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. (© 2022 Wiley‐VCH GmbH)