Phonon Scattering by Local Off‐Centering in Diamond‐Like Cu _2−X Ag _x In ₂ Se ₄ Chalcopyrites: High Carrier Mobility and Ultralow Thermal Conductivity

Diamond‐like Cu 2−x Ag x In 2 Se 4 ( x = 0–0.3) chalcopyrites were synthesized by arc‐melting and investigated as sustainable, tellurium‐free thermoelectric (TE) materials. Synchrotron X‐ray diffraction reveals that partial Ag substitution induces lattice expansion and local off‐centering distortions within the tetrahedral Cu/Ag‐Se network, resulting in pronounced anisotropic atomic displacement parameters. This dynamic lattice disorder, reminiscent of emphanitic behavior, enhances phonon scattering and drives a marked reduction of the lattice thermal conductivity down to ≈1.25 W m −1 K −1 at 748 K. Despite a moderate carrier concentration (∼10 18 cm −3 ), the alloys maintain high carrier mobility up to 400 cm 2 V −1 s −1 , yielding a power factor of 0.33 mW m −1 K −2 for Cu 1.9 Ag 0.1 In 2 Se 4 . Hybrid‐functional density functional theory calculations confirm progressive bandgap widening with unaltered conduction‐band curvature, consistent with the transport trends. These findings reveal a direct nanoscale structure–property relationship in Ag‐alloyed CuInSe 2 chalcopyrites, where local off‐centering acts as an intrinsic phonon‐scattering mechanism. The work establishes a sustainable, tellurium‐free platform for designing high‐mobility TEs through controlled lattice disorder.