High-Performance Thermoelectric α-Ag 9 Ga 1-x Te 6 Compounds with Ultralow Lattice Thermal Conductivity Originating from Ag 9 Te 2 Motifs.

We have determined the complex atomic structure of high-temperature α-Ag 9 GaTe 6 phase with a hexagonal lattice (P6 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.