Structure Evolution and Bonding Inhomogeneity toward High Thermoelectric Performance in Cu₂CoSnS_4-x Se _x Materials

Lightweight diamond-like structure (DLS) materials areexcellentcandidates for thermoelectric (TE) applications due to their low costs,eco-friendly nature, and property stability. The main obstacles restrictingthe energy-conversion performance by the lightweight DLS materialsare high lattice thermal conductivity and relatively low carrier mobility.By investigating the anion substitution effect on the structural,microstructural, electronic, and thermal properties of Cu2CoSnS4-x Se x , we show that the simultaneous enhancement of the crystalsymmetry and bonding inhomogeneity engineering are effective approachesto enhance the TE performance in lightweight DLS materials. Particularly,the increase of x in Cu2CoSnS4-x Se x makes the DLS structurewith the ideal tetrahedral bond angles of 109.5 degrees favorable, leadingto better crystal symmetry and higher carrier mobility in sampleswith higher selenium content. In turn, the phonon transport in theinvestigated DLS materials is strongly disturbed due to the bondinginhomogeneity between anions and three sorts of cations inducing largelattice anharmonicity. The increase of Se content in Cu2CoSnS4-x Se x only intensified this effect resulting in a lower latticecomponent of the thermal conductivity (kappa(L)) for Se-richsamples. As a result of the enhanced power factor S (2)rho(-1) and the low kappa(L), the dimensionless thermoelectric figure of merit ZT achieves a high value of 0.75 for Cu2CoSnSe4 DLS material. This work demonstrates that crystal symmetry and bondinginhomogeneity play an important role in the transport properties ofDLS materials and provide a path for the development of new perspectivematerials for TE energy conversion.