We propose a novel design principle for achieving ultralow thermal conductivity in crystalline materials via a "heavy-light and soft-stiff" structural motif. By combination of heavy and light atomic species with soft and stiff bonding networks, both particle-like (κp) and wave-like (κc) phonon transport channels are concurrently suppressed. First-principles calculations show that this architecture induces a hierarchical phonon spectrum: soft-bonded heavy atoms generate dense low-frequency modes that enhance scattering and reduce κp, while stiff-bonded light atoms produce sparse high-frequency optical branches that disrupt the coherence and lower κc. High-throughput screening identifies Tl4SiS4 (κp = 0.10, κc = 0.06 W/mK) and Tl4GeS4 (κp = 0.09, κc = 0.06 W/mK) as representative candidates with strongly suppressed transport in both channels. A minimal 1D triatomic chain model further demonstrates the generality of this mechanism, offering a new paradigm for phonon engineering beyond the conventional κp-κc trade-off.
Wu et al. (Sun,) studied this question.