Searching for and designing materials with ultralow thermal conductivity is vital for thermoelectrics and thermal management. Using first-principles calculations and a unified theory of thermal transport, we systematically investigate the lattice dynamics of orthorhombic $\mathrm{Be}{X}_2$ ($X$ = Cl, Br, I), accounting for anharmonic phonon renormalization (APR) and three- and four-phonon scattering. Due to their quasi-one-dimensional (quasi-1D) structure, these compounds exhibit pronounced thermal anisotropy. While the chain-direction ($a$ axis) lattice thermal conductivity (${\kappa }_L$) follows a standard $1/T$ scaling, the transverse ($b$ and $c$ axes) ${\kappa }_L$ shows anomalous nonmonotonic temperature dependence. This behavior originates from competition between particlelike phonon transport and wavelike coherent phonon transport. With increasing temperature, enhanced phonon scattering broadens the phonon linewidths, increases the spectral overlap between neighboring modes, and strengthens the off-diagonal coupling, thereby promoting the coherent transport channel, which eventually dominates ${\kappa }_L$. Four-phonon scattering substantially suppresses the particlelike contribution, particularly in compounds containing heavier halogens. APR hardens selected low-frequency acoustic modes and increases the corresponding direction-resolved phonon group velocities, but it also modifies the anharmonic scattering rates and phonon lifetimes through the temperature-dependent interatomic force constants. The synergistic effect of APR on ${\kappa }_L$ is therefore direction dependent and reflects competition between these harmonic and anharmonic factors. Overall, our findings elucidate the interplay between coherent phonon tunneling and higher-order anharmonicity in quasi-1D $\mathrm{Be}{X}_2$ and provide theoretical guidance for thermal management and the design of ultralow thermal conductivity materials.

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• Heat transfer
• Lattice thermal conductivity
• 1-dimensional systems
• Density functional calculations