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Applied Materials Today 18 (2020) 100532

Contents lists available at ScienceDirect

Applied Materials Today

j ourna l h o mepage: www.elsev ier .com/ locate /apmt

ressure-induced band-gap closure and metallization in two-dimensionalransition metal halide CdI2

hipeng Yana, Ketao Yinb, Zhenhai Yuc, Xin Lia, Mingtao Lia, Ye Yuana, Xiaodong Lid, Ke Yange,iaoli Wangb,∗, Lin Wangf,∗

Center for High Pressure Science and Technology Advanced Research (HPSTAR), Shanghai 201203, ChinaSchool of Physics and Electronic Engineering, Linyi University, Linyi, Shandong 276005, ChinaSchool of Physical Science and Technology, Shanghai Tech University, Shanghai 201210, ChinaInstitute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, ChinaShanghai Synchrotron Radiation Facility (SSRF), Shanghai 201800, ChinaCenter for High Pressure Science (CHiPS), State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, Hebei 066004, China

r t i c l e i n f o

rticle history:eceived 25 October 2019eceived in revised form9 November 2019ccepted 11 December 2019

eywords:wo-dimensional transition metal halidedI2

igh pressureand gap closure

a b s t r a c t

In this letter, we present the pressure-induced evolution of the band-gap, conductance, and crystal struc-ture of layered transition metal dihalide (TMH), CdI2, an insulator at ambient conditions, using electricaltransport measurement, UV–vis absorption spectroscopy, X-ray diffraction, and Raman scattering spec-troscopy. We found that the band gap shrinks gradually following a sharp drop at 34.5 GPa. Meanwhile, thetemperature-dependent resistance indicated an insulator-to-semiconductor and then metal transitionoccurred at 36 and 62 GPa, respectively. Both X-ray diffraction and Raman scattering measurements indi-cate that the CdI2 underwent a first-order transition from a hexagonal to monoclinic phase at ∼32 GPadue to collapses in the c-lattice parameter and volume. The second structural phase transition (SPT)from monoclinic-to-tetragonal occurred at 48 GPa. The pressure-induced insulator-to-semiconductor-to-metal of CdI2 is attributed to the structural transition from hexagonal-to-monoclinic-to-tetragonal.

etallization Our first-principle calculations further confirm the sequence of the SPTs and the semiconducting andmetallic band structure of the phases, respectively. Namely, the metallization is observed due to thefilled 5p-iodide to shift and overlap with the filled 5s-cadmium band in metal phase. These findings pavethe way for investigating crystal structure evolution, and the optical and electrical properties in CdI2-typecompounds under extreme conditions.

© 2019 Elsevier Ltd. All rights reserved.

. Introduction

Transition metal dihalides (TMDs) MX2, where M stands for transition metal and X presents a halide (F, Cl, Br, I) haveeceived considerable research interests due to their pseudo-wo-dimensional structure, a variety of physical properties fromnsulating to superconducting, and a rich scenario in their phaseiagrams under external variables such as temperature and pres-ure [1–3]. Among TMDs, CdI2 has good structural stability. Itrystallizes into a hexagonal structure with I-Cd-I slabs, in which a

lice of the Cd atoms is sandwiched between two slices of I atoms inn octahedral coordination. These slabs stack along the c-axis with

Van der Waals interaction. The coulomb force is dominant in the

∗ Corresponding authors.E-mail addresses: wxl@lyu.edu.cn (X. Wang), linwang@ysu.edu.cn (L. Wang).

ttps://doi.org/10.1016/j.apmt.2019.100532352-9407/© 2019 Elsevier Ltd. All rights reserved.

interatomic binding of each layer, which is much stronger than theinterlayer interaction [4]. The significant difference of the interac-tions between the intralayer and interlayers suggests an anisotropiccompressional behavior and accompanying properties changes ofthe material.

