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Article pubs.acs.org/JPCC

Structure Evolutions and Metallic Transitions in In2Se3 Under High Pressure Jinggeng Zhao*,† and Liuxiang Yang*,‡ †

Natural Science Research Center, Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150080, China ‡ Geophysical Laboratory, Carnegie Institution of Washington, Washington, District of Columbia 20015, United States S Supporting Information *

ABSTRACT: Indium selenide (In2Se3) could be used as the phase-change random access memory device and thermoelectric material. The high-pressure investigations are important to the applications on In2Se3 and other A2B3-type materials. In this study, we performed the in situ angle-dispersive X-ray diffraction and Raman spectra experiments and the first-principle calculations on In2Se3 under high pressure, and observed a series of structure phase transitions from experiments and metallized phenomena from calculations. In2Se3 transforms from the original rhombohedral structure (phase I) to a distorted monoclinic structure (phase II) and further to a Bi2Te3-type structure (phase III) at about 0.81 and 5.02 GPa, respectively. And then, phase III′ of In2Se3 adopts a similar structure with phase III from about 20.6 GPa. At pressures above about 32.1 GPa, In2Se3 starts to crystallize into a defective Th3P4-type structure (phase IV). According to the first-principle calculations, the structural transitions in the compression process induce that In2Se3 transforms from an insulator in phase I, across a semimetal in phase II and III, to a novel metal in the body-centered cubic structure (phase IV). The pressure-induced structure and conducting evolution on In2Se3 are helpful to understand the properties of other selenides upon compression.

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experiments, the d-spacing is limited to a small range,20 which may bring influence in obtaining the high-pressure crystal structure. Up to now, no other crystal structure investigations under higher pressure for In2Se3 have been reported. The systematic structural evolution information upon compression is important to its applications, since the lattice mismatch exists generally between the films and substrates.21−23 In this work, by performing the in situ angle-dispersive synchrotron X-ray diffraction (AD-XRD) and Raman spectra measurements using a diamond anvil cell (DAC) technique, we discovered a series of pressure-induced structural phase transitions in In2Se3. Before transforming to β phase, In2Se3 crystallizes into a monoclinic β′ phase. Finally, it adopts a defective Th3P4-type structure. By using the ab initio calculations, we also found the metallic transitions in the compression process of In2Se3 modulated by the change in the crystal structure.

INTRODUCTION Very recently, the quantum anomalous Hall (QAH) effect was observed in Cr-doped (Bi, Sb)2Te3 thin films,1 which may bring the great development of low-power-consumption electronics. The related topological insulators Bi2Te3, Sb2Te3, and Bi2Se3 exhibit excellent thermoelectric properties.2−4 The similar compound, indium selenide (In2Se3), composed of III and VI group elements, is also a thermoelectric material.5,6 On the other hand, In2Se3 can be used as a phase-change random access memory (PRAM) device.7−9 The studies of structure phase transitions for In2Se3 are important in the applications of PRAM device. There are several crystal structures at different temperatures in this compound.10−14 Under ambient conditions, In2Se3 adopts a layered rhombohedral structure (α phase), with the space group of R3m.10−14 The relatively large interspaces exist between the two neighboring Se−In−Se−In− Se quintuple layers in this phase. So its structure and physical behaviors could be adjusted significantly by compression. β-In2Se3 (space group R-3m), which comes from α phase at about 200 °C,10−14 is isostructural with ambient Bi2Te3, Sb2Te3, and Bi2Se3.15,16 These topological insulators undergo several structure phase transitions under high pressure.17−19 Because of the similar structure and chemical composition with abovementioned materials, pressure could also play an important role in modulating the structure of In2Se3. Recently, Rasmussen et al. found that α-In2Se3 transforms into β phase at 0.7 GPa,20 with a maximum experimental pressure of 10 GPa. In their © 2014 American Chemical Society

