Indium-Assisted Gallium Solubility Expansion and Defect Hierarchy Evolution in Thermoelectric ZnO Ceramics

Ga-In co-doped ZnO ceramics were synthesized by a solid-state reaction to elucidate the role of defect chemistry and dopant solubility in governing thermoelectric transport. Compared with pristine ZnO and Ga singly doped ZnO, the In-Ga co-doped sample (IGZO) exhibits a distinct evolution in microstructure and defect configuration. A minor fraction of Ga₂O₃(ZnO)₉ spinel phase is present, contributing to phonon scattering as part of a composite-like microstructure. Structural analyses reveal that In incorporation suppresses the formation of Ga₂O₃(ZnO)₉ spinel clusters, indicating an expansion of Ga solubility in the ZnO lattice. This solubility-assisted effect leads to a transition from secondary-phase-dominated microstructures toward point-defect-dominated configurations. The competition between spinel clusters and point defects plays a key role in determining phonon scattering behavior, where atomic-scale defects become dominant in the IGZO ceramic. At the same time, the enhanced substitution of Ga, assisted by In, modifies the electronic structure through increased carrier concentration and density-of-states effective mass. As a result, the IGZO ceramic exhibits reduced lattice thermal conductivity and modified charge transport behavior under conventional bulk processing conditions, with a ZT of 0.182 at 1073 K. These findings highlight that solubility-assisted defect engineering provides an effective pathway to tune defect hierarchy and transport mechanisms in ZnO-based thermoelectric materials.