CdI2 has been widely studied as a catalyst and optical device.It has been found that the band gap and optical properties areclosely related to the grain size and crystallinity of CdI2 [5–7].Tiyagi et al. reported that the optical band gap decreases with theincrease of grain size and film thickness [7]. The decreasing bandgap was suggested to be attributed to the grain size dependentgrain boundary barrier height for film thickness > 200 nm. Besides,after being quenched at liquid nitrogen temperature, the amor-phous CdI2 films including the Cd-I covalent bonds have splendid

transmittance below the absorption edge. This gives insights intounderstanding the anomalous behavior of crystallinity in the films[8]. Rawat et al. studied the effect of ion irradiation on the band gap

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Z. Yan, K. Yin, Z. Yu et al. / Appli

f CdI2 with compressive residual stress. They found that the directptical energy gap Eg of the ion-irradiated films, mainly in the rangef 3–3.3 eV, was dependent on the compressive residual stress ofhe film. The change in the iodine–iodine distance in the unit cellue to compressive stress could be responsible for the observedhange in the optical energy gap [9,10]. These phenomena indicatehe external stress could have a significant impact on the proper-ies of CdI2. However, all the studies have been limited to the filmhickness and grain size of CdI2. High pressure has been used as anffective and clean tool to tune the structural and physical proper-ies of materials, including 3D and 2D functional materials [11–16].o date, no study of CdI2 has been reported by compression, andhe high-pressure properties of this material are still unknown.his motivates us to present a comprehensive investigation of theorrelation between the structure and physical properties of thisaterial under high pressure.Herein, we investigated the structural transitions, optical, and

lectrical properties of 2H-CdI2 (space group P63mc [17]) up to2 GPa using diamond anvil cells. The pressure-induced narrow-

ng and eventual closure of the optical band gap was observedy ultra-vis absorption spectroscopy and transport measurements.he structural phase transitions were determined by a combinationf synchrotron powder X-ray diffraction, Raman scattering, and ab

nitio calculations. The resistance versus temperature confirmedhe metallic state at 62 GPa, which supports the pressure-induced

etallization of CdI2. This work not only certifies a variety of sig-ificant pressure effects on 2H-CdI2 crystal structure and physicalroperties, but also sheds light on the correlations of structure-roperty in two-dimensional transition metal dihalides.

. Materials and computational methods

Commercial 2H-CdI2 powder (99.5 %) was purchased and useds obtained for this work. Pressure was generated with symmetriciamond-anvil cells (DAC) using Re gaskets. The ruby fluores-ence method was employed for pressure calibration [18]. In situigh-pressure resistance measurements used an impedance sys-em below 36 GPa (Solartron-1296&1260), and the four-probesechnique by a 2410-KEITHLEY-source meter from 36 GPa to 62 GPa19]. A mixture of epoxy and cubic boron nitride was used as annsulating layer on the steel gasket to ensure electrical isolationetween the different electrodes. Four platinum electrodes andopper wires were set up to contact the sample in the chamber.o pressure medium was used to do the high-pressure electrical

esistance measurement [20]. High-pressure ultra-visible absorp-ion measurements were performed by a UV–vis Absorption andransmission Spectrometer System in a DAC with a type-II diamondp to 52 GPa [21]. Silicon oil was used as a pressure-transmittingedium. XRD measurements confirmed that the obtained powder

ample had a hexagonal structure. Representative monochromaticoom temperature (RT) high-pressure XRD patterns of CdI2 werenvestigated at the Beijing Synchrotron Radiation Facility (BSRF,W2 beamline) and Shanghai Synchrotron Radiation Facility (SSRF,5U beamline) China. Both series of experiments were performedt � =0.6199 Å and the image data were integrated using the Diop-as [22] software. Raman measurements were performed with aenishaw (UK) micro-Raman spectrometer of [23]. The system con-isted of a solid-state laser (� =532 nm) with NdYAG crystal and

thermoelectric cooled CCD detector, in which the spectral res-lution was less than 1 cm−1. Silicon oil was used as a pressureransmitting medium, and the ruby fluorescence peak position was

sed to mark the pressure [18].

The structures mentioned here were searched by the swarm-ntelligence CALYPSO method [24,25]. The total energy calculationsnd structure optimization were carried out using the plane wave

terials Today 18 (2020) 100532

basis, projected augmented wave (PAW) potentials, and gener-alized gradient approximation (GGA) with the Perdew-Burke-Ernzerh (PBE) of exchange-correlation functional as implementedin the Vienna ab initio simulation package (VASP) [26–30]. More-over, we recalculated the total energy and performed structureoptimization using the strongly constrained and appropriatelynormed (SCAN) meta-GGA functional [30,31]. The frozen-core all-electron PAW potentials were used with 4s23d10 (cutoff radius 2.3a.u.) and 5s25p5 (cutoff radius 2.3 a.u.) treated as valence electronsfor Cd and I, respectively. The Van der Waals density functional,namely optB86b-vdW, was adopted to treat dispersion forces.