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EXPERIMENTAL SECTION The in situ high-pressure angle-dispersive synchrotron X-ray diffraction experiments were carried out at room temperature at the X17C beamline of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory with the incident Xray wavelength of 0.4067 Å. The incident X-ray monochromatic Received: July 31, 2013 Revised: February 11, 2014 Published: February 17, 2014 5445

dx.doi.org/10.1021/jp4076383 | J. Phys. Chem. C 2014, 118, 5445−5452

The Journal of Physical Chemistry C

Article

completely above about 1.81 GPa, indicating the finished transformation of phase I into II. A small quantity of impurity exists at lower pressure, with two weak diffraction peaks indicated by asterisks. The peak at d-spacing of about 2.9398 Å lies in the same location with that of phase II. However, the other peak at d-spacing of about 3.2922 Å does not belong to phase II, and vanishes completely above about 2.02 GPa. So the impurity in phase I does not belong to phase II and may become amorphous under compression. The high-temperature structure experiments showed that phase II adopts the β′-In2Se3 structure, which emergences in the cooling process of β phase from 200 to 60 °C.10−14 The XRD pattern of β′ phase is very similar to that of β-In2Se3, except the splitting peaks at high angle. As shown in Figure S2a of the Supporting Information, the splitting peaks at d-spacing of about 1.6087 Å merge to one peak from about 5.02 GPa, which indicates that In2Se3 starts to become another rhombohedral structure (phase III, i.e., βIn2Se3; space group R-3m). The schematic view of crystal structure of phase II is shown in Figure 2, in which a

beam was slit-collimated down to 25 × 25 μm2. The In2Se3 sample used in this work is the business product purchased from Alfa Aesar Company. The bulks were ground for about 30 min to a fine powder sample. Then the samples were loaded into the sample chamber in the gasket. The pressure was measured by using the ruby fluorescence method.24 Silicone oil or methanol−ethanol (4:1) mixture was used as the pressuretransmitting medium, which generates a quasi-hydrostatic pressure environment.25,26 Two-dimensional diffraction patterns were collected on a charge-coupled device (CCD) detector. The recorded images were integrated using the program Fit2D.27 The XRD patterns under high pressure were analyzed with Rietveld refinements by using the General Structure Analysis System (GSAS) program package.28,29 The in situ high-pressure Raman spectra of In2Se3 were recorded with exposure times of 60 s and a laser power of 20 mW, by using neon gas as the pressure-transmitting medium. The electronic band structure calculations were carried out using WIEN2K software within the Perdew−Burke−Ernzerhof (PBE) generalized gradient approximation (GGA).30 In this work, 1000 k points were set in the Brillouin zone (BZ) for phase I, III, and IV, and 980 k points were set for phase II. A plane-wave cutoff was defined by RKmax = 7.0.

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RESULTS AND DISCUSSION Figure 1 shows the selected XRD patterns of In2Se3 up to 59.5 GPa at room temperature. Two peaks of impurity exist in the

Figure 2. Schematic view of crystal structure of phase II of In2Se3. The black bold lines show the unit cell of phase II, and the purple bold lines show the “unit cell” of a pseudorhombohedral structure, which is similar to that of phase III.

pseudorhombohedral structure could be captured from the lattice of phase II. The structural evolutions of In2Se3 under compression display a close relationship to that in the heating and cooling process, as shown in Figure S3 of the Supporting Information. Although phase III of In2Se3 is isostructural with ambient Bi2Te3, Sb2Te3, and Bi2Se3, it keeps within a large pressure range, being different from the compression process of these topological insulators.17−19 All the diffraction peaks shifts toward the high-angle direction with the increasing pressure, and no splitting or merging peaks are observed. However, the weak peaks gradually disappear upon compression and only the strong peaks leave in the XRD patterns from about 20.6 GPa, as

Figure 1. Angle-dispersive X-ray diffraction patterns of In2Se3 at room temperature up to 59.5 GPa.

original rhombohedral structure (phase I; space group R3m), indicated with asterisks in Figure 1. At about 0.81 GPa, In2Se3 starts the transition to a monoclinic structure (phase II; space group C2/m), because of the new peaks at d-spacing of about 1.6439 Å, indicated with an arrow in Figure S1 of the Supporting Information. The peaks of phase I vanish 5446



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shown in Figure S2b of the Supporting Information. The in situ high-pressure Raman spectra of In2Se3 in the range of 1.2−26.9 GPa are shown in Figure S4 of the Supporting Information. There are three main Raman modes of ν1, ν2, and ν3, indicated in Figure S4. All spectra were fitted using Lorentz functions, in order to calculate the phonon frequency ω. The relationships of Raman shift versus pressure are shown in Figure 3. A phonon

