The Seebeck coefficient 𝑆, electrical conductivity 𝜎 and electronic thermal conductivity 𝜅 el depend on the electronic structure of the material and can be fine-tuned by chemical doping. All three are interdependent through the carrier concentration 𝑛 and must be balanced to optimize the 𝑍𝑇 at a target operating temperature. The lattice thermal conductivity 𝜅 latt is the transport through phonons, which depends on the structure and bonding in the material and can in principle be optimized independently of the electrical properties. The requirement for a favorable electronic structure and low 𝜅 latt has led to a number of high-performance TEs based on "heavy" chalcogenides with strongly-anharmonic lattice dynamics and intrinsically low thermal conductivity. PbTe is generally considered as a standard for hightemperature thermoelectric applications and can reach a 𝑍𝑇 of 2.2 at 915 K with hierarchical nanostructuring. Its high performance has been attributed to anharmonic lattice dynamics and to a convergence of the electronic bands at elevated temperature leading to enhanced band degeneracy and a larger Seebeck coefficient. More recently, SnSe was shown to have a very high bulk 𝑍𝑇 of up to 2.6 at 923 K, due to an ultra-low 𝜅 latt associated with the high-temperature phase transition. With a 𝑍𝑇 of ~1.0 between 300-400 K, Bi2Te3 is the commercial standard for room-temperature TEGs at present, due to its favorable electronic properties and inherently low 𝜅 latt , 2,15 the latter of which has been attributed to weak chemical bonding and highly anharmonic lattice dynamics. However, the low terrestrial abundance of Te (ca. 1 ppb) limits large-scale adoption. Therefore, there has been substantial investigation into variants of Bi2Te3 in the same tetradymite family (e.g. Bi2Se2Te) and using strategies such as alloying to reduce or eliminate Te. Doping and alloying are common design strategies for enhancing the 𝑍𝑇 of thermoelectrics, as they can both improve the Seebeck coefficient by promoting band convergence and reduce the 𝜅 latt by introducing point defects that scatter phonons through mass-and strain-field fluctuations. A number of recent studies on bismuth chalcogenide alloy systems, including Bi2(S,Se)3, Bi2(S,Te)3 and Bi2(Se,Te)3, have reported a 𝑍𝑇 of 0.5-1.3 (Table ). Table Reported thermoelectric figures of merit 𝑍𝑇 of some bismuth chalcogenide alloys.