after 7 days, the furnace was closed and slowly cooled to room temperature. Characterization. Single-crystal and powder X-ray diffraction (XRD) experiments were performed with a PANalytical X’pert diffractometer using Cu Kα1 radiation (λ = 0.15406 nm). All of the Raman measurements were performed from 5 to 400 K by using the 780 nm laser line in a DXR Raman microscope (Thermo Scientific) with a single exposure of the CCD with a spectral resolution of 1 cm−1. Low-temperature Raman spectra were obtained on a Raman microscope (Horiba JY T64000) equipped with Janis ST-500 microscopy cryostat. Electrical transport measurements were carried out by using the Physical Properties Measurement System (PPMS-9 T) for 1.8 K < T < 400 K and H < 9 T. Calculation. The calculations of phonon vibrations were carried out using the QUANTUM-ESPRESSO package28 based on density functional perturbation theory (DFPT).29 The pseudopotentials we used were ultrasoft pseudopotentials, where the exchange-correlation interaction was treated with the general gradient approximation (GGA) with Perdew−Burke−Ernzerhof functional. An energy cutoff of 60 Ry (1500 Ry) for plane wave (charge density) and an 8 × 8 × 8 mesh of k-points were used for obtain a convergence of 10−6 Ry (5.0 × 10−4 Ry/Å) for energy (force). Before the phonon frequencies were used, we used the Broyden−Fletcher−Goldfarb−Shanno (BFGS) quasi Newton algorithm to optimize the lattice constants and ionic positions. The symmetries of the phonon vibration modes at the Γ point were analyzed using the IR Raman and Hyper-Raman Modes in Bilbao Crystallographic Server.30 9037

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Figure 2. (a, b) Collinear (XX) Raman spectra of bulk Ta2NiSe5 single crystals at 5 and 350 K, respectively. The eight peaks examined in this study are labeled by Ag1, Ag2, B2g, Ag4, Ag5, Ag6, B3g, and Ag8. (c, d) Temperature-dependent collinear (XX) Raman spectra of Ta2NiSe5 measured in the temperature range from 5 to 400 K. TC is indicated by the red dashed line. The change of the vibration mode at TC is showed by the red dashed lines. (e, f) Some typical Raman peaks above and below TC. (g−v) Temperature-dependent positions of the Raman peak and fwhm. The solid lines are fitting data according to eq 1.

As shown in Figures 2c,d, with increasing temperature from 5 to 400 K, all the Raman peaks are softened. To get more information on the Raman spectra, the Raman shifts and the

peaks from 5 to 400 K. There is an obvious variety around the TC. The following text will discuss the origin of the variety in Raman spectra. 9038

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fwhms of all peaks were obtained by the Lorentz fits. There is an obvious decrease in number of phonon modes in Ag1 and Ag2, which is consistent with an increase in the lattice symmetry due to the monoclinic to orthorhombic structural phase transition in Ta2NiSe5.17,23 The temperature-dependent Raman shifts and fwhm sof all the Raman peaks are shown in Figure 2g−v. Interestingly, it can be found that all the peaks show an unusual blue shift around the TC value. Especially, the fwhms of Ag1 and Ag2 show a modest temperature dependence, which indicates that the phonon−phonon interaction is weak for these modes.34 In addition four other Ag peaks are steadily broadened with the increasing temperature and show a sudden narrowing of fwhm at TC, suggesting that the line widths of all Ag peaks are sensitive to the structural phase transition. The temperature-dependent shifts in Raman frequencies have been further analyzed by the theoretical model. The temperature dependence of the phonon frequency can be well described by35

Table 1. Atomic Species and the Contribution of Each Atom to the Γ-Point Phonons and the Corresponding Raman Tensors for Ta2NiSe5 and Calculated and Experimental Phonon Energies for the Orthorhombic (Cmcm) Structure above TC and Monoclinic Structure (C2h) below TC atoms

irreducible representations

Ta1, Ta2, Ta3, Ta4, Ni1, Ni2, Se1, Se2, ···, Se10 Raman Tensors of C2h

11Ag + 11Au + 13Bg + 13Bu

ji 0 0 e zy R̂ Bg = jjjj 0 0 f zzzz je f 0z k {

ia d 0y R̂ A g = jjjj d b 0 zzzz k0 0 c { Raman active orthorhombic (Cmcm)

monoclinic (C2h)

symmetry

calcd (cm−1)

