Materials with anisotropic thermal conductivity enable directional heat transfer and are critical for advanced thermal management. Transition-metal pentatellurides, especially $\mathrm{HfT}{\mathrm{e}}_5$, have attracted increasing interest due to the recently reported record-high in-plane thermal conductivity anisotropy ratio ($\sim 3.5$) at room temperature. However, the microscopic mechanisms underlying this extreme anisotropy, particularly the lattice dynamics and phonon properties, have not been systematically investigated. Here, we reveal that the strong bonding inhomogeneity—featuring robust chainlike bonds along the $a$-axis and weak interchain bonds along the $c$-axis—leads to pronounced anisotropy in phonon dispersions, group velocities, and mean free paths. Notably, low-energy optical phonons along the $a$-axis are strongly activated, acquiring high group velocities and long mean free paths, and contribute up to 57% of the total thermal conductivity, while $c$-axis optical phonons remain flat with limited transport. As a result, optical phonons contribute nearly twice as much as acoustic phonons to the in-plane thermal conductivity anisotropy. These findings reveal the crucial role of optical phonons in directional heat transport and provide a framework for designing materials to precisely control heat flow.

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• Phonons
• Thermal conductivity
• Thermal transport
• Layered crystals
• Boltzmann theory
• Density functional calculations
• Inelastic X-ray scattering