Se‐Induced Phase Stabilization of CsAg 5 Te 3 ‐Dominant Composites for Enhanced Thermoelectric Performance

Silver chalcogenides, characterized by soft chemical bonding and strong lattice anharmonicity, have emerged as promising thermoelectric materials. However, obtaining phase‐pure bulk CsAg 5 Te 3 remains challenging because Ag‐rich secondary phases readily form during consolidation. Here, we demonstrate that an isoelectronic Se substitution strategy effectively stabilizes CsAg 5 Te 3 as the dominant phase in spark plasma sintered bulks, while simultaneously constructing a controllable CsAg 5 Te 3 /Ag 2 Te composite architecture. X‐ray diffraction with quantitative phase analysis reveals a clear Se‐dependent phase evolution. The pristine sample is Ag 2 Te‐rich, whereas increasing Se content progressively shifts the phase equilibrium toward CsAg 5 Te 3 ‐dominant composites, maintaining CsAg 5 Te 3 as the primary phase across the investigated compositions. Microstructural characterization further reveals heterogeneous cation distribution and abundant phase boundaries, confirming the in situ formation of a composite microstructure. This engineered composite architecture promotes synergistic transport behavior, in which Se‐substituted samples exhibit improved electrical transport properties together with significantly reduced thermal conductivity. A minimum lattice thermal conductivity of 0.14 W m −1 K −1 at 623 K is achieved for CsAg 5 Te 2.85 Se 0.15 . Consequently, a maximum ZT (dimensionless thermoelectric figure of merit) of 0.82 at 673 K is obtained for CsAg 5 Te 2.85 Se 0.15 , representing a 164.5% enhancement and highlighting the effectiveness of Se‐driven phase composition evolution combined with in situ composite engineering in synergistically optimizing phonon and charge transport.