Cation Off-Centering Drives Unusual Synthesizability and Extraordinarily Low Thermal Conductivity in Metastable AgPbSbS₃

Understanding and controlling phase stability in compositionally complex chalcogenides remains a central challenge in solid-state chemistry. Among these systems, AgPbSbS₃ is a particularly elusive composition that has resisted stabilization as a pure crystalline phase, thus preventing reliable assessment of its intrinsic lattice and transport behavior. In this study, we identify the microscopic origin of its synthetic difficulty and demonstrate a viable strategy to access this otherwise unattainable phase. Atomic-resolution electron microscopy and pair distribution function analyses reveal pronounced local symmetry breaking and cation off-centering, generating strong distortions from ideal coordination polyhedra and destabilizing long-range order. We introduce a small amount of iodine with partially charge-compensated by sulfur and employ rapid pressure-assisted densification, enabling the synthesis of cubic AgPbSbS_2.96I_0.06 as a single phase. The resulting material exhibits extraordinarily low thermal conductivity of ∼0.5 W m^–1 K^–1 at 300 K, even lower than the corresponding heavier analogues, with minimal temperature dependence within this structure. Sound-velocity measurements indicate substantial phonon softening, consistent with significant bond anharmonicity and the resulting nanoscale structural heterogeneity. This work illustrates how targeted chemical modulation combined with nonequilibrium processing can unlock metastable multicomponent chalcogenides and expand design opportunities for crystalline materials exhibiting ultralow thermal conductivity.