Ag Ag1 B2g1 Ag2 B2g2 Ag4 Ag5 Ag6 B3g Ag8

32.0 95.4 107.5 119.9 150.0 171 184.2 202.5 244.6 286.6

exptl (cm−1) (below TC) 93.8 119.2 171.2 185.8 209.8 283.23

symmetry

calcd (cm−1)

exptl (cm−1) (above TC)

Ag Ag1 Ag Ag2 Ag3 Ag4 Ag5 Ag6 Ag7 Ag8

32.3 91.7 108.7 119.9 149.5 184.2 202.9 222.3 244.8 286.5

98 106.5 122.5 147.1 179.9 195.5 217.9 235.9 294.3

i 2 ω(T) = ωB + Ajjj1 + x e + k

where x =

Ag1 Ag2 Ag3 Ag4 Ag5 Ag6 Ag7 Ag8

A (cm−1) −0.635 −0.425 −0.338 −1.745 −1.889 −2.081 −2.365 −4.447

± ± ± ± ± ± ± ±

0.02 0.01 0.02 0.04 0.04 0.08 0.01 0.11

ωB (cm−1) 99.66 122.44 147.37 180.13 195.67 219.00 237.87 297.07

± ± ± ± ± ± ± ±

(1)

and ωB is the harmonic phonon frequency at 0

K. The factor A represents an anharmonic contribution to the frequency resulting from the decay of an optical phonon into two acoustic phonons. When T = 0 K, ω(0) is equal to ωB + A, which indicates that A represents a third-order correction to the phonon self-energy. According to eq 1, the Raman shifts of all the peaks can be well fitted from 5 to 350 K; the fitting results are displayed in Figure 2g−j,o−r (the red lines of the figures). The extracted values of A and ωB are summarized in Table 2. In combination with the significant change in fwhm, large values of A in Ag4−Ag8 modes reflect the strong coupling in Ta2NiSe5, which has been proved by optical conductivity spectra.20 Large values of A also suggest that phonon interactions dominate the anharmonic coupling. Furthermore, it is clear that most of the modes present a kink around TC ≈ 328 K and deviate from eq 1 fitting above TC, which indicates that the structural phase transition is apparently optically sensitive.26 Let us try to understand the possible origin of the abnormal Raman spectra in Ta2NiSe5. First, the Ag2 mode splits into two modes below TC = 328 K and Ag3 and Ag7 appear below TC.

Table 2. Fitting Parameters of A and ωB for All of the Raman Active Modes mode

ℏωB 2kBT

yz zz 1{

0.1 0.04 0.06 0.17 0.14 0.35 0.26 0.16

Figure 3. Calculated Raman vibration modes of Ag1, Ag2, Ag3, Ag4, Ag5, Ag6, Ag7, and Ag8. 9039

