Anomalous carrier transport enhances thermoelectricity in HfSe₂/SnX₂ (X = S, Se)-based heterostructures

Crystal structure and chemical bonding are well recognized key factors governing heat transport, yet strategies that leverage bonding principles to reduce lattice thermal conductivity (κ_l) remain relatively unexplored. Herein, considering the experimental synthesizability of 1T-HfSe₂, we use first-principles based density functional theory to present a bonding-anomaly-driven approach in a homo-bilayer of HfSe₂, where tailored cation/anion modulation weakens interatomic interactions, suppressing κ_l and enhancing charge transport for superior thermoelectric performance. We obtain ultralow κ_l values of 1.10 W m^-1 K^-1 and 1.87 W m^-1 K^-1 for cation and dual-site substituted HfSe₂/SnSe₂ and HfSe₂/SnS₂ bilayer heterostructures (BLHs), respectively, at 300 K. These values arise from distinct lattice dynamics induced by bunching of low-lying optical (LLO) phonon branches and softened ZA mode that enable strong AAO and AOO phonon scattering pathways, driven by a pronounced charge localisation anomaly around Sn atoms triggering their rattling motion and fosters the formation of extended antibonding states below the Fermi level. Furthermore, to avoid mobility overestimation via deformation potential theory (DPT), we include the Fröhlich interaction to account for longitudinal optical (LO) phonon scattering, yielding a more reliable mobility estimate. Despite S incorporation, HfSe₂/SnS₂ shows high electrical conductivity (141.9 × 10⁴ S m^-1 and 144.2 × 10⁴ S m^-1 for p- and n-type carriers, respectively, at 800 K) arising from suppressed LO phonon scattering, reduced Seebeck coefficient, and a narrow bandgap anomaly. Harnessing these bonding induced thermal and electronic transport anomalies yields outstanding n- and p-type thermoelectric performance, with maximum ZT of 2.91 and 2.53 for HfSe₂/SnSe₂ and 1.93 and 2.10 for HfSe₂/SnS₂ BLHs, respectively, at 800 K.