Four-Phonon Scattering and Phonon Coherence in Monolayer Penta-CdO2 with Ultralow Lattice Thermal Conductivity

Abstract Two-dimensional (2D) pentagonal materials have emerged as promising candidates for low-lattice-thermal-conductivity applications owing to their extraordinary geometry structures and strong intrinsic anharmonicity. However, the roles of high-order phonon scattering and phonon coherence in their thermal transport remain elusive. Herein, we investigate the lattice thermal conductivity κL of the newly predicted monolayer penta-CdO2 by employing the unified theory of thermal transport, which rigorously incorporates four-phonon scattering alongside both particle-like propagation and wave-like phonon tunneling. Penta-CdO2 exhibits an ultra-low and highly anisotropic κL of 0.394 W m–1 K–1 along the x-direction and 0.789 W m–1 K–1 along the y-direction at 300 K, undercutting most reported pentagonal counterparts. Remarkably, four-phonon (4ph) scattering, dominated by redistribution processes, dramatically suppresses the particle-like thermal conductivity by up to 65%. Crucially, this strong high-order scattering induces a non-monotonic temperature dependence of the total κL, accompanied by a crossover to a phonon-coherence-dominated regime. The microscopic mechanism behind this behavior is attributed to the non-monotonic evolution of coherent contributions by quasi-degenerate phonon modes, driven by the competition between rapidly increasing scattering rates and the narrow frequency spacings of optical phonons. This work clarifies the impact of 4ph scattering and phonon coherence on thermal transport in two-dimensional (2D) pentagonal materials, providing new microscopic insights for designing low-κL materials for thermoelectric and other technological applications.