s‐Orbital Mediated Metavalent Bonding Enables State‐Of‐The‐Art n‐Type AgBiSe ₂ Thermoelectrics

Abstract Tailoring chemical bonds offers an innovative way to design materials for a wide range of applications. Metavalent bonding is conducive to excellent thermoelectric performance in p‐bonded chalcogenides with octahedral coordination. However, the requirement to form a bond through only a single p‐electron between adjacent atoms (half of an electron pair), such as in PbTe and Bi 2 Te 3 , limits the number of possible materials. Here, it is shown that the essence of metavalent bonding is a half‐filled single‐electron σ‐bond, which can also be formed with a significant s‐orbital contribution. This is illustrated for AgBiSe 2 , which crystallizes in three different phases: hexagonal, rhombohedral, and cubic. Quantum chemical calculations and bond‐breaking behavior reveal that all three octahedrally coordinated AgBiSe 2 phases utilize metavalent bonding. In addition, PbTe alloying is used to tune the chemical bonding and Br doping to optimize the carrier concentration. With these modifications, a record‐high zT max value of 1.1 is achieved in n‐type cubic (AgBiSe 2 ) 0.75 (PbTe) 0.25 −0.01BiBr 3 at 798 K. The understanding and tailoring of chemical bonds achieved in AgBiSe 2 can be easily extended to other AgVVI 2 compounds.