Tunable Phonon Scattering and Phonon Coherence in Bilayer Penta-PdPSe

Stacking engineering has emerged as an effective strategy for tuning thermal transport in layered materials by reshaping interlayer phonon dynamics. Motivated by the recent experimental synthesis of the puckered pentagonal penta-PdPSe sheet, we systematically investigate stacking-dependent thermal transport in bilayer penta-PdPSe using first-principles calculations combined with the unified theory of thermal transport. Our results demonstrate that both AA and AA-mirror stacking structures are semiconducting with indirect band gaps of 0.91 and 0.94 eV, respectively, at the HSE06 level. Under room-temperature conditions, the low density of thermally excited carriers suggests that heat conduction is primarily governed by phonons. Despite similar electronic structures, their lattice thermal conductivities differ markedly. The AA stacking exhibits anisotropic thermal conductivities of 0.97 and 1.13 W m^-1 K^-1 along the x- and y-directions, respectively, arising from the coexistence of particle-like transport (0.63 and 0.75 W m^-1 K^-1) and substantial phonon coherence contributions (0.34 and 0.38 W m^-1 K^-1). The AA-mirror stacking, on the other hand, shows enhanced lattice thermal conductivities of 1.22 and 1.29 W m^-1 K^-1, with particle-like contributions of 0.95 and 0.97 W m^-1 K^-1 and coherence components of 0.27 and 0.32 W m^-1 K^-1. This enhancement arises from interlayer coupling, which increases phonon group velocities and suppresses anharmonic scattering, thereby reinforcing particle-like heat transport. Meanwhile, the reduced phonon dephasing length in the AA-mirror stacking leads to lower coherence contributions. These results demonstrate that interlayer stacking order provides an effective strategy for engineering phonon transport in layered two-dimensional pentagonal materials.