Introduction

The A2B3 (A = Sb, Bi; B = S, Se, Te) series consists of layered chalcogenide semiconductors that have attracted considerable interest, mainly due to their exceptional thermoelectric properties1. More recently, three members of this series, namely the Bi2Te3, Sb2Te3 and Bi2Se3 compounds, were shown to exhibit topological properties2,3. This discovery has revitalized the scientific interest in these materials4,5.

At ambient conditions, the A2B3 (A = Sb, Bi; B = S, Se, Te) family is divided into two structural classes: the heavier Bi2Se3, Bi2Te3 and Sb2Te3 compounds adopt a rhombohedral structure (SG R-3m, Z = 3), which is composed of layers of AB6 octahedra stacked perpendicular to the long c-axis6; the lighter Bi2S3, Sb2S3 and Sb2Se3 materials on the other hand, crystallize in an orthorhombic phase (SG Pnma, Z = 4, U2S3-type), made up of AB7 and AB7+1 polyhedra ( Fig. 1 and Supplementary Fig. S1 online).

Figure 1
figure 1

(Left) The unit cell of Sb2Se3 at ambient conditions (SG Pnma, Z = 4). The blue, green and red spheres correspond to Sb(1), Sb(2) and Se ions, respectively. The Sb(1)Se7 (blue) and Sb(2)Se7+1 (green) polyhedra are also displayed. (Right) Sketch of the coordination environment around the Sb(1) and Sb(2) cations. The various Sb-Se bonds are depicted by different colors.

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Pressure provides a means for “tuning” the physical properties of these compounds. As an example, their thermoelectric properties are enhanced by application of external pressure1, a feature attributed to an electronic topological transition. More recent and detailed high-pressure investigations on Bi2Te36,7,8,9,10,11,12,13, Bi2Se314,15,16,17,18 and Sb2Te39,19,20,21,22,23 have revealed a plethora of phase transitions. In particular, novel structures with higher cationic coordinations6,7,14,15,18,19,20,21, insulator-to-metal transitions9,16 and even superconductivity8,12,13,23 have been observed upon increasing pressure. Quite surprisingly, the compounds crystallizing in the R-3m structure do not adopt the denser Pnma phase upon compression. The only known exception is Bi2Se3, which can adopt the Pnma structure at combined high-pressure and high-temperature conditions24 or, as recently shown, at ambient conditions after high-pressure treatment15.

The present study focuses on the effect of pressure on the structural and vibrational properties of Sb2Se3, which is an insulator with a ~1 eV band gap25. Even though it does not exhibit the topological properties of the Sb2Te3, Bi2Te3 and Bi2Se3 compounds, Sb2Se3 has attracted significant interest over the years as a potential candidate for optical storage devices26, as well as in photovoltaic27 and thermoelectric28 applications. Two recent studies on Sb2Se329,30 indicated the possibility of a pressure-induced electronic topological transition, thus transforming the material from a normal band insulator into a topological insulator upon pressure application. These investigations, however, considered the R-3m structure (isostructural to Bi2Te3) as an ambient-pressure phase for Sb2Se3, a modification which has not been observed experimentally. In addition, a most recent high-pressure Raman study on Sb2Se3 indicated several phase transitions taking place up to ~25 GPa29. Therefore, a detailed structural investigation of Sb2Se3 is necessary in order to clarify any potential pressure-induced phase transitions. The key findings of this work can be summarized as follows:

  1. I

    A novel binary alloy between Sb and Se atoms was observed. In particular, the ambient-pressure Pnma phase of Sb2Se3 persists up to ~51 GPa; beyond that pressure, Sb2Se3 begins to transform into a disordered cubic bcc phase (SG Im-3m, Z = 2), similar to the Bi2Te36,7 and Sb2Te319,20 compounds. The transition, which is reversible, is not fully completed up to the highest pressure reached here, i.e. 65 GPa.

  2. II

    To our knowledge, a direct structural transition from the Pnma phase into any other structure type has not previously been observed for any A2B3 materials. Therefore, this study adds a new structural “path” to the P-T phase diagrams of A2B3 compounds.

