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 \(_2\) ) in its 1T (CdI \(_2\) -type, \(P\bar{3}m1\) ) 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 ( \(\sim 5.0\times 10^3\) 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 \(_2\) as a 2D material whose vibrational softness naturally enforces ultralow lattice thermal conductivity, underscoring its potential for thermoelectric applications.
