s-Orbital Mediated Metavalent Bonding Enables State-Of-The-Art n-Type AgBiSe₂ Thermoelectrics

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 Bi₂Te₃, 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 AgBiSe₂, which crystallizes in three different phases: hexagonal, rhombohedral, and cubic. Quantum chemical calculations and bond-breaking behavior reveal that all three octahedrally coordinated AgBiSe₂ 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 (AgBiSe₂)_0.75(PbTe)_0.25−0.01BiBr₃ at 798 K. The understanding and tailoring of chemical bonds achieved in AgBiSe₂ can be easily extended to other AgVVI₂ compounds.