  3. III

    The high-pressure Raman study on Sb2Se3 could not reproduce the recently reported results29. This discrepancy prompted a more detailed investigation of the Raman response of Sb2Se3 from our side, under both ambient- and high-pressure conditions. Considering the available literature, we offer a thorough discussion on the vibrational properties of Sb2Se3.

Results

At ambient conditions, Sb2Se3 adopts an orthorhombic Pnma structure (U2S3-type, Fig. 1 ). There are two non-equivalent Sb cation sites in this phase, denoted as Sb(1) and Sb(2) in Fig. 1 . The Sb(1) cation is coordinated by seven Se anions; on the other hand, the coordination around the Sb(2) site can be described as of (7 + 1) with respect to the Se ions. A more detailed description of the Pnma structure is provided in the Supplementary Information online.

Overall, Sb2Se3 undergoes one structural transition under pressure ( Fig. 2 ): the initial Pnma phase begins to transform into a disordered bcc structure (SG Im-3m, Z = 2) above 51 GPa. The transition to the bcc phase is almost completed at 65 GPa. Upon pressure release, the bcc phase persists down to 35 GPa; below that pressure, Sb2Se3 reverts to the ambient-pressure Pnma phase (see Supplementary Fig. S2 online). In the recovered sample, the XRD peaks are significantly broadened (bottom XRD pattern in Supplementary Fig. S2 online), thus hindering any reliable refinement. The reversibility of the ambient-pressure Pnma structure is verified by our Transmission Electron Microscopy (TEM) study on the recovered Sb2Se3 sample (see Supplementary Figs. S4&S5 online).

Figure 2
figure 2

(a) XRD patterns of Sb2Se3 at selected pressures (T = 300 K, λ = 0.4246 Å). The various phases are indicated by different colors: black for the starting Pnma structure, red for the high-pressure Im-3m modification and orange for the coexistence regime. Asterisks mark the strongest Bragg peak of the rhenium gasket. Background has been subtracted for clarity. (b) Refined XRD patterns of Sb2Se3 at 9.2 GPa (Pnma, bottom) and at 65 GPa (Im-3m, top). Dots correspond to the measured spectra and the red solid lines represent the best refinements. The difference spectra between the measured and the refined patterns are depicted too (blue lines). Vertical ticks mark the Bragg peak positions.

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For the initial Pnma phase, Rietveld refinements were possible up to ~35 GPa; both the unit cell constants, as well as the interatomic parameter evolution as a function pressure could be obtained ( Fig. 3 ). An example of a refined XRD pattern is displayed in Fig. 2(b) , whereas the extracted crystallographic parameters are listed in Table 1 . The Bragg peaks corresponding to the Pnma phase broaden significantly above 40 GPa [ Fig. 2(a) ]; therefore, only the lattice parameters could be obtained beyond that pressure.

Table 1 Refined crystallographic data for the starting Pnma structure and the high-pressure Im-3m modification of Sb2Se3 [Fig. 2(b)]
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Figure 3
figure 3

(a) Lattice constants and (b) unit cell volume per formula unit as a function of pressure for the two phases of Sb2Se3. The closed and open symbols correspond to data collected upon compression and decompression cycles, respectively. The dashed rectangle marks the transition regime for the Pnma-Im-3m structural transition. The red solid lines represent the fitted Birch-Murnaghan EOS functions to the measured P-V data. (c, d) Pressure-induced changes of the Sb(2)-Se and the Sb(1)-Se bond lengths within the Sb(2)Se7+1 and Sb(1)Se7 polyhedra, respectively, for the Pnma phase of Sb2Se3. The various Sb-Se bond distances are displayed in Fig. 1 for clarity.

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The pressure dependence of the lattice parameters for the Pnma structure is plotted in Fig. 3(a) . The long orthorhombic a- and c-axes, which lie very close in values throughout the investigated pressure range, exhibit a similar non-linear pressure-induced behavior, with a change in their slopes at about 20 GPa. We should note here that a structural transition has been claimed to take place at about 20 GPa for Sb2Se3 from a recent Raman investigation29. Since we do not detect any changes in our XRD patterns and considering that our Raman spectra do not reproduce the pressure-induced Raman response of Sb2Se3 reported in Ref. 29 as we discuss in more detail below, we attribute this change in the pressure dependence of both the a- and c-axes to a decrease in the compressibility of Sb2Se3 along these two directions beyond 20 GPa. On the other hand, the short b-axis exhibits an almost linear (within error) pressure-change up to 48 GPa.

