Electric field-induced phase transitions in ferroelectrics provide a reversible and effective strategy for tuning the lattice thermal conductivity. In this study, we systematically investigate phonon thermal transport in bilayer SnSe across its antiferroelectric (AFE) and ferroelectric (FE) phases using accurate neural network potentials combined with the phonon Boltzmann transport equation. Our results reveal that the AFE-to-FE transition dramatically reduces the lattice thermal conductivity, achieving ultralow values of $0.21$ and $0.15\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ along the $a$ and $b$ axes at $300\mathrm{K}$, respectively. This suppression is primarily attributed to a downward shift of low-frequency optical branches due to weakened interlayer interactions, together with enhanced phonon anharmonicity induced by bonding heterogeneity. Furthermore, the AFE and FE phases of SnSe exhibit marked differences in four-phonon scattering processes and optical phonon transport behavior. While four-phonon processes are negligible in the AFE phase, they become crucial in the FE phase, reducing the lattice thermal conductivity by up to 50%. Moreover, in the FE phase, optical phonons contribute up to 78.0% and 90.7% of the thermal conductivity along the $a$ and $b$ axes, respectively, which deviates from the conventional acoustic phonon-dominated paradigm. Further analysis shows that this phenomenon is driven by the hardening of optical phonons and the enhanced anharmonicity of acoustic phonons. This work not only deepens the understanding of phonon thermal transport in ferroelectric materials but also provides crucial insights for its efficient regulation.

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• Electric polarization
• Phonons
• Thermal conductivity
• Bilayer films
• Density functional calculations
• Machine learning