First-principles study of the lattice thermal conductivity in NbOCl₂

Recently, a layered ferroelectric semiconductor, NbOCl₂, has been successfully synthesized (Guo et al 2023 Nature 613 53). Understanding the thermal transport mechanisms in layered ferroelectric materials is critical for advancing their applications and elucidating their microscopic properