Forced bond ionization–driven design of ultralow lattice thermal conductivity materials for flexible thermoelectrics

The search for development strategies that yield low κ_lat has become the focus of thermoelectrics and barrier coatings. Here, we propose a "forced bond ionization" strategy by integrating conflicting coordination environments (planar three coordination versus tetrahedral four coordination of Cu) to form pseudo-tetrahedral structures. This approach induces partial ionization of Cu─I bonds in Cu₅TeS₃I₃ (CTSI), yielding a record-low κ_lat of 0.17 W/(m·K) for dense inorganic polycrystals. The pseudo-tetrahedral configuration triggers shear modes, markedly reducing the transverse speed of sound (ν_T = 839 m/s) and amplifying anharmonicity (Grüneisen parameter γ = 2.76). Theoretical analysis reveals that coordination preference competition provides Cu atoms a metastable site, promoting the disordered behavior. The corresponding vibrations of I atoms and disordered Cu atoms dominate the phonon scattering while the material having remarkable flexibility and certain thermoelectric potential. This work establishes a bond ionization-driven design paradigm for ultralow κ_lat materials, marking a leap toward potential flexible thermoelectric applications.