High‐Performance Thermoelectric α‐Ag<sub>9</sub>Ga<sub>1−<i>x</i></sub>Te<sub>6</sub> Compounds with Ultralow Lattice Thermal Conductivity Originating from Ag<sub>9</sub>Te<sub>2</sub> Motifs
Yingfei Tang, Yimeng Yu, Na Zhao, Keke Liu, Haijie Chen, Constantinos C. Stoumpos, Yixuan Shi, Shuo Chen, Lingxiao Yu, Jinsong Wu, Qingjie Zhang, Xianli Su, Xinfeng Tang
Abstract
Abstract We have determined the complex atomic structure of high‐temperature α‐Ag 9 GaTe 6 phase with a hexagonal lattice ( P 6 3 mc space group, a = b =8.2766 Å, c =13.4349 Å). The structure has outer [GaTe 4 ] 5− tetrahedrons and inner [Ag 9 Te 2 ] 5+ clusters. All of the Ag ions are disorderly distributed in the lattice. Seven types of the Ag atoms constitute the cage‐like [Ag 9 Te 2 ] 5+ clusters. The highly disordered Ag ions vibrate in‐harmonically, producing strong coupling between low frequency optical phonons and acoustic phonons. This in conjunction with a low sound velocity of 1354 m s −1 leads to an ultralow thermal conductivity of 0.20 W m −1 K −1 at 673 K. Meanwhile, the deficiency of Ga in Ag 9 Ga 1− x Te 6 compounds effectively optimizes the electronic transport properties. Ag 9 Ga 0.91 Te 6 attains a highest power factor of 0.40 mW m −1 K −2 at 673 K. All these contribute to a much‐improved ZT value of 1.13 at 623 K for Ag 9 Ga 0.95 Te 6 , which is 41 % higher than that of the pristine Ag 9 GaTe 6 sample.