Figure 4. Schematic views of (a) crystal structure of phase IV of In2Se3 and (b) the InSe8 dodecahedron and SeIn6 octahedron. Figure 3. Pressure dependences of Raman shift for the main modes in In2Se3. The lines are linear fits giving dω/dP.

obvious disordered atom arrangement in the crystal lattice could be observed in phase IV of In2Se3, because of the large difference of radius between In and Se atoms according to the Shannon table.35 The CN for all atoms is 8 in the disordered Bi2Te3 and Sb2Te3.17,18 So it is possible for In2Se3 to adopt another structure with larger CN for Se atoms under higher pressure. The pressure dependences of volume per In2Se3 chemical formula unit (V/Z) is plotted in Figure 5. According to the corresponding crystal structure, the value of Z is equal to 3, 2, 3, 3, and 16/3 for phase I, II, III, III′, and IV, respectively. In the compression process, the volume (V/Z) decreases with pressure, with the volume collapse of about 4.1% and 2.1% for the phase transitions of I−II and III′−IV, respectively. So

softening phenomenon exists in the ν3 mode below about 20 GPa. With the increasing pressure, ν3 mode shifts to the lowfrequency direction, which is contrary to ν1 and ν2 modes. The pressure derivatives of frequency (dω/dP) are obtained by linear fitting the ω-P relationships in Figure 3, as summarized in Table S1 of the Supporting Information. A large discontinuity happens at about 4 GPa in the ω-P relationship for ν3 mode, which is corresponding to the transition pressure from phase II to III. At pressures above 20 GPa, ν3 mode starts the shift to the high-frequency direction, with the weaker intensity, and almost disappears at about 26.9 GPa. The values of dω/dP for ν1 and ν2 modes above 20 GPa are different from those below 20 GPa. However, no obvious crystal structure phase transition could be observed from XRD results in Figure 1. Therefore, combining the in situ high-pressure XRD and Raman data, the structure of In2Se3 above about 20.6 GPa should be considered as phase III′, in order to distinguish it with the previous phase III. The evolution from phase III to III′ is similar to an isostructural phase transition. At about 32.1 GPa, In2Se3 transforms to a body-centered cubic (BCC) structure (phase IV; space group I-43d). The peaks of phase III′ vanish completely above about 50 GPa, indicating the finished transition of phase III′ into IV. And then, phase IV remains a stable structure up to the maximum experimental pressure of 59.5 GPa. Phase IV of In2Se3 adopts a defective Th3P4-type structure, which is isostructural with Ln2Se3 (Ln = Gd, Sm, Nd, Pr, Ce, La).31−34 The schematic view of crystal structure of phase IV is shown in Figure 4, as well as the InSe8 dodecahedron and SeIn6 octahedron. In and Se ions occupy the 12a and 16c positions, respectively, with the site occupation factor (sof) in the In position of 8/9 in order to satisfy the ratio of 2:3 for In:Se in In2Se3. The coordination numbers (CN) for In and Se atoms are 8 and 16/3, respectively. Being different from the disordered BCC structure in the high-pressure phases of Bi2Te3 and Sb2Te3,17,18 no

Figure 5. Pressure dependence of volume per In2Se3 chemical formula (V/Z). The solid lines are the fitting results according to the secondorder Birch−Murnaghan equation of state (EoS). The inset shows the ambient pressure isothermal bulk modulus (B0) and fitted ambient volume (V0/Z) for these phases. 5447



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they belong to the first-order phase transitions. No obvious volume collapse occurred in the phase transitions of II−III and III−III′ and no two-phase coexistence regions was found in these two transformations, which shows the second-order phase transitions for them. In Figure 5, the solid lines are the fitting results for these phases of In2Se3 using the second-order Birch−Murnaghan equation of state (EoS).36 The ambient pressure isothermal bulk modulus B0 (B0′ = 4) for these phases is shown in the inset of Figure 5, as well as the fitted ambient volume (V0/Z). The B0 of phase II is almost equal to that of phase I, and is smaller than the value of phase III, although it adopts the similar atom arrangements with the latter in the crystal lattice. The V−P relationships of phase III and III′ can not be fitted by using one curve, which shows their different compressibility. The larger B0 of phase III′ indicates that it is more difficult to compress it than phase III. Comparing with these layer structures, the cubic In2Se3 has smaller compressibility. In phase I−III′, In and Se atoms in one Se−In−Se−In−Se quintuple layer connect by the chemical bonds with more ionic bond character,37,38 and Se atoms between the two neighbor quintuple layers connect by the van der Waals’ forces. In phase IV, all In and Se atoms connect by the chemical bonds with more ionic bond character, without the van der Waals’ forces. This is an important reason for the larger B0 in phase IV. The typical Rietveld refinements results are shown in Figure 6 for phase I, II, III, and IV of In2Se3 at 0.39, 1.81, 6.05, and 59.5 GPa, respectively. There are two possible crystal structures (A and B) for phase I of In2Se3, with their schematic views shown in Figure S5a of the Supporting Information. Comparing with the structure A, the In(1) and Se(1) positions exchange to each other in the structure B. In refs 11 and 38, α-In2Se3 was considered to adopt structures A and B, respectively. However,