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along the ac plane; there is an obvious kink related to the EI state around TC.16,17 Above TC, the resistivity is modestly temperature dependent, which means a nearly zero energy gap with an order of ∼0.05 eV. Below TC, the resistivity increases rapidly, suggesting that an energy gap is opening. An instructive data processing of the activation energy (with the form of Eρ = −kBT2(∂ ln ρ/∂T)) shows the EI transition. The temperature-dependent Eρ is shown in the right axis of Figure 4a. Eρ reflects the activation energy EA and shows a clear jump at TC, indicating a gap in the order of an eV opening due to the formation of an EI state. With regard to the transition from an orthorhombic phase at high temperature to a monoclinic phase at low temperature, the angle between b and c axes changes from 90° to 90.53°. Furthermore, the Ta atoms are displaced along the Ta chains, as shown in Figure 4b,c. Because the displacement does not influence the interlayer coupling, the layer-dependent Raman spectra do not show any changes.22 For the abnormal Raman spectra, other factors should be considered. According to the first-principles calculations, the Raman mode changes from B2g to Ag with a change from the orthorhombic phase to the monoclinic phase. We pay attention to the Raman mode of Ag3, which translates into the B2g mode above TC. As shown in Figure 4d−g, with a decrease in temperature, the vibrations of Ni atoms strengthen and the vibrations of nearest Se atoms deflect, resulting in the distortion of NiSe4 tetrahedra. In other words, the vibrations of TaSe6 octahedra nearly remain “frozen” through the whole temperature range. This means that the distortion of NiSe4 tetrahedra is sensitive to the optical response, indicating that there exists strong electron-optical-phonon coupling in Ta2NiSe5, which are also observed by other measurements.20,26 More importantly, the structural phase distortion in Ta2NiSe5 is known to result from the overall distortion of Ta chains.23 In comparison with TaSe6 octahedra, NiSe4 tetrahedra have been much more distorted because both sides of the Ta atoms are displaced, which is verified by our temperature-dependent XRD experiments (Supporting Information). Therefore, the distortion of NiSe4 tetrahedra plays a vital role in the strong EPC in Ta2NiSe5, while the strong EPC, coupled with electron−hole interactions, opens the energy gap to form the EI state in Ta2NiSe5. This result is also verified by a recent STM/STS study.27

Figure 4. (a) Temperature-dependent longitudinal resistivity of Ta2NiSe5 single crystals. The temperature dependence on the activation energy Eρ obtained from Eρ = −kBT2(∂ ln ρ/∂T) is shown in the right axis. The inset shows the schematic diagram of the opening gap through the EI state. (b, c) Schematic representations of the three-chain Hubbard model with Ta chain distortion corresponding to the structural transition temperature. (d−g) The Raman active vibration modes of Ag3 and B2g.

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CONCLUSION In summary, the phonon vibrations in Ta2NiSe5 have been studied by measuring the temperature-dependent Raman spectra. An abnormal blue shift around TC is observed, which originates from the monoclinic to orthorhombic structural phase transition and the appearance of an EI state. Meanwhile, the splitting of one mode and two new Raman modes at 147 and 235 cm−1, respectively, have been observed with the formation of EI state. On the basis of first-principles calculations and temperature-dependent XRD experiments, the TaSe6 octahedra are “frozen” and the NiSe4 tetrahedra are much more distorted below TC. It is suggested that the distortion of NiSe4 tetrahedra plays an important role in revealing the EI state in Ta2NiSe5.

There are many possibilities for the appearance of other vibration modes in Raman spectra: first, there exist some novel Raman modes from monolayers to multilayers, such as MoSe2;36 second, the formation of an another state, such as a CDW state, will also lead to a fresh Raman peak emergence. For example, there are two fresh peaks at 70 (Eg) and 110 (A1g) cm−1 in 1T-TiSe2 below the CDW temperature.37 The layer-dependent Raman spectra do not show any changes;22 on the other hand, an EI state appears in Ta2NiSe5 below TC. Thus, we deduce the splitting of the Ag2 mode is related to the appearance of the EI state in Ta2NiSe5. Second, we obtained the Raman vibration modes from the first-principles calculations, which are shown in Figure 3. Third, the transport properties of Ta2NiSe5 have also been considered. Figure 4a shows the temperature-dependent longitudinal resistivity ρ