The P-V data for the Pnma phase of Sb2Se3 are shown in Fig. 3(b) . By employing the measured zero-pressure volume per formula unit V0/Z = 136.4 Å3, the Birch-Murnaghan EOS fitting yields a bulk modulus B0 = 30(±1) GPa and its pressure derivative B′0 = 6.1(±0.2). The obtained B0 and B′0 values are consistent with those of isostructural Sb2S331 and Bi2S332 compounds.

The effect of pressure on the interatomic Sb-Se bond lengths for both Sb(1) and Sb(2) cations within the Sb(1)Se7 and Sb(2)Se7+1 polyhedral cages is displayed in Figs. 3(c,d) . In the low-pressure regime, the Se-coordination environment around both Sb(1) and Sb(2) cations is highly asymmetric, as indicated by the large dispersion of the Sb-Se bond length values within the respective polyhedra. The effect of pressure is more pronounced for the longer Sb-Se bonds; the shorter Sb-Se bonds on the other hand, remain almost unaffected upon increasing pressure [ Figs. 3 (c, d) ]. The second observation is that pressure “forces” the various Sb-Se bond lengths of the Sb(1)Se7 polyhedra to adopt similar values at ~35 GPa [ Fig. 3(d) ]. This effect can be translated into a pressure-induced “symmetrization” of the Sb(1)Se7 polyhedral units. Actually, the distortions of the coordination polyhedra around the Sb3+ cations reflect the stereochemical activity of the lone electron pairs of Sb3+, as discussed in more detail in the Supplementary Information.

Above 51 GPa, new Bragg peaks appear in the XRD patterns at about 11°, 15° and 19° ( Fig. 2 ). Upon further pressure increase, these new peaks gain in intensity, indicative of a pressure-induced structural transition. The high-pressure phase can be assigned to a simple, yet disordered, bcc structure (Im-3m, Z = 2). Within the Im-3m phase, the Sb and Se atoms are randomly distributed on the bcc lattice sites, forming a Sb-Se substitutional alloy. A similar phase transition has been observed for the Bi2Te36,7 and Sb2Te319,20 compounds above ~15 GPa and ~22 GPa, respectively.

The only high-pressure studies conducted on U2S3-type compounds up to now are those on Sb2S331 and Bi2S332 up to 10 GPa and on the Bi2Se3-II phase up to 26 GPa15. In all of these studies, no structural transition was observed under pressure. Therefore, a direct pressure-induced structural transition from the U2S3-type phase to another structure type (Im-3m) has been observed here for the first time in the case of Sb2Se3. This result supplements further the already rich P-T phase diagrams of A2B3 compounds.

The transition into the bcc phase is not fully completed at 65 GPa for Sb2Se3, the highest pressure reached in this study, as some remnant Pnma phase can still be observed in the XRD patterns [ Fig. 2(b) ]. The fitting of the P-V data for the bcc phase with a Birch-Murnaghan EOS form [ Fig. 3(b) ] yields the following values for the elastic parameters: VTr/Z = 87.25(±0.1) Å3 and BTr = 217(±11) GPa, with B′Tr = 4 (fixed value) and Z = 0.419. It should be also noted here that the P-V data employed for the bcc EOS fitting correspond to both compression [filled symbols in Fig. 3(b) ] and decompression [open symbols in Fig. 3(b) ] cycles. Therefore, the transition pressure point PTr for the Pnma-Im-3m transition, where the obtained elastic parameters are evaluated, is assumed to occur at 36.2 GPa for the EOS fitting purposes.