as shown in Figure S5b of the Supporting Information, the fitting XRD pattern from the structure B is more suitable to the experimental one at 0.12 GPa. So the structure B is selected as phase I of In2Se3 in this work. The fitted atomic coordinates are summarized in Table 1 from the refinement results, as well as the corresponding lattice parameters and conventional Rietveld R-factors. Table 1. Fitted Atomic Coordinates of Phase I, II, III, and IV for In2Se3 atom

site

In(1) In(2) Se(1) Se(2) Se(3)

3a 3a 3a 3a 3a

In Se(1) Se(2)

4i 4i 2a

In Se(1) Se(2)

6c 6c 3a

In Se

12a 16c

x

y

Phase I at 0.39 GPaa 0 0 0 0 0 0 0 0 0 0 Phase II at 1.81 GPab 0.5993(3) 0 0.7872(3) 0 0 0 Phase III at 6.05 GPac 0 0 0 0 0 0 Phase IV at 59.5 GPad 0.375 0 0.0668(4) 0.0668(4)

z 0.5334(4) 0.7121(3) 0.2396(3) 0.8120(5) 0 0.7925(5) 0.3593(5) 0 0.4022(1) 0.2106(2) 0 0.25 0.0668(4)

a

Space group R3m; a = 4.0286(2) Å, c = 28.731(6) Å; Rp = 2.54%, Rwp = 4.21%. bSpace group C2/m; a = 6.8268(3) Å, b = 3.9731(2) Å, c = 9.394(1) Å, β = 103.30(1)°; Rp = 1.04%, Rwp = 1.27%. cSpace group R3m; a = 3.8680(1) Å, c = 26.388(3) Å; Rp = 1.50%, Rwp = 2.21%. d Space group: I-43d; a = 7.7243(3) Å; Rp = 1.47%, Rwp = 2.03%; sof(In)=8/9.

The pressure dependences of atom distances for phase I, II, and III below 10.3 GPa are plotted in Figure 7, in which the corresponding labels are denoted in Figure S6 of the Supporting Information. The Se−Se distance between the two neighbor Se−In−Se−In−Se quintuple layers decreases with pressure, with the discontinuous change when the phase

Figure 6. Experimental (cross) and fitted (line) X-ray diffraction patterns for phase I, II, III, and IV of In2Se3 at different pressures, in which the experimental (circle) and fitted (line) XRD patterns were plotted.

Figure 7. Pressure dependences of atom distances for phase I, II, and III of In2Se3 below 10.3 GPa. The lines are guide for the eyes. The corresponding labels are indicated in Figure S6. 5448



