Pentagonal transition-metal dichalcogenides $\mathrm{Pd}{X}_2 (X=\mathrm{S},\mathrm{Se},\mathrm{Te})$, characterized by corrugated pentagonal lattices, have attracted considerable attention owing to their unconventional physical properties. In this work, we systematically investigate the anharmonic lattice dynamics and thermal transport properties of monolayer $\mathrm{Pd}{X}_2$ using first-principles calculations combined with harmonic approximation (HA), self-consistent phonon (SCP) theory and Boltzmann transport equation. Our results demonstrate that accurate evaluation of the lattice thermal conductivity (${\kappa }_{\mathrm{L}}$) for pentagonal $\mathrm{Pd}{X}_2$ monolayers requires effective-thickness correction, without which the predicted values markedly deviate from experimental measurements. In addition, cubic and quartic anharmonic phonon renormalization and strong four-phonon (4ph) scattering play decisive roles in determining ${\kappa }_{\mathrm{L}}$ in pentagonal $\mathrm{Pd}{X}_2$. Specifically, the SCP dispersions reveal temperature-driven hardening of acoustic and low-frequency optical modes, indicating strong temperature dependence of phonon transport and enhancing group velocities. When 4ph scattering is included, ${\kappa }_{\mathrm{L}}$ suppressed significantly. For example, ${\kappa }_{\mathrm{L}}$ of $\mathrm{Pd}{\mathrm{S}}_2$ along the $b$ axis decreases by 25.93% (300 K) and 43.65% (900 K) under HA+3,4ph relative to HA+3ph, and by 31.96% (300 K) and 47.38% (900 K) under SCP+3,4ph relative to SCP+3ph. For both crystallographic directions and theoretical levels, ${\kappa }_{\mathrm{L}}$ follows the order: $\mathrm{Pd}{\mathrm{S}}_2>\mathrm{PdS}{\mathrm{e}}_2>\mathrm{PdT}{\mathrm{e}}_2$. Owing to the larger atomic mass of Te, $\mathrm{PdT}{\mathrm{e}}_2$ exhibits strongly corrugated structure, enhanced anharmonicity and pronounced rattling modes, together with stronger phonon localization, smaller specific heat capacity, and lower phonon group velocities. These features significantly suppress phonon thermal transport and contribute to lower ${\kappa }_{\mathrm{L}}$. Moreover, $\mathrm{Pd}{X}_2$ display strongly softened acoustic modes and lower ${\kappa }_{\mathrm{L}}$ along the $a$ axis than those along the $b$ axis, reflecting the strong in-plane anisotropy. The pronounced anisotropy originates from the corrugated and flat $\left[\mathrm{Pd}{X}_4\right]$ units along the $a$ and $b$ axis, respectively. Our research highlights the importance of effective-thickness correction on ${\kappa }_{\mathrm{L}}$ for 2D materials and reveal the crucial role of strong anisotropy, lattice anharmonicity, and 4ph scattering in the phonon thermal transport of pentagonal $\mathrm{Pd}{X}_2$.

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• Lattice thermal conductivity
• 2-dimensional systems
• Transition metal dichalcogenides
• First-principles calculations