3. Results and discussions

3.1. Electrical resistance

CdI2 is known to be an insulator or semi-insulator at ambi-ent conditions with very large resistivity (> 1010�·m). Impedancespectroscopy was employed to study its transport behavior in alow-pressure range. The Nyquist plots and fitted results using anequivalent circuit model are presented in Fig. 1(a)-(d) by compres-sion cycles. As shown in Fig. 1(a), at 1.7 GPa, the Nyquist plot iscomposed of an obvious semicircle in the high-frequency range andan upward slope in the low frequency range [32]. The semicirclededucted from the Z ’ -axis intercept is the resistance of charge-transfer conduction (Rg), and the slope representative ion transferconduction (Ri). The equivalent circuit model is shown in the inset,in which C1 is the capacitor, CPE1 is the constant phase angle ele-ment, and R2 is the contact resistance between the sample andelectrodes. The Zview software was used to fit the impedance spec-troscopy to extract the Rg(P). Here, the Nyquist plot was fittedby only considering the grain contribution. Below 7.8 GPa, thoseNyquist curves are shown in Fig. 1(b). With increasing pressure,the semicircle radii gradually increases and the upward slope dis-appears simultaneously. This is due to the fact that the contributionof resistance is gradually dominated by charge-transfer conduction.Above 9.2 GPa, the spectroscopy of the Nyquist plots are shown atdifferent pressures in Fig. 1(c) and (d). The intercept with the Z ’

-axis of the semicircles decreases with increasing pressure, andthe fitted curve is in good agreement with scattering data. Thus,the contribution of resistance is completely determined by charge-transfer conduction. Above 19.6 GPa, the representative radius ofthe Rg value semicircle exhibits a rapid decrease with increasingpressure up to 38.1 GPa.

The pressure dependence of the electrical resistance Rg(P) ofCdI2 is plotted in Fig. 2, in which the four-probe technique wasadopted above 46.8 GPa and up to 61 GPa, as denoted by the blackpoints. As seen in Fig. 1(a), the increase in Rg(P) (green points)indicates that the dominant contribution to conductivity switchesfrom ion diffusion to grain below 7.8 GPa. Above 9.2 GPa, the Rg(P)decreases slowly with increasing pressure and reaches a plateauup to 19.6 GPa during compression. At >19.6 GPa, the R under-goes dramatic decrease due to the collapse of c-axis. With furthercompression, the Rg(P) decreased rapidly until 46.8 GPa. To elim-inate the contact resistance R2 and obtain the Rg(P) values higherthan 46.8 GPa, we performed four-probe measurements. The Rg(P)(black points) decreased by six orders of magnitude with pres-sure up to 62 GPa, and it approached a stable value of less than4 �, suggesting the sample was metallic. To verify the conductingbehavior, the temperature-dependent resistance of the sample at

62 GPa was measured and is shown in the inset, confirming metallicbehavior. In contrast, the temperature-dependent resistance of thesample at 36 GPa shows clear semiconducting behavior, indicat-ing a pressure-induced semiconductor-to-metal transition under

Z. Yan, K. Yin, Z. Yu et al. / Applied Materials Today 18 (2020) 100532 3

Fig. 1. Pressure-dependent Nyquist plot (Z ′′ vs Z′ ) for CdI2 at (a) 1.7 GPa, (b) 1.7–7.8 GPa,

for fitting. Solid (open) symbols are for observed (fitted) results.

Fig. 2. Pressure-induced variation of electrical resistance Rg (P) of CdI2 with threedistrict phases, phase I (blue), phase II (yellow) and phase III (orange). Data forRdi

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g (P) were collected with two-probes (green dots) and four-probes (black and wineots) methods. Rg (T) − P curves with semiconducting (Left inset) and metallic (Right

nset) behaviors.

igh pressure. After decompression, the Rg (P) almost returns to itsriginal value, indicating a reversible electrical property.

.2. Ultra-visible absorption spectroscopy

Ultra-visible absorption spectroscopy was used to determinehe evolution of the band gap of CdI2 at high pressure. Fig. 3(a)hows the selected ultra-visible absorption spectrum. A sharpbsorption edge is detected at about 390.8 nm (3.17 eV)at 0.6 GPa,hich is close to that of previous reports at ambient conditions

33–35]. With increasing pressure, the absorption edge is sup-ressed to a lower-energy region (red shifts of the absorption edge),

ndicating the narrowing of the optical band gap. The absorptiondges become more and more flattened above 34.5 GPa, and even-ually shift out of the detection range of the spectrometer above

4.6 GPa.

The obtained Eg values versus fitted pressure is plotted inig. 3(c). The band gap narrows slowly until 10.2 GPa, following

slightly faster decrease with further compression up to 22 GPa.

