figure-of-merit (zT) of a TE material, which is given by zT = S 2 σT/κ, where S, σ, T, and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. [3] Nevertheless, the competing interplay between electrical and thermal properties has stagnated the rise of zT for decades. [4] Ternary ABX 2 -type (A = group I, B = group V, X = group VI elements) compounds have gained considerable attention as promising candidates for TE applications. [5] Lone-pair electrons of B-site cations contribute to strong phonon-phonon interactions and lower the lattice thermal conductivity. [6] Therefore, several ABX 2 compounds show temperature-independent thermal conductivities unlike those of conventional TE materials. [7] This intriguing property provides a venue for modulating electronic properties with more degrees of freedom because ABX 2 compounds are relatively free from the complicated trade-off relationship between electrical and thermal properties. However, utilizing ABX 2 compounds has been limited by serious problems: an absence of rational design rules, complex phase stability, and poor reproducibility. [8,9] For example, an undoped hexagonal AgBiSe 2 shows a wide range of Seebeck coefficients from −450 to 250 µV K −1 depending on the synthesis condition (Figure 1a,b). In addition, the B-site cation determines the sign of the Seebeck coefficient, where most Sb-based ABX 2 compounds are intrinsically p-type and Bi-based Cation disordering is commonly found in multinary cubic compounds, but its effect on electronic properties has been neglected because of difficulties in determining the ordered structure and defect energetics. An absence of rational understanding of the point defects present has led to poor reproducibility and uncontrolled conduction type. AgBiSe 2 is a representative compound that suffers from poor reproducibility of thermoelectric properties, while the origins of its intrinsic n-type conductivity remain speculative. Here, it is demonstrated that cation disordering is facilitated by Bi Ag charged antisite defects in cubic AgBiSe 2 which also act as a principal donor defect that greatly controls the electronic properties. Using density functional theory calculations and in situ Raman spectroscopy, how saturation annealing with selenium vapor can stabilize p-type conductivity in cubic AgBiSe 2 alloyed with SnSe at high temperatures is elucidated. With stable and controlled hole concentration, a peak is observed in the weighted mobility and the density-ofstates effective mass in AgBiSnSe 3 , implying an increased valley degeneracy in this system. These findings corroborate the importance of considering the defect energetics for exploring the dopability of ternary thermoelectric chalcogenides and engineering electronic bands by controlling self-doping.
