Two-Dimensional-Like Phonons in Three-Dimensional-Structured Rhombohedral GeSe-Based Compounds with Excellent Thermoelectric Performance
Jingjing Cui, Chenghao Xie, Weiwei Hu, Hao Luo, Qicai Mei, Songlin Li, Weibin Xu, Zhibin Gao, Jinsong Wu, Qingjie Zhang, Xinfeng Tang, Gangjian Tan
Abstract
The coupling of charge and phonon transport in solids is a long-standing issue for thermoelectric performance enhancement. Herein, two new narrow-gap semiconductors with the same chemical formula of GeSe 0.65 Te 0.35 (GST) are rationally designed and synthesized: one with a layered hexagonal structure (H-GST) and the other with a non-layered rhombohedral structure (R-GST). Thanks to the three-dimensional (3D) network structure, R-GST possesses a significantly larger weighted mobility than H-GST. Surprisingly, 3D-structured R-GST displays an extremely low lattice thermal conductivity of ∼0.5 W m –1 K –1 at 523 K, which is comparable to that of layered H-GST. The two-dimensional (2D)-like phonon transport in R-GST stems from the unique off-centering Ge atoms that induce ferroelectric instability, yielding soft polar phonons, as demonstrated by the Boson peak detected by the low-temperature specific heat and calculated phonon spectra. Furthermore, 1 mol % doping of Sb is utilized to successfully suppress the undesired phase transition of R-GST toward H-GST at elevated temperatures. Consequently, a peak ZT of 1.1 at 623 K is attained in the rhombohedral Ge 0.99 Sb 0.01 Se 0.65 Te 0.35 sample, which is 1 order of magnitude larger than that of GeSe. This work demonstrates the feasibility of exploring high-performance thermoelectric materials with decoupled charge and phonon transport in off-centering compounds.