(c) 9.2–22.5 GPa, (d) 36.0 GPa, and 38.1 GPa. Inset (a): the equivalent circuit model

Once compression reaches beyond 34.5 GPa, the Eg value shows arapid decrease from the visible region to the near-infrared region.During the whole compression process, the band gap of CdI2 under-goes a series of pressure-induced narrowing. Within the rangeof our spectrometer, Eg decreased down to 0.2 eV at 44.6 GPa,which agrees with the considerable decrease of Rg(P) by sevenorders of magnitude up to 46.8 GPa in the electrical measurement.These obvious changes imply the emergence of significant struc-tural changes, resulting in the dramatic observed electrical andoptical properties [36,37]. To monitor the evolution of the trans-parency of the experimental CdI2, we took a series of photographsof the sample under different pressures, as shown in Fig. 3(b). Uponcompression, the sample clearly transformed from transparent toopaque along with increasing pressure from 0 GPa to 38 GPa, andit changed to fully opaque at 52 GPa, confirming the band gap nar-rowing.

3.3. High-pressure X-ray diffraction study

To elucidate the unusual changes of the electrical and opticalproperties induced by pressure, which may be caused by SPTs,in situ synchrotron XRD experiments were carried out under highpressures. The selected XRD patterns up to 61 GPa are shown inFig. 4(a). At 0.3 GPa, the XRD pattern reveals that CdI2 possesses ahexagonal structure with space group P63mc(a = 4.24 Å, c = 13.67 Å),the same as in ambient conditions [38]. During compression, sev-eral new peaks were observed at 2� angle of 6◦, 13◦, 20.5◦, and22◦ (indicated by black narrows) at 34 GPa. The inset shows thedetailed changes at 2� of 6◦. The emergence of new reflections indi-cates the SPT, denoted as phase II. The new phase was determinedto be a monoclinic structure with lattice parameters of a =5.42 Å, b=4.45 Å, c =6.58 Å, � = 123.93◦, and V = 131.62 Å3. This phase tran-sition is highly consistent with the sharp narrowing of the bandgap, suggesting the abrupt band gap change at 34.5 GPa is due to astructural transition from hexagonal-to-monoclinic.

Another new peak appears at 15.1◦ (indicated by the black nar-row) under pressure up to 51 GPa, and the intensity of the new peakdramatically increases with pressure. This indicates the appearanceof the new SPT, denoted as phase III. The third phase is found to have

tetragonal symmetry with a = 3.28, c = 10.16, and V = 109.57 Å3. Thisphase is stable up to 61 GPa, where the sample was found to bemetallic, suggesting that the semiconductor-to-metal transition isdue to the phase transition from monoclinic to tetragonal.

4 Z. Yan, K. Yin, Z. Yu et al. / Applied Materials Today 18 (2020) 100532

Fig. 3. (a) Selected optical absorption spectra of CdI2 under high pressure. (b) Optical microscopic images for CdI2 upon compression. (c) Band gap evolution of CdI2 underhigh pressure up to 40 GPa. The black (red) symbols are for compression (decompression).

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ig. 4. (a)Room temperature synchrotron (� =0.6199 Å) XRD patterns for CdI2 underf new peak around 32 GPa. (b)The calculated XRD patterns with three space group

.4. Raman vibrational spectroscopy

To confirm the SPTs as observed in XRD, Raman measurementsere performed under high pressure. For layered CdI2 at ambi-

nt conditions, the Raman modes corresponding to the in-plane E

ression up to 61 GPa. The phase transition is indicated by arrows. Inset: Emergencec, C2/m and I4/mmm. Re is marked as peak from gasket.

(E22g , E1

2g , E1g) and the out-of-plane A (A11g , A2

1g)were predicted from

a group-theoretic analysis at � point23. Here, we observed threeobvious and strong Raman peaks E2

2 (20.4 cm−1), E11 (49 cm−1), and

A11 (117.9 cm−1), and two weak peaks appearing at 19.7 cm−1 and

Z. Yan, K. Yin, Z. Yu et al. / Applied Materials Today 18 (2020) 100532 5

Fig. 5. (a) The in-plane phonon modes E22g

, E12g

, E1g and the out-of -plane phonon mode A1g at ambient conditions.(b)Raman spectra of CdI2 at various pressures up to 50.1 GPaduring both compression and decompression.

