Motivated by the experimental synthesis of the first ternary penta-PdPSe sheet [P. Li, et al., Adv. Mater. 33, 2102541 (2021)], we investigate its high-order phonon scattering and phonon coherence, along with their impact on the lattice thermal conductivity using first-principles calculations combined with the unified theory of thermal transport. The penta-PdPSe sheet is semiconducting with an indirect band gap of 1.94 eV at the HSE06 level. Our calculated results reveal that four-phonon scattering plays an important role in suppressing the lattice thermal conductivity. When four-phonon scattering is considered, the particle-like thermal conductivity ${\kappa }_L^P$ at 300 K is reduced by 54.70% along the $x$-direction and 59.05% along the $y$-direction. This reduction is primarily due to the strong scattering of acoustic phonons, which contributes approximately 88.50% to the particle-like lattice thermal transport. The phonon lifetime of acoustic modes of phonon is greater than the Wigner limit in time, resulting in a small contribution from phonon coherent transport. The total lattice thermal conductivity, incorporating both phonon scattering and coherence effects, exhibits anisotropic values of 6.20 $\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ along the $x$-direction and 7.68 $\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ along the $y$-direction at 300 K. The low lattice thermal conductivity arises from the large mass contrast between Pd and P/Se atoms, weak interatomic bonding, and puckered pentagonal geometry, which collectively enhance anharmonicity and phonon scattering. Our work provides new insights into the low lattice thermal transport properties of the first synthesized ternary penta-sheet.

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• Acoustic phonons
• Lattice thermal conductivity
• 2-dimensional systems
• First-principles calculations