Sb₂Se₃ under pressure

Abstract

Selected members of the A₂B₃ (A = Sb, Bi; B = Se, Te) family are topological insulators. The Sb₂Se₃ compound does not exhibit any topological properties at ambient conditions; a recent high-pressure study, however, indicated that pressure transforms Sb₂Se₃ from a band insulator into a topological insulator above ~2 GPa; in addition, three structural transitions were proposed to occur up to 25 GPa. Partly motivated by these results, we have performed x-ray diffraction and Raman spectroscopy investigations on Sb₂Se₃ under pressure up to 65 GPa. We have identified only one reversible structural transition: the initial Pnma structure transforms into a disordered cubic bcc alloy above 51 GPa. On the other hand, our high-pressure Raman study did not reproduce the previous results; we attribute the discrepancies to the effects of the different pressure transmitting media used in the high-pressure experiments. We discuss the structural behavior of Sb₂Se₃ within the A₂B₃ (A = Sb, Bi; B = Se, Te) series.

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.

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.

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.

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.