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transitions have happened. In the transition process from phase I to II, the atom arrangement of In and Se in the Se−In−Se− In−Se quintuple layer transforms from the zinc blende type to the rock salt one, which induces the increasing In−Se distances. The discontinuous pressure dependences of Se−Se atom distance are consistent with the structure phase transitions of I−II and II−III in In2Se3. Under high pressure, some simple compounds tend to become the analogous structure that includes larger anions or cations. Considering the similar atom ratio and valence of cations, the structure evolution of ambient Ln2Se3 (Ln = lanthanide elements) with the increasing radius of Ln cations could be viewed as a comparison with the compression behavior of In2Se3. For the smaller Ln cations (Ln = Lu, Yb, Er, Ho), Ln2Se3 crystallizes into a face-centered orthorhombic structure (space groups Fddd) at ambient,32,39−41 which is a defective NaCl-like structure, with CN of 6 for Ln cations. In the local layer structure of phase III and III′ of In2Se3, In and Se atoms form two interpenetrating face-centered cubic (FCC) lattices, which is similar to that in the rock salt structure. The Se−In−Se−In−Se quintuple layer is the infinite two-dimensional structure paralleled the ab-plane, in which the atoms stack five layers along the c-axis. For the larger Ln cations (Ln = Gd, Sm, Nd, Pr, Ce, La), Ln2Se3 adopts the isostructural cubic lattice with phase IV of In2Se3.31−34 Therefore, the transformation from phase III′ to IV in In2Se3 under compression is similar to the evolution of Ln2Se3 at ambient pressure with the increasing Ln cations radius, but without the intermediate primitive orthorhombic structure (Ln = Dy, Tb, Gd, Sm; space group Pnma).32,34,42,43 Here the fitted ambient structure parameters of cubic In2Se3 were listed with those of isostructural Ln2Se3 (Ln = Gd, Sm, Nd, Pr, Ce, La) together. Figure 8 summarizes the relationships of (a) ambient volume per chemical formula unit (V0/Z; Z = 16/3) and Se position x, and (b) mean (d̅M−Se = (d1 + d2)/2) and difference (ΔdM−Se = d1 − d2) of the two nonequivalent M−Se distances, versus M-site ionic radius (rM) with CN of 8 for the cubic M2Se3 (M = In, Gd−La).35 The V0/Z decreases with the shrinkage of rM. The ΔdM−Se increases with the decreasing rM, which shows the increasing distortion degree of MSe8 and SeM6 polyhedrons from La to In atoms. The approximately linear dependences to rM of these structure parameters in Figure 8 indicate that phase IV of In2Se3 could be classified to a series with these cubic Ln2Se3. And their structure is modulated only by cation radius. For these cubic Ln2Se3, no corresponding electrical properties have been reported up to now, as well as the first-principle calculation results. And there is also no structural data upon compression in previous works. Because of the similar crystal structure and element constitution, the structure and physical properties of phase IV of In2Se3 could be as a reference to these cubic Ln2Se3. The calculated total density of state (DOS) for In2Se3 within phase I, II, III, and IV at 0.39, 1.81, 6.05, and 59.5 GPa, respectively, are presented in Figure 9, in which the crystal structure parameters are obtained from Table 1. The corresponding band structure plots around the Fermi energy (EF) are shown in Figure 10. The energy gap of about 0.24 eV at 0.39 GPa indicates that phase I is an insulator, in consistent with previous experiment results at ambient pressure.44−46 The DOS of phase II is similar to that of phase III, since its crystal structure is analogous with that of the later. According to the band structures, phase II and III looks like semimetals, because of the partial overlap of conduction bands and valence bands in

Figure 8. Ionic radius dependences of (a) ambient volume per chemical formula unit (V0/Z) and Se position x, and (b) mean (d̅M−Se) and difference (ΔdM−Se) of the two nonequivalent M−Se distances for cubic M2Se3 (M = In and Gd−La).24−27 The lines are guide for the eyes.

Figure 9. Calculated total density of state (DOS) for phase I, II, III, and IV of In2Se3 at different pressures. The vertical lines mark the Fermi energy (EF).

the Fermi level. So pressure induces the change of conductivity of In2Se3 when the phase transitions have happened. The transition from layered structure in phase I, II, III, and III′ to 5449



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Figure 10. Band structure around the Fermi energy (EF) for phase I, II, III, and IV of In2Se3.

ambient,5,6 the cubic In2Se3 will become a good thermoelectric material if EF moves to the range of E1 − E2, that is, the energy “gap”. Modulating the lattice parameter could induce the shift of DOS and let EF locate in the energy “gap”. From Figure 5, the fitting ambient value of a-axis is larger than that at high pressure. The XRD patterns in the unloading process of In2Se3 from high pressure were also collected. Below about 10 GPa, phase IV disappears and an amorphous state emergences down to 0 GPa. So the cubic In2Se3 could not be obtained at ambient by only cold compression. The high-pressure sintering could be a feasible method to make this structure remain at ambient conditions. On the other hand, doping other cations with larger radius in In site could also enlarge the lattice parameter of cubic In2Se3, according to the structural evolutions in Figure 8. In the next works, the synthesis of materials based on cubic In2Se3 will

