Bi₂Te₃-Sb₂Te₃ alloys ((Bi_1−XSb_x)₂Te₃, BST-x) have been extensively studied due to their outstanding thermoelectric properties at near-room temperature ranging from 300-500K. At ambient conditions, Bi₂Te₃, Sb₂Te₃ and BST alloys adopt the rhombohedral tetradymite crystal structure (space group R − 3m (166)), formed by monoatomic sublayers of Bi/Sb, and Te atoms, independently of the relative concentration of Bi and Sb. The quintuple layers of Te-Bi(Sb)-Te-Bi(Sb)-Te are stacked in a sequence along c-axis direction.
The high pressure studies of pure Bi₂Te₃ and Sb₂Te₃, mainly stemmed from the possibility of using pressure to improve their higher thermoelectric conversion efficiency and the increasing interest of the superconducting phase. Bi₂Te₃ and Sb₂Te₃, exhibit, at least, three phase transitions under pressure. Zhu et al. [1] clarified that Bi₂Te₃ transforms from the rhombohedral R − 3m (six-fold) phase (α-Bi₂Te₃ or phase I) to a monoclinic C2/m (seven-fold) phase at 8 GPa (β-Bi₂Te₃ or phase II) and then to a C2/c (eight-fold) phase at 14 GPa (γ-Bi₂Te₃ or phase III). At higher pressures, Bi₂Te₃ adopts a disorder solid- solution body-center cubic (BCC, A2) Im − 3m structure (phasse IV), with a 40% and 60% occupancy of the 2a Wyckoff Position (WP) for Bi and Te atoms, respectively [1,2]. Identical structural evolution was concluded for Sb₂Te₃ [3,4], albeit with slightly different critical pressures.
Studies on the structural evolution under pressure of Bi_1−XSb_xTe₃ alloys are extremely limited. Indeed, Bai et al. [5] studied Bi_0.5Sb_1.5Te₃ (BST-0.75) only inside the pressure stability range of the ambient phase, <10 GPa. Motivated by the above, we have performed a detailed comparative in − situ synchrotron angle-dispersive powder x-ray diffraction (ADXRD) study of three different BST-X alloys: x=0.2, 0.7 and 0.9, up to 20+ GPa. Our main motivation was to explore the structural evolution under pressure as a function of varying Sb concentration.
Our results document that the three BST alloys follow the same phase sequence with the pure Bi₂Te₃ and Sb₂Te₃ under pressure, without any noticeable effect of the Sb concentration on the critical pressures for the first two phase transitions. On the other hand, it is apparent that the increase of the Sb concentration results to an increase of the critical pressure for the formation of the BCC phase. According to previous high-pressure studies of the ambient phase of Bi₂Te₃, and this was attributed to an electronic topological transition (ETT), that mainly affects the intralayer electronic dis- tribution of the strong ionocovalent bonds, making a-axis more compressible [6]. The same crossover behavior of the c/a ratio was observed for all the BST aloys of this study. Although defining the exact pressure for the crossover is challenging, no clear trend was observed with varying Sb concentration. Thus, we conclude that the appearance of the EET is universal in BST aloys and independent of Sb concentration.
In the case of BST-0.7, our study was extended to ≈ 180 GPa, with the aim of exploring the stability of the BCC phase under Mbar pressures. Surprisingly, we observe that the BCC solid-solution phase remains stable up to the highest pressure of this study. Finally, for the same BST-0.7, in − situ electrical transport measurements as a function of pressure and temperature were performed. A clear indication for metallization was observed above ≈ 12 GPa.

 [1] L. Zhu, H. Wang, Y. Wang, J. Lv, Y. Ma, Q. Cui, Y. Ma, and G. Zou, Substitutional Alloy of Bi and Te at High Pressure, Phys. Rev. Lett. 106, 145501 (2011).
[2] M. Einaga, A. Ohmura, A. Nakayama, F. Ishikawa, Y. Yamada, and S. Nakano, Pressure- induced phase transition of Bi2Te3 to a bcc structure, Phys. Rev. B 83, 092102 (2011).
[3] Y. Ma, G. Liu, P. Zhu, H. Wang, X. Wang, Q. Cui, J. Liu, and Y. Ma, Determinations of the high-pressure crystal structures of Sb2Te3, J. Phys.: Condens. Matter 24, 475403 (2012).
[4] S. M. Souza, C. M. Poffo, D. M. Triches, J. C. de Lima, T. A. Grandi, A. Polian, and M. Gauthier, High pressure monoclinic phases of Sb2Te3, Phys. B: Condens. Matter 407, 3781 (2012).
[5] F.-X. Bai, H. Yu, Y.-K. Peng, S. Li, L. Yin, G. Huang, L.-C. Chen, A. F. Goncharov, J.-H. Sui, and F. Cao, Electronic topological transition as a route to improve thermoelectric performance in Bi0.5Sb1.5Te3, Adv. Sci. 9, 2105709 (2022).
[6] A. Polian, M. Gauthier, S. M. Souza, D. M. Trichˆes, J. a. Cardoso de Lima, and T. A. Grandi, Two-dimensional pressure-induced electronic topological transition in Bi2Te3, Phys. Rev. B 83, 113106 (2011).

 

Thermoelectric Effect,Bi2Te3,electrical transport properties