Correlating synthesis, microstructural engineering and thermoelectric performance in BiCuSeO

Layered BiCuSeO chalcogenides have attracted attention as potential oxide thermoelectric material due to inherently low thermal conductivity values, however, accomplishment of simultaneous enhancement in electronic and thermal transport properties remains challenging. In this work, we methodically investigate the effect of high-energy ball milling duration and consolidated ball milling–vacuum sealing processing on the microstructure, transport properties, and material efficiency of polycrystalline BiCuSeO. A distinguishing aspect of this work is the establishment of direct correlation among factors like processing-induced grain refinement, porosity evolution, directional phonon/charge transport and grain connectivity via an integrated experimental and computational approach. The microstructural analysis of the samples prepared using High Energy Ball Milling (HEBM) revealed that short-duration milling cycle produced refined grains and defect-rich structures that aid in suppression of lattice thermal conductivity. The lowest lattice thermal conductivity is recorded for BM6 sample (~0.23 W m⁻¹ K⁻¹) along with the highest thermoelectric performance with a peak ZT of 0.63 at 873 K. In contrast, vacuum sealing process improved the microstructural morphology in the prepared sample, yielding a highest power factor of 0.32 mW m⁻¹ K⁻² at 873 K. First-principles calculations suggest that antibonding Cu–Se states near the valence band maximum leads to softened lattice along with the generation of highly anharmonic low-energy phonon modes, thus, inducing ultralow thermal conductivity. The calculations further signify a strong decoupling between electronic and phonon transport, transforming into phonon-glass electron-crystal–like behavior. These findings provide mechanistic insight into the interplay between processing induced microstructural evolution and directional charge/phonon transport, establishing microstructural engineering as a viable route for tuning the thermoelectric response of undoped layered BiCuSeO.