the nonlayered structure in phase IV remarkably influences the physical properties of In2Se3. In phase IV, the density of states at Fermi level (N(EF)) of 4.77 state/f.u./eV indicates In2Se3 is a metal at 59.5 GPa. The band structure in Figure 10 also shows the metallic conductivity. The N(EF) of phase IV is much larger than that of phase II and III. The better metallic properties of phase IV are due to the chemical bonds with more ionic bond character between all the neighbor atoms, unlike the van der Waals’ forces in Se atoms between the two neighbor quintuple layers in phase I, II, III, and III′. In phase IV, there is a “gap” of about 0.45 eV in the energy range of −0.92 (E1) to −0.47 (E2) eV, and the DOS rapidly changes nearby E1 or E2. The materials with a δ-shape DOS near energy gap may have a higher electronic conductivity (σ).47,48 Considering the thermoelectric properties in phase I at 5450



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Realization of a Three-Dimensional Topological Insulator, Bi2Te3. Science 2009, 325, 178−181. (4) Wang, G. F.; Cagin, T. Investigation of Effective Mass of Carriers in Bi2Te3/Sb2Te3 Superlattices via Electronic Structure Studies on Its Component Crystals. Appl. Phys. Lett. 2006, 89, 152101(1−3). (5) Cui, J. L.; Liu, X. L.; Zhang, X. J.; Li, Y. Y.; Deng, Y. Bandgap Reduction Responsible for the Improved Thermoelectric Performance of Bulk Polycrystalline In2‑xCuxSe3 (x = 0∼0.2). J. Appl. Phys. 2011, 110, 023708(1−5). (6) Cui, J. L.; Zhang, X. J.; Deng, Y.; Fu, H.; Yan, Y. M.; Gao, Y. L.; Li, Y. Y. Modified Structures and Improved Thermoelectric Property in Ag-added Polycrystalline In2Se3. Scr. Mater. 2011, 64, 510−512. (7) Lee, H.; Kang, D.-H.; Tran, L. Indium Selenide (In2Se3) Thin Film for Phase-Change Memory. Mater. Sci. Eng., B 2005, 119, 196− 201. (8) Yu, B.; Ju, S.; Sun, X. H.; Ng, G.; Nguyen, T. D.; Meyyappan, M.; Janes, D. B. Indium Selenide Nanowire Phase-Change Memory. Appl. Phys. Lett. 2007, 91, 133119(1−3). (9) Matheswaran, P.; Sathyamoorthy, R.; Asokan, K. Schottky Nature of InSe/Cu Thin Film Diode Prepared by Sequential Thermal Evaporation. Electron. Mat. Lett. 2012, 8, 417−421. (10) Popović, S.; Tonejc, A.; Gržeta-Plenković, B.; Č elustka, B.; Trojko, R. Revised and New Crystal Data for Indium Selenides. J. Appl. Crystallogr. 1979, 12, 416−420. (11) Osamura, K.; Murakami, Y.; Tomiie, Y. Crystal Structures of αand β-Indium Selenide, In2Se3. J. Phys. Soc. Jpn. 1966, 21, 1848−1848. (12) Ye, J. P.; Soeda, S.; Nakamura, Y.; Nittono, O. Crystal Structures and Phase Transformation in In2Se3 Compound Semiconductor. Jpn. J. Appl. Phys. 1998, 37, 4264−4271. (13) Pfitzner, A.; Lutz, H. D. Redetermination of the Crystal Structure of γ-In2Se3 by Twin Crystal X-Ray Method. J. Solid State Chem. 1996, 124, 305−308. (14) Manolikas, C. New Results on the Phase Transformations of In2Se3. J. Solid State Chem. 1988, 74, 319−328. (15) Nakajima, S. The Crystal Structure of Bi2Te3−xSex. J. Phys. Chem. Solid 1963, 24, 479−485. (16) Anderson, T. L.; Krause, H. B. Refinement of the Sb2Te3 and Sb2Te2Se Structures and Their Relationship to Nonstoichiometric Sb2Te3‑ySey Compounds. Acta Crystallogr. 1974, B30, 1307−1310. (17) Zhu, L.; Wang, H.; Wang, Y. C.; Lv, J.; Ma, Y. M.; Cui, Q. L.; Ma, Y. M.; Zou, G. T. Substitutional Alloy of Bi and Te at High Pressure. Phys. Rev. Lett. 2011, 106, 145501(1−4). (18) Ma, Y. M.; Liu, G. T.; Zhu, P. W.; Wang, H.; Wang, X.; Cui, Q. L.; Liu, J.; Ma, Y. M. Determinations of the High-Pressure Crystal Structures of Sb2Te3. J. Phys.: Condens. Matter 2012, 24, 475403(1−8). (19) Liu, G. T.; Zhu, L.; Ma, Y. M.; Lin, C. L.; Liu, J.; Ma, Y. M. Stabilization of 9/10-Fold Structure in Bismuth Selenide at High Pressures. J. Phys. Chem. C 2013, 117, 10045−10050. (20) Rasmussen, A. M.; Teklemichael, S. T.; Mafi, E.; Gu, Y.; McCluskey, M. D. Pressure-Induced Phase Transformation of In2Se3. Appl. Phys. Lett. 2013, 102, 062105(1−4). (21) Kim, K. C.; Choi, W. C.; Kim, H. J.; Lyeo, H. K.; Kim, J. S.; Park, C. Thermoelectric Properties of Bi2Te3-In2Se3 Composite Thin Films Prepared by Co-Sputtering. J. Nanosci. Nanotech. 2012, 23, 3633−3636. (22) Sreekumar, R.; Sajeesh, T. H.; Abe, T.; Kashiwaba, Y.; Kartha, C. S.; Vijayakumar, K. P. Influence of Indium Concentration and Growth Temperature on the Structural and Optoelectronic Properties of Indium Selenide Thin Films. Phys. Status Solidi B 2012, 250, 95−102. (23) Emziane, M.; Marsillac, S.; Bernède, J. C. Preparation of Highly Oriented α-In2Se3 Thin Films by a Simple Technique. Mater. Chem. Phys. 2000, 62, 84−87. (24) Mao, H. K.; Xu, J. A.; Bell, P. M. Calibration of the Ruby Pressure Gauge to 800 kbar Under Quasi-Hydrostatic Conditions. J. Geophys. Res. 1986, 91, 4673−4676. (25) Shen, Y. R.; Kumar, R. S.; Pravica, M.; Nicol, M. F. Characteristics of Silicone Fluid as a Pressure Transmitting Medium in Diamond Anvil Cells. Rev. Sci. Instrum. 2004, 75, 4450−4454.

