The quest for intrinsically low lattice thermal conductivity (${\ensuremath{\kappa}}_{\mathrm{L}}$) has gained widespread attention in thermoelectrics and refractories. The reduction of ${\ensuremath{\kappa}}_{\mathrm{L}}$ in low-dimensional systems typically arises from anharmonicity and anisotropic chemical bonding. Due to the structural instability and the technical complexity, the thermal functional application of practical low-dimensional materials confronts great challenges. In this context, the investigations of quasi-low-dimensional bulk materials, rather than the real low-dimensional materials, hold paramount importance from both fundamental and practical perspectives. Here, we employ the ab initio self-consistent phonon (SCPH) theory in combination with the compressive sensing lattice dynamics method to explore the lattice dynamics and microscopic mechanisms of thermal transport in emerging pentanary thiophosphates ${A}_{2}\mathrm{PdP}{\mathrm{S}}_{4}\mathrm{I}$ ($A=\mathrm{K}$, Rb, Cs). These compounds feature covalent one-dimensional (1D) chains, providing bonding heterogeneity and resulting in flat phonon bands and low phonon group velocities (${V}_{\mathrm{ph}}$). The strengthened anharmonicity stems from the cooperative rattlinglike vibrations of the interchain alkali metal atoms and I atoms. In K- and Rb-containing systems, the cooperative vibrations of the rattling unit (K/Rb-I) result in strong anharmonicity, while in Cs-containing system, the abnormally enhanced particlelike ${\ensuremath{\kappa}}_{\mathrm{L}}$ (${\ensuremath{\kappa}}_{\mathrm{p}}$) is due to the antibonding interaction breaking the cooperative vibration of the rattling unit (Cs-I), thereby weakening the anharmonicity. Upon heating, the ${\ensuremath{\kappa}}_{\mathrm{p}}$ show decreasing behavior. However, the wavelike ${\ensuremath{\kappa}}_{\mathrm{L}}$ (${\ensuremath{\kappa}}_{\mathrm{c}}$) exhibits a positive temperature dependence, highlighting the nonnegligible coherent phonon contributions to the ${\ensuremath{\kappa}}_{\mathrm{L}}$ and triggering anomalous temperature-dependent behavior of ${\ensuremath{\kappa}}_{\mathrm{L}}$. Our study uncovers the multifaceted microscopic mechanisms behind the low ${\ensuremath{\kappa}}_{\mathrm{L}}$ in three quasi-one-dimensional (Q-1D) systems of ${A}_{2}\mathrm{PdP}{\mathrm{S}}_{4}\mathrm{I}$ and provides fresh insights for the discovery and design of Q-1D low ${\ensuremath{\kappa}}_{\mathrm{L}}$ materials.
