Probing of Strategic Disorder to Tune Thermoelectric Properties in Sb, Zn Co-doped Mg2Si0.3Sn0.7 through Extended X-ray Absorption Fine Structure Spectroscopy

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Abstract
Defect engineering in thermoelectric materials is a key approach for controlling lattice vibrations. Introducing defects in the lattice brings disorder, which facilitates scattering of phonons and suppresses heat transport. Although this can be beneficial for enhancing the material’s thermoelectric performance, increased disorder may also reduce charge carrier mobility, thereby lowering the electrical conductivity and overall thermoelectric efficiency. Therefore, optimizing the efficiency of thermoelectric materials requires careful management of the delicate interplay between structural order and disorder. In our recent work, controlled disorder was achieved in Mg2(Si0.3Sn0.7)0.98Sb0.02 through Zn doping. This approach simultaneously enhanced carrier mobility (∼90 cm2/Vs) and weighted mobility (∼660 cm2/Vs) at 300 K while suppressing lattice thermal conductivity. In the present work, we investigate the nature of this disorder by employing a combination of experimental techniques including extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), positron annihilation lifetime spectroscopy (PALS), powder X-ray diffraction (XRD), and transmission electron microscopy (TEM). EXAFS analysis reveals that Zn occupies Mg-sites (ZnS) and non-Mg (ZnNS) sites in nearly equal proportions. XANES results confirm that ZnNS sites promote p-d orbital hybridization, resulting in broader electronic bandwidth and enhanced charge transport. XRD and TEM analyses confirm that the dislocation density (ND), predominantly associated with edge-type dislocations, increases monotonically with Zn doping, thereby enhancing phonon scattering. PALS measurements further support these findings by revealing the evolution of defects and the formation of vacancy clusters. Thus, controlled disorder provides an effective strategy for designing high performance thermoelectric materials.

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