High-Performance Thermoelectric α-Ag₉ Ga_1-x Te₆ Compounds with Ultralow Lattice Thermal Conductivity Originating from Ag₉ Te₂ Motifs

We have determined the complex atomic structure of high-temperature α-Ag₉ GaTe₆ phase with a hexagonal lattice (P6₃ mc space group, a=b=8.2766 Å, c=13.4349 Å). The structure has outer [GaTe₄ ]^5- tetrahedrons and inner [Ag₉ Te₂ ]⁵⁺ clusters. All of the Ag ions are disorderly distributed in the lattice. Seven types of the Ag atoms constitute the cage-like [Ag₉ Te₂ ]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 Ag₉ Ga_1-x Te₆ compounds effectively optimizes the electronic transport properties. Ag₉ Ga_0.91 Te₆ 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 Ag₉ Ga_0.95 Te₆ , which is 41 % higher than that of the pristine Ag₉ GaTe₆ sample.