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00432. 9040

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(10) Kuneš, J. Excitonic condensation in systems of strongly correlated electrons. J. Phys.: Condens. Matter 2015, 27, 333201. (11) Ejima, S.; Kaneko, T.; Ohta, Y.; Fehske, H. Order, Criticality, and Excitations in the Extended Falicov-Kimball Model. Phys. Rev. Lett. 2014, 112, 026401. (12) Neuenschwander, J.; Wachter, P. Pressure-driven semiconductor-metal transition in intermediate-valence TmSe1‑xTex and the concept of an excitonic insulator. Phys. Rev. B: Condens. Matter Mater. Phys. 1990, 41, 12693. (13) Cercellier, H.; Monney, C.; Clerc, F.; Battaglia, C.; Despont, L.; Garnier, M. G.; Beck, H.; Aebi, P.; Patthey, L.; Berger, H.; et al. Evidence for an Excitonic Insulator Phase in 1T-TiSe2. Phys. Rev. Lett. 2007, 99, 146403. (14) Zenker, B.; Fehske, H.; Beck, H.; Monney, C.; Bishop, A. R. Chiral charge order in 1T-TiSe2: Importance of lattice degrees of freedom. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 075138. (15) Kaneko, T.; Zenker, B.; Fehske, H.; Ohta, Y. Competition between excitonic charge and spin density waves: Influence of electron-phonon and Hund’s rule couplings. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 115106. (16) Wakisaka, Y.; Sudayama, T.; Takubo, K.; Mizokawa, T.; Arita, M.; Namatame, H.; Taniguchi, M.; Katayama, N.; Nohara, M.; Takagi, H. Excitonic Insulator state in Ta2NiSe5 probed by photoemission spectroscopy. Phys. Rev. Lett. 2009, 103, 026402. (17) Wakisaka, Y.; Sudayama, T.; Takubo, K.; Mizokawa, T.; Saini, N. L.; Arita, M.; Namatame, H.; Taniguchi, M.; Katayama, N.; Nohara, M.; et al. Photoemission Spectroscopy of Ta2NiSe5. J. Supercond. Novel Magn. 2012, 25, 1231. (18) Larkin, T. I.; Yaresko, A. N.; Pröpper, D.; Kikoin, K. A.; Lu, Y. F.; Takayama, T.; Mathis, Y. L.; Rost, A. W.; Takagi, H.; Keimer, B.; et al. Giant exciton Fano resonance in quasi-one-dimensional Ta2NiSe5. Phys. Rev. B: Condens. Matter Mater. Phys. 2017, 95, 195144. (19) Sugimoto, K.; Kaneko, T.; Ohta, Y. Microscopic quantum interference in excitonic condensation of Ta2NiSe5. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 041105. (20) Sugimoto, K.; Nishimoto, S.; Kaneko, T.; Ohta, Y. Strong Coupling Nature of the Excitonic Insulator State in Ta2NiSe5. Phys. Rev. Lett. 2018, 120, 247602. (21) Werdehausen, D.; Takayama, T.; Höppner, M.; Albrecht, G.; Rost, A. W.; Lu, Y.; Manske, D.; Takagi, H.; Kaiser, S. Coherent order parameter oscillations in the ground state of the excitonic insulator Ta2NiSe5. Sci. Adv. 2018, 4, eaap8652. (22) Kim, S. Y.; Kim, Y.; Kang, C.-J.; An, E.-S.; Kim, H. K.; Eom, M. J.; Lee, M.; Park, C.; Kim, T.-H.; Choi, H. C.; et al. Layer-confined Excitonic Insulating phase in ultrathin Ta2NiSe5 crystals. ACS Nano 2016, 10, 8888. (23) Kaneko, T.; Toriyama, T.; Konishi, T.; Ohta, Y. Orthorhombicto-monoclinic phase transition of Ta2NiSe5 induced by the Bose− Einstein condensation of excitons. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 035121. (24) Seki, K.; Wakisaka, Y.; Kaneko, T.; Toriyama, T.; Konishi, T.; Sudayama, T.; Saini, N. L.; Arita, M.; Namatame, H.; Taniguchi, M.; et al. Excitonic Bose−Einstein condensation in Ta2NiSe5 above room temperature. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90, 155116. (25) Murakami, Y.; Golež, D.; Eckstein, M.; Werner, P. Photoinduced enhancement of excitonic order. Phys. Rev. Lett. 2017, 119, 247601. (26) Nakano, A.; Hasegawa, T.; Tamura, S.; Katayama, N.; Tsutsui, S.; Sawa, H. Antiferroelectric distortion with anomalous phonon softening in the excitonic insulator Ta2NiSe5. Phys. Rev. B: Condens. Matter Mater. Phys. 2018, 98, 045139. (27) Lee, J.; Kang, C -J; Eom, M. J.; Kim, J. S.; Min, B. I.; Teom, H. W. Strong interband interaction in the excitonic insulator phase of Ta2NiSe5. Phys. Rev. B 2019, 99, 075408. (28) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L; Cococcioni, M.; Dabo, I.; Dal Corso, A.; de Gironcoli, S.; Fabris, S.; Fratesi, G.; Gebauer, R.;

