Microscopic insights to the ultralow thermal conductivity of monolayer 1T-SnTe₂

Two-dimensional (2D) metallic systems with intrinsically low lattice thermal conductivity are rare, yet they are of great interest for next-generation energy and electronic technologies. Here, we present a comprehensive first-principles investigation of monolayer tin telluride (SnTe[Formula: see text]) in its 1T (CdI[Formula: see text]-type, [Formula: see text]) structure. Our calculations establish its energetic and dynamical stability, confirmed by large cohesive (10.9 eV/atom) and formation (- 4.06 eV/atom) energies and a phonon spectrum free of imaginary modes. The electronic band structure reveals metallicity arising from strong Sn-Te p orbital hybridization. Most importantly, phonon dispersion analysis uncovers a microscopic origin for the ultralow lattice thermal conductivity: the heavy mass of Te atoms, weak Sn-Te bonding, and flat acoustic branches that yield exceptionally low and anisotropic group velocities ([Formula: see text] m/s), together with the absence of a phonon bandgap that enhances Umklapp scattering. These features converge to suppress phonon-mediated heat transport. Complementary calculations of the optical dielectric response and joint density of states reveal pronounced interband transitions and a plasmonic resonance near 4.84 eV, suggesting additional optoelectronic opportunities. These findings establish monolayer SnTe[Formula: see text] as a 2D material whose vibrational softness naturally enforces ultralow lattice thermal conductivity, underscoring its potential for thermoelectric applications.