Phonon Spectral Characteristics Outweigh Bonding Hierarchy in MSe0.5Te0.5/MSe Thermoelectrics

Abstract Low lattice thermal conductivity (κl) in crystalline solids is often attributed to bonding hierarchy arising from the coexistence of weak and strong chemical bonds. However, the extent to which the bonding hierarchy alone can dictates the intrinsic heat transport remains unclear. Here, using first-principles calculations combined with density functional theory, we investigate the lattice dynamics and carrier transport properties of MSe0.5Te0.5/MSe (M = Ga, In, and Tl) bilayer heterostructures with broken out-of-plane mirror symmetry. The interfacing of Janus MSe0.5Te0.5 layer onto a pristine MSe monolayer provides an ideal platform to disentangle the relative roles of bonding hierarchy and phonon spectral characteristics in determining κl. All the studied heterostructures are found to be energetically and dynamically stable. TlSe0.5Te0.5/TlSe exhibits an ultralow κl of 0.76 W/mK at 300 K, originating from strong phonon softening and bunching together with enhanced Tl rattling vibrations induced by localized charge accumulation that weakens Tl-centered bonds. More importantly, despite possessing a weaker bonding environment, InSe0.5Te0.5/InSe exhibits a higher κl than GaSe0.5Te0.5/GaSe BH. This anomalous trend is attributed to pronounced phonon branching and the absence of a phonon band gap in the intermediate optical range of Ga-based heterostructure, which activate additional three-phonon scattering channels and substantially reduce phonon lifetimes. These findings demonstrate that phonon spectral characteristics can outweigh conventional bonding hierarchy effects in governing lattice thermal transport. Furthermore, the Janus interface preserves favorable carrier transport while suppressing the Mexican-hat-like valence-band dispersion. Consequently, maximum thermoelectric figures of merit of 2.08 (p– type), 2.82 (n– type), and 0.38 (p– type) are achieved for GaSe0.5Te0.5/GaSe, InSe0.5Te0.5/InSe, and TlSe0.5Te0.5/TlSe, respectively, at 800 K.