In addition, we have probed the high-pressure vibrational features of Sb2Se3 by employing Raman spectroscopy. For the starting Pnma phase, a sum of thirty first-order Raman-active modes are expected33:

At ambient conditions [bottom spectra in Fig. 4(a) ], the Raman spectrum of Sb2Se3 consists of eight relatively sharp peaks (labeled 1–8) and some broad Raman features, such as the low-intensity “wings” of peak 3. Upon pressure application, these broad bands become more prominent and can be clearly resolved (D1–D3, Fig. 4 ). Given the distinct width differences between the sharp and broad Raman features in the Sb2Se3 Raman spectra, we speculate that the scattering mechanism and, therefore, the origin of these Raman bands are different. We assign the sharp Raman features to first-order Raman-active modes of Sb2Se3 (except for mode 8), whereas the broad bands to defect-induced Raman scattering. Our arguments are the following:

  1. a

    Structural defects are known to affect the Raman response of crystalline materials, by introducing broad bands in the measured Raman spectra34,35. This kind of defect-induced Raman scattering mechanism has been explored in detail for several binary semiconductors34.

  2. b

    Most of the reported ambient-pressure Sb2Se3 Raman spectra exhibit only sharp Raman features36,37,38, which are consistent with our observed (1–7) Raman peaks. In addition, additional TEM measurements performed on these Sb2Se3 materials36,37,38 established their high crystalline quality. On the other hand, the Raman spectra of Sb2Se3 reported in Refs. 29, 39 are quite different. The main feature in both of these studies is a broad Raman band located at ~190 cm−1; since no other micro-structural studies were provided in those investigations, the crystalline quality of the measured samples is unclear and questionable. Nevertheless, this apparent discrepancy prompted us to conduct additional Raman measurements on Sb2Se3 (see Supplementary Information).

  3. c

    As part of these detailed Raman investigations, we have studied the effect of laser annealing on the Raman response of Sb2Se3 at ambient pressure. After the laser annealing process, the background in the Sb2Se3 Raman spectrum becomes completely flat, whereas the broad features are eliminated completely [see Supplementary Fig. S6(a) online]. Given that such laser-annealing/high-temperature treatment induces analogous effects, i.e. improves the quality of the 3C-SiC Raman spectra due to the suppression of microstructural disorder40, we can reasonably assume that a similar effect is taking place in the case of Sb2Se3 as well.

Figure 4
figure 4

(a) Raman spectra of Sb2Se3 at various pressures (λ = 532 nm, T = 300 K). (b) Raman mode frequency evolution against pressure. Solid circles and triangles correspond to crystalline and disorder-induced Raman features of Sb2Se3, respectively (see text). The open symbols correspond to data collected upon decompression. Solid lines represent least square fits to the measured data. The vertical dashed line denotes the loss of the sharp Raman features in the Sb2Se3 Raman spectra.

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The measured zero-pressure frequencies ω0 for the first-order Raman modes of Sb2Se3 (Supplementary Table S1 online) are in good agreement with the reported values36,37,38. The Raman mode assignment (1–7) is achieved by comparing the (almost identical) Raman spectra of isostructural Sb2Se3 and Sb2S3 compounds (see Supplementary Fig. S7 online). All of the observed sharp Raman peaks (1–7) are thus identified as first-order Raman modes. The only notable exception is mode 8 located at ~705 cm−1; interestingly, this mode is present in both Sb2Se3 and Sb2S3 compounds, as well as in the cubic Sb2O3 (see Supplementary Fig. S7 online). Although such mode is expected in the case of Sb2O3 due to the presence of the light oxygen anion41, it is not anticipated for the heavier S- and Se-bearing compounds. By taking also into account its (almost) identical pressure dependence with the respective F2g mode of Sb2O341, we attribute mode 8 to an impurity Raman band.

Upon compression, most of the first-order Raman-active modes of Sb2Se3 (1–7) exhibit “normal” behavior, i.e. their frequencies shift to higher values with increasing pressure ( Fig. 4 ). The effect of pressure is more pronounced for the 1, 3, 6 and 7 Raman modes, implying that the respective force constants are more sensitive against pressure application. This larger pressure dependence results in the merging of both 1 and 3 modes with their neighboring 2 and 4 bands above 10 and 20 GPa, respectively. On the contrary, mode 5 exhibits pressure-induced softening (even though marginal) up to 32 GPa. Since this mode is assigned to a Sb-Se stretching motion, it most likely reflects the pressure-induced behavior of the shorter (and almost incompressible) Sb-Se bonds [ Figs. 3 (c, d) ].