F phasec ction

9bplttbstA

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ig. 6. (a)The schematic crystal structure of layered CdI2 structures for phase-I,

ompression. (c) The enthalpy and (d) volume data of three phases for CdI2 as a fun

8 cm−1, respectively. Our result is consistent with previous reportsy Cingolani et al. [23]. The E modes vibrations are formed by thelane between Cd and I atoms. The vibration mode of the rigid-

ayer corresponds to the Van der Waals force at the � point. Hence,he Raman peaks should be at a lower frequency than the vibra-ion mode of the rigid-layer. The A modes reflect the vibration ofoth the I atoms along with the c-axis [39]. We summarized thechematic graphs in Fig. 5(a).The E2

2g , E12g, and E1g modes are from

he vibrations in both the Cd and I atoms in the inter-layer, and the11g , A2

1g modes are only from the I atoms along the c-axis.As shown in Fig. 5(b), there is no obvious change below

0 GPa. Two originally overlapped E2g vibration modes start toe well separated under high pressure, suggesting the anisotropicompressional behavior of 2D material. The new peaks appeart 17.6 cm−1, 28.1 cm−1, and 42.2 cm−1 when pressure reaches2.4 GPa. The emergence of new Raman modes signifies the occur-

ence of SPT, which is consistent with the first SPT observed inRD under the same pressure. The three strongest peaks show bluehift with increasing pressure, showing the lattice becomes stiffens.

-II, and phase-III. (b) The schematic diagram of the phase transition path duringof pressure calculated by first-principles methods.

All of the Raman peaks become much weaker above 34.5 GPa andfinally disappear above 50.1 GPa. In general, there are two inter-pretations of the disappearance of Raman modes: (i) a structuraltransition to new phase in which no Raman modes is allowed bysymmetry; or (ii) an electronic transition to a metal in which theefficiency of the Raman mode becomes extremely small due to thelimited penetration depth of the exciting laser [40]. According tothe aforementioned XRD measurements, the second SPT occursat 48 GPa. Also, the electrical measurements confirm the metallicbehavior in Rg(T) at 61 GPa. Consequently, the diminished Ramanpeaks not only signify the second SPT but also support the pressure-induced metallization in the third phase.

3.5. Ab initio calculation

Using CALYPSO, we searched the structures of CdI2 with simu-

lation cell sizes of 1–4 formula units(f.u.) in the pressure range of0−70 GPa. As shown in Fig. 6(a), our structure simulations uncov-ered a hexagonal phase (space group: P63mc phase I) at ambientpressure, which is consistent with our experiment. In the P63mc

6 Z. Yan, K. Yin, Z. Yu et al. / Applied Ma

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ig. 7. Band structures and density of states (DOS) by DFT calculations for the CdI2

a) phase-I at 10 GPa. (b) phase-II at 30 GPa. (c) phase-III at 50 GPa.

tructure, Cd is six-coordinated to I with AB stacking. At ∼35 GPa, hexagonal phase transforms to a monoclinic phase (space group:2/m, phase II) with AA stacking, which remains six-coordinated,s shown in Fig. 6(b). At ∼48 GPa, the monoclinic phase transformso a tetragonal phase (space group: I4/mmm, phase III) with eight-oordination. As shown in Fig. 6(c) and (d), in the range of 0−35 GPa,he volume decreases sharply as a function of pressure, indicat-ng the shortening of inter-layer bonding. Above 35 GPa, there is arop in the continuous decreasing volume. Intra-layers interactionslay a key role in volume changes; the volume decreases gradually

eading to a coordination number increase.To further understand the pressure effects on the electronic

tructure of 2H-CdI2, we performed band structure calculations at0 GPa, 30 GPa, and 50 GPa, respectively. Fig. 7(a)-(c) show the bandtructures under high pressures. The values of calculated band gapre smaller than the observed ones, as the calculations of bandtructures and density of states are done at 0 K. Importantly, therend of band narrowing and closure is consistent with the calcu-ation. For instance, the band gap closure is predicted to occur atround 50 GPa, which is in good agreement with our experiments.he calculations show that both phase-I and phase-II are semicon-uctors, which possess an indirect band gap of 1.69 eV and 0.35 eV,espectively. The indirect band gap is indicated by the differenceetween the valence-band (VB) and conduction-band (CB) maxima.t 50 GPa, the VB and CB have overlapped, confirming the band gaplosure and metallization in phaseIII.