Temperature-dependent XRD experiments, Rietveld refinement, structural diagram below (LT) and above (HT) TC, atomic coordinates for Ta2NiSe5, and experimental Ta−Se distances and Ni−Se distances (PDF) Accession Codes

CCDC 1899479 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for X.L.: [email protected]. *E-mail for Y.S.: [email protected]. ORCID

Xuan Luo: 0000-0002-0482-4096 Xuebin Zhu: 0000-0001-9484-8253 Author Contributions #

J.Y. and R.X. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the National Key Research and Development Program under contract 2016YFA0300404, the National Natural Science Foundation of China under contracts 11674326 and 11874357, the Joint Funds of the National Natural Science Foundation of China and the Chinese Academy of Sciences’ (CAS) Large-Scale Scientific Facility under contract U1832141, the Users with Excellence and Scientific Research Grant of Hefei Science Center of CAS (2018HSC-UE011), and the Key Research Program of Frontier Sciences, CAS (QYZDB-SSW-SLH015). The authors thank Dr. Chen Sun for her assistance in editing this manuscript.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.9b00432 Inorg. Chem. 2019, 58, 9036−9042

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Inorganic Chemistry Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; MartinSamos, L.; Marzari, N.; Mauri, F.; Mazzarello, R.; Paolini, S.; Pasquarello, A.; Paulatto, L.; Sbraccia, C.; Scandolo, S.; Sclauzero, G.; Seitsonen, A. P.; Smogunov, A.; Umari, P.; Wentzcovitch, R. M.; et al. QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials. J. Phys.: Condens. Matter 2009, 21, 395502. (29) Baroni, S.; de Gironcoli, S.; Dal Corso, A.; Giannozzi, P. Phonons and related crystal properties from density-functional perturbation theory. Rev. Mod. Phys. 2001, 73, 515. (30) Aroyo, M. I.; Orobengoa, D.; de la Flor, G.; Tasci, E. S.; PerezMato, J. M.; Wondratschek, H. Brillouin-zone database on the Bilbao Crystallographic Server. Acta Crystallogr., Sect. A: Found. Adv. 2014, 70, 126. (31) Sunshine, S. J. I. C. Structure and physical properties of the new layered ternary chalcogenides tantalum nickel sulfide (Ta2NiS5) and tantalum nickel selenide (Ta2NiSe5). Inorg. Chem. 1985, 24, 3611. (32) Di Salvo, F. J.; Chen, C. H.; Fleming, R. M.; Waszczak, J. V.; Dunn, R. G.; Sunshine, S. A.; Ibers, J. A. Physical and structural properties of the new layered compounds Ta2NiS5 and Ta2NiSe5. J. Less-Common Met. 1986, 116, 51. (33) See a detailed analysis in http://gernot-katzers-spice-pages. com/character_tables/D2h.html. (34) Joshi, J.; Stone, I. R.; Beams, R.; Krylyuk, S.; Kalish, I.; Davydov, A. V.; Vora, P. M. Phonon anharmonicity in bulk TdMoTe2. Appl. Phys. Lett. 2016, 109, 031903. (35) Balkanski, M.; Wallis, R. F.; Haro, E. Anharmonic effects in light scattering due to optical phonons in silicon. Phys. Rev. B: Condens. Matter Mater. Phys. 1983, 28, 1928. (36) Soubelet, P.; Bruchhausen, A. E.; Fainstein, A.; Nogajewski, K.; Faugeras, C. Resonance effects in the Raman scattering of monolayer and few-layer MoSe2. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 155407. (37) Duong, D. L.; Ryu, G.; Hoyer, A.; Lin, C.; Burghard, M.; Kern, K. Raman characterization of the charge density wave phase of 1TTiSe2: From bulk to atomically thin layers. ACS Nano 2017, 11, 1034.

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DOI: 10.1021/acs.inorgchem.9b00432 Inorg. Chem. 2019, 58, 9036−9042