As for the disorder-induced D1, D2 and D3 modes, their frequencies increase with increasing pressure. Mode D3 broadens significantly above 7.5 GPa; two bands (D4 and D5 in Fig. 4 ) are employed for the fitting of this feature beyond this pressure point. Upon further compression, an additional broad band (D6) appears in our Raman spectra above 29 GPa. Coincidentally, the strongest sharp peak of Sb2Se3 (mode 4) starts to decline in intensity, whereas the broad Raman features (D1, D3 and D5) enhance above that pressure intensity-wise. Mode 4 can be resolved up to 40 GPa; the Raman spectra are dominated by the four D1, D3, D5 and D6 broad bands beyond that pressure ( Fig. 4 ).

The total loss of the sharp Raman peaks above 40 GPa coincides roughly with the broadening of the Bragg peaks evidenced in the XRD patterns ( Fig. 2 ). Since the Pnma-Im-3m structural transition initiates at a higher pressure, i.e. above 51 GPa, the loss of the sharp peaks in the Raman spectra arises probably due to pressure-induced structural disorder rather than a structural transition. However and since the high-pressure Im-3m structure is both a disordered and a Raman-inactive phase, this utter loss of the sharp Raman features above 40 GPa might also be a signature for the Pnma-Im-3m structural transition. The original phase/Raman spectrum of Sb2Se3 is recovered upon decompression ( Fig. 4 ), but only after full pressure release.

Before finishing this Section, a direct comparison between our high-pressure Raman study and that of Bera et al.29 is in order. As mentioned before, our Sb2Se3 Raman spectra are quite different from those reported in Ref. 29, where the main feature of the Raman spectra is a broad band located at ~190 cm−1. In order to resolve this discrepancy, we have probed the Raman activity of Sb2Se3 in different environments and experimental conditions [see Supplemental Fig. S6(b) online]. In particular, the form of the sample (single-crystalline or grounded powder) does not influence the Raman response at ambient conditions. On the contrary, the choice of the pressure transmitting medium (PTM) employed for the high-pressure measurements affects the Raman signal of Sb2Se3 significantly. More precisely, the Raman spectrum of Sb2Se3 measured with helium as PTM (the PTM employed for this study) is consistent with the Raman spectra obtained from the sample at ambient conditions. Employing mixtures of methanol-ethanol-water (M/E/W) 16:3:1 and methanol-ethanol (M/E) 4:1 as PTM, however, alters the Raman response of Sb2Se3 drastically: the strongest Raman band is now located at ~190 cm−1, whereas the intense Raman peak at ~250 cm−1 is absent from the Raman spectra in both cases. The Raman spectra obtained in this case resemble that of Ref. 29. It should be noted, however, that the Raman spectra obtained with M/E/W and M/E exhibit also distinct differences between each other, e.g. completely different relative intensities of the Raman peaks [Supplemental Fig. S6(b) online]. Therefore, it becomes clear that the choice of PTM affects the Raman response of Sb2Se3. Since the Raman response of Sb2Se3 at ambient conditions matches that of Sb2Se3 embedded in the helium PTM [Supplemental Fig. S6(b) online], we are confident that helium is the proper PTM for conducting high-pressure Raman investigations. In the case of the alcohol mixtures on the other hand, the different Raman spectra imply a possible reaction between the material and the PTM.

Discussion

As mentioned before, the series of A2B3 (A = Sb, Bi; B = S, Se, Te) compounds can be structurally divided into two classes: the heavier Sb2Te3, Bi2Te3 and Bi2Se3 materials adopt a rhombohedral R-3m structure at ambient conditions, with sixfold coordination around the A cations. On the other hand, the lighter Sb2Se3, Sb2S3 and Bi2S3 systems crystallize in the denser Pnma phase with mixed cationic coordinations (sevenfold and eightfold).

