Exploiting mismatch strain and the β–α phase transition for microstructural engineering in thermoelectric Ag2Se Liu, Yu ; https://orcid.org/0000-0001-7313-6740 Kleinhanns, Tobias ; https://orcid.org/0000-0003-1537-7436 Spadaro, Maria Chiara Genç, Aziz Horta, Sharona Navita, Navita ; https://orcid.org/0000-0001-7408-8197 Costanzo, Tommaso ; https://orcid.org/0000-0001-9732-3815 Dutkiewicz, Ewelina Arbiol, Jordi Hong, Min Ibáñez, Maria ; https://orcid.org/0000-0001-5013-2843 ddc:540 Silver selenide (Ag2Se) is a promising near-room-temperature thermoelectric material, but its narrow stoichiometric window and β–α phase transition complicate reproducible microstructure control. Here, we present a mismatch-assisted microstructure engineering strategy in which Ag2Se particles are treated with polyanionic ZnSe complexes and consolidated through the β–α transition to introduce ZnSe nanoprecipitates, Ag2Se/ZnSe interfaces, and local strain fields. The crystallographic mismatch between ZnSe and Ag2Se, together with the Zn2+/Ag+ size difference, amplifies phase-transition-induced deformation and promotes high-density dislocations with periodic strain modulations. This defect architecture suppresses grain coarsening, removes excess Ag, limits Ag-interstitial formation, and reduces lattice thermal conductivity through lattice softening and multiscale phonon scattering. Ag2Se–4%ZnSe nanocomposites achieve a peak zTmax of 1.13 at 369 K and a zTavg of 1.08 from 300 to 380 K, demonstrating mismatch-driven defect engineering through the β–α phase transition as a route for optimizing Ag2Se-based thermoelectrics. American Chemical Society 2026 info:eu-repo/semantics/article doc-type:article text http://purl.org/coar/resource_type/c_6501 https://research-explorer.ista.ac.at/record/22734 https://research-explorer.ista.ac.at/download/22734/22736 Liu Y, Kleinhanns T, Spadaro MC, et al. Exploiting mismatch strain and the β–α phase transition for microstructural engineering in thermoelectric Ag2Se. <i>ACS Energy Letters</i>. 2026;11(8):5752-5762. doi:<a href="https://doi.org/10.1021/acsenergylett.6c01499">10.1021/acsenergylett.6c01499</a> eng info:eu-repo/semantics/altIdentifier/doi/10.1021/acsenergylett.6c01499 info:eu-repo/semantics/altIdentifier/e-issn/2380-8195 info:eu-repo/semantics/openAccess