Bonding hierarchy and coordination interaction leading to high thermoelectricity in wide bandgap TlAgI2

High thermoelectric properties are associated with the phonon-glass electron-crystal paradigm. Conventional wisdom suggests that the optimal bandgap of semiconductor to achieve the largest power factor should be between 6 and $10{\kappa }_{B}T$. To address challenges related to the bipolar effect and temperature limitations, we present findings on Zintl-type ${\mathrm{TlAgI}}_2$, which demonstrates an exceptionally low lattice thermal conductivity of 0.30 W ${\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 300 K. The achieved figure of merit ($ZT$) for ${\mathrm{TlAgI}}_2$, featuring a 2.40 eV bandgap, reaches a value of 1.53 for $p$-type semiconductor. This remarkable $ZT$ is attributed to the existence of extended antibonding states [Ag-I] in the valence band. Furthermore, the bonding hierarchy, influencing phonon anharmonicity, and coordination bonds, facilitating electron transfer between the ligand and the central metal ion, significantly contribute to the electronic transport. This finding serves as a promising avenue for the development of high $ZT$ materials with wide bandgaps at elevated temperatures.