Regarding the former, recent high-pressure studies of the Bi2Te36,7,8,9,10,11,12,13, Bi2Se314,15,16,17,18 and Sb2Te39,19,20,21,22,23 compounds have revealed several interesting effects. In particular, a pressure-induced electronic topological transition has been observed for all of these materials in the R-3m phase10,11,18,22. This feature has been associated with an enhancement of their thermoelectric properties under pressure1,42,43. In addition, pressure-induced superconductivity has been reported for both Bi2Te312,13 and Sb2Te323 systems.

Regarding the reported structural transitions, all Bi2Te36,7,8,9,10,11,12,13, Bi2Se314,15,16,17,18 and Sb2Te39,19,20,21,22,23 compounds adopt a monoclinic phase (SG C2/m, Z = 4) above 10 GPa, accompanied by changes in their electronic properties. In this structure, the coordination of the A cations increases to sevenfold with respect to the chalcogenide anions. Upon further pressure increase, the structural evolution varies: both Bi2Te36 and Sb2Te320 compounds adopt another monoclinic structure (SG C2/c, Z = 4) with eightfold coordination at ~13 GPa, whereas a disordered bcc phase (SG Im-3m, Z = 2) appears above 15 GPa and 22 GPa, respectively. In addition, an intermediate bcc-like monoclinic phase (C2/m) has been reported for Sb2Te319. These structural transitions are reversible upon pressure release.

Regarding the pressure-induced structural behavior of Bi2Se3, however, things are more complex: in Ref. 15, Bi2Se3 was shown to transform from the first high-pressure C2/m phase directly into a body-centered tetragonal structure (SG I4/mmm, Z = 2) above 25 GPa15; upon decompression, an amorphous phase is recovered. Given sufficient relaxation time, the orthorhombic Bi2Se3-II phase (isostructural to Sb2Se3), forms from the amorphous material15. This phase has been observed before under combined high-pressure and high-temperature conditions24. Regarding the stability of Bi2Se3-II under pressure, the orthorhombic Pnma phase was reported to persist up to 26 GPa15. In Ref. 14 on the other hand, Bi2Se3 is found to exhibit the same structural sequence as the Bi2Te3 and Sb2Te3 materials, i.e. R-3m- > C2/m- > C2/c. Upon further compression, however, instead of a disordered bcc phase, a novel monoclinic phase (SG C2/m, Z = 4) was observed. In this structure, the Bi cations adopt a mixed ninefold and tenfold coordination with respect to the Se anions. The absence of the high-pressure bcc phase was attributed to the large ionic radii difference between the Bi and Se ions. Given this structural diversity in the high-pressure behavior of Bi2Se3, it appears that the choice of PTM (M/E and Si oil in Ref. 15, M/E in Ref. 14), as well as the sample quality (as-synthesized in Ref. 15, commercial in Ref. 14) may significantly influence the structural evolution of these materials under pressure. Even though a plethora of results is available, it certainly appears that more systematic high-pressure structural investigations are needed in order to establish the connection between the structural evolution of these A2B3 materials and the experimental parameters.

Turning now to the material at hand, the disordered high-pressure Im-3m phase of Sb2Se3 represents a novel binary structure/alloy between Sb and Se atoms; the only known binary crystalline phase between Sb and Se is the Pnma structure with A2B3 composition. The formation mechanism of the bcc alloy is not easy to understand intuitively, since the atomic radii of Sb and Se atoms (rSb = 1.33 Å, rSe = 1.03 Å) are quite different at ambient pressure. A plausible explanation may be that charge transfer from the Sb cations towards the Se anions is taking place under pressure, thus “equalizing” the two ionic radii and favoring the disordered structure, as in Bi2Te36. Such pressure-induced charge transfer is probably correlated with the gradual suppression of the Sb3+ lone electron pair stereochemical activity in the Pnma phase [see Supplementary Fig. S6(b) online].

