Co-alloying of Sn and Te enables high thermoelectric performance in Ag9GaSe6

As a typical liquid-like material, the argyrodite-type Ag9GaSe6 has attracted considerable attention in the past decade due to its liquid-like characteristics, complex crystal structures, and high carrier mobility. However, the carrier concentration of pristine Ag9GaSe6 is still not optimal while the thermal conductivity remains to be further reduced. Herein, we simultaneously optimize the electrical and thermal properties of Ag(9)GaSe(6)via alloying Sn at the Ga sites and Te at the Se sites. It is found that the crystal symmetry is well maintained, while the phase transition temperature is reduced and local chemical bonding is altered after alloying. Due to the nominally reduced Ag content and the change of chemical bonding, the carrier concentration is reduced by two orders of magnitude, giving rise to a largely improved Seebeck coefficient. Meanwhile, the lattice thermal conductivity kappa(L) of Ag9-x(Ga1-xSnx)(Se1-xTex)(6) is significantly suppressed by the strong phonon scattering from point defects. An ultralow kappa(L) of similar to 0.25 W m(-1) K-1, which approaches the theoretical minimum kappa(L), is attained at room temperature in the alloyed samples. Consequently, the zT value is boosted to 1.4 for the x = 0.1 sample, which represents an improvement of 75% over that of pristine Ag9GaSe6.