be performed in order to obtain the new thermoelectric materials. At the same time, the further first-principle calculation investigations will be also carried out.

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CONCLUSIONS In conclusion, the in situ high-pressure angle-dispersive X-ray diffraction and Raman experiments are operated on In2Se3 at room temperature up to 59.5 and 26.9 GPa, respectively. During the compression process, four crystal structural phase transitions were discovered, including two first-order and two second-order transitions. The modulations of crystal lattice in the structural phase transition process influence the electrical properties of In2Se3. Structural phase transitions induce the transformation to a semimetal from the original insulator, and further to a metal under higher pressure, obtained from the first-principle calculation results. These high-pressure studies for In2Se3 in this work are helpful to understand the universal structure evolution and physical property patterns for these A2B3-type selenides at ambient and high pressure.

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ASSOCIATED CONTENT

S Supporting Information *

XRD patterns below 2.02 GPa (Figure S1), details of XRD patterns (Figure S2), structure evolutions in the heating and cooling, and pressing process (Figure S3), in situ high-pressure Raman spectra (Figure S4), values of dω/dP of phase II, II, and III′ (Table S1), schematic views of two possible structures for phase I and fitting XRD patterns (Figure S5), schematic views of atom distances in phase I and phase II&III (Figure S6), and the complete references 1 and 3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

* E-mail: [email protected] (J.G.Z.). * E-mail: [email protected] (L.X.Y.). Notes

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS Use of the National Synchrotron Light Source is supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0298CH10886. The X17 beamline is supported by the Consortium for Materials Properties Research in Earth Sciences (COMPRES). We also thank the High Performance Computing Center in Harbin Institute of Technology (HIT), and the support by the National Natural Science Foundation of China (Grant No. 10904022), the Postdoctoral Science-research Developmental Foundation of Heilongjiang Province (Grant No. LBH-Q12095), and the Fundamental Research Funds for the Central Universities (Grant No. HIT.NSRIF.2013054).

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REFERENCES

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