Given the disordered nature of the high-pressure Bi2Te3, Sb2Te3 and Sb2Se3 bcc phases, we speculate that these modifications will most likely transform into ordered crystalline structures upon sufficient heating (unless decomposition occurs). A plausible structural candidate after such high-temperature treatment might be the defect cubic Th3P4-type phase (SG I-43d, Z = 4), with eightfold cationic coordination. This structure is a common polymorph for rare-earth based A2B3 chalcogenides (γ-phase) at high-temperature and/or high-pressure conditions44. Furthermore, this cubic modification is structurally resilient against pressure increase45. The disordered high-pressure bcc modifications of the heavier Bi2Te3 and Sb2Te3 compounds are more suitable candidates for testing this suggestion, mainly due to their lower transition pressures.

Finally, we would like to add some thoughts on the electronic properties of Sb2Se3 in the high-pressure bcc modification. At ambient conditions, Sb2Se3 is an insulator with a band gap of ~1 eV25. Given that (a) the high-pressure bcc phases of both Bi2Te36,13 and Sb2Te323 are metallic and superconducting and (b) the respective high-pressure CsCl-type/bcc modifications for several binary compounds are also metallic46 and even superconducting47, it is reasonable to assume that the high-pressure bcc phase of Sb2Se3 will also exhibit metallic and possibly superconducting behavior. This assumption, however, needs to be checked by appropriate high-pressure electrical transport measurements.

Methods

Sample and high-pressure technique details

The Sb2Se3 compound was available in single-crystalline form (Alfa-Aesar, 99.999% purity). Both the XRD and TEM measurements at ambient conditions did not detect any impurity phases. For the x-ray measurements, the single crystals were grinded into fine powder.

Pressure was generated by a symmetric diamond anvil cell (DAC) with 300 μm diamond culet diameter. A 150 μm diameter hole was drilled in the middle of a preindented rhenium gasket of 40 μm thickness and served as the sample chamber. Liquid helium was employed as a PTM for both high-pressure x-ray diffraction (XRD) and Raman investigations. The helium loading took place at the gas-loading facility of GeoSoilEnviroCARS/Sector 1348, located at the Advanced Photon Source (APS) of Argonne National Laboratory (ANL). The ruby luminescence method was employed for pressure measurement49.

Angle-dispersive powder x-ray diffraction

The monochromatic angle-dispersive powder XRD measurements under pressure were performed at the 16BM-D beamline of the High Pressure Collaborative Access Team, at APS-ANL. The x-ray beam size was ~8 μm × 12 μm and the x-ray wavelength was λ = 0.4246 Å. The XRD patterns were collected with a MAR 345 Image Plate detector. The intensity. vs. 2θ patterns were obtained using the FIT2D software50. Refinements of the measured XRD patterns were performed using the GSAS + EXPGUI software packages51,52. The measured P-V data for both the ambient-pressure Pnma and high-pressure Im-3m phases were fitted with a Birch-Murnaghan equation of state (EOS)53. Crystal-chemical calculations were performed with the IVTON software54. We should emphasize here that there exist several available settings for SG Pnma; here we choose to employ the same Pnma setting as in isostructural Sb2S331 and Bi2S332 compounds.

Transmission electron microscopy

In order to verify the reversibility of the original phase of Sb2Se3 from the XRD study, we have performed transmission electron microscopy (TEM) studies on the starting Sb2Se3 sample (crushed powder) and the quenched Sb2Se3 powder, i.e. the Sb2Se3 sample recovered after full decompression from the aforementioned XRD study. For this purpose, the samples were dispersed onto holey-carbon TEM grids and were analyzed using a JEOL 3011 microscope by means of high-resolution TEM images (HRTEM) and selected area electron diffraction (SAED) patterns.

High-pressure Raman spectroscopy

The high-pressure Raman experiments were conducted on single-crystalline Sb2Se3 samples with a solid-state laser (λ = 532 nm) coupled to a single-stage Raman spectrometer (Andor S500i) and a charge-coupled device. The spectral resolution was 2 cm−1 and the lowest resolvable frequency was ~75 cm−1. The size of the laser spot on the sample surface was approximately 30 μm, whereas the laser power was 2 mW outside the DAC (unless specified otherwise). In order to investigate the effect of PTM, as well as the effect of the incident laser power on the sample, additional Raman measurements on Sb2Se3 were conducted. These extra Raman studies are presented in the Supporting Information (SI).