. Discussion

Both XRD and Raman results show that two transitions occur inhe materials at similar pressures. Both experiments support therst-order phase transitions in the material under high pressure.ompared with the 2H (P63mc) structure, phase II can be viewed

s a monoclinic structure, and forms a new octahedral arrange-ent in the intra-layer plane, so the volume change and atomic

isplacement are discontinuous. This structural phase transitions associated with an electrical structure transition, as observed

terials Today 18 (2020) 100532

by high-pressure impedance measurements. In the phase III, theresistance and band gap values show a fast decrease with increas-ing pressure. Therefore, the simultaneous occurrence of these twophenomena indicate that the electronic transition is more sensitiveto pressure than the structural phase transition. As the pressureincreases up to 48 GPa, the structural phase transition is observedagain, and all of the Raman peaks fade away. The interactionbetween the I-I atoms increases and the Cd-I bonds are furthercompressed. The resistance and band gap maintain a continuousand rapid decrease. We demonstrated that layered structural CdI2is converts into a metal at pressure up to 62 GPa. Furthermore, ourtheoretical calculations also indicate that the band bap is closed inthe phase III (I4/mmm). Therefore, pressure-induced metallizationoccurs in the third phase. The discovery of this polytypes transfor-mation of CdI2 has not been reported under high pressure.

As one of the TMDs, FeCl2 was found to have no symme-try change under high pressure, however metallic behavior wasobserved around 47 GPa due to the p-d correlation breakdown[41], which is different to the metallization mechanism of CdI2.The pressure-induced structural phase transition was reported inrutile-type MnF2. This study showed that the following structuralphase sequence was rutile type→SrI2 type (3 GPa) → �-PbCl2 type(13 GPa) [42]. Layered antiferromagnetic FeI2 was found to undergotwo structural phase transtions under high pressure [43], whichformed a new Fe sublattice and tended to disorder in the high-pressure phase. Pressure-induced metallization was observed inlayered NiI2, and the pressure effect caused the filled 5p-iodideband to shift and overlap with the partially-filled 3d-nickel band[44]. In this work, the metallization observed was due to the filled5p-iodide shift and overlap with the filled 5s-cadmium band inphase III, as shown in Fig. 7. Our results show a clear diagram ofthe high-pressure phase and pressure-induced metallization. Thiswork will provide new guidance for electronic structural transi-tions resultant from or induced by high-pressure structural phasetransformations in the CdI2-type structure.

5. Conclusions

In summary, we have presented a systematic investigation ofthe structural, vibrational, and electronic properties of layeredCdI2 under high pressure by a combination of experiments andtheoretical calculations. We found that resistance can be dramat-ically reduced in the monoclinic phase, which is associated withstacking faults accumulation along the c axis. Also, the bandgapshows an unprecedented narrowing deriving from the structuralphase transition. Using electrical resistance measurements andband structure calculations, we discovered pressure-induced met-allization in CdI2 in the tetragonal phase, implying a new electroniclandscape. Our experimental and theoretical results suggest thatpressure can be used to modify the electrical and optical proper-ties of transition metal dihalides materials. Our results lead furtherexploration of potential applications for layered two-dimensionalhalides compounds under extreme conditions.

Author contributions

L.W. designed the project and supervised the experimental mea-surements. X.W. supervised the theoretical calculations and theanalysis. Z.Y., Z.Y., X.L., M.L., Y.Y., X.L., K.Y. did the experiments.K.Y. did the calculations. L.W., Z.Y., and X.W. analyzed the date andwrote the paper.

Declaration of Competing Interest

The authors declare no competing financial interests.

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cknowledgments

This work is mainly supported by Natural Science Founda-ion of China (Grant No. 11874076), National Science Associatedunding (NSAF, Grant No. U1530402), and Science Challenging Pro-ram (Grant No. TZ2016001). M. T. Li thanks the financial supportrom Natural Science Foundation of China (Grant No. 11804011)..W. was supported by the National Natural Science Foundationf China (Grant No. 11974154 and 11674144) and the Naturalcience Foundation of Shandong Province (Grant No. JQ201602,019GGX103023 and ZR2018MA038). Portions of this work wereerformed at the BL15U1 beamline, Shanghai Synchrotron Radia-ion Facility (SSRF) in China. The authors also would like to thankhe Beijing Synchrotron Radiation Facility (BSRF) (4W2 beamline)or use of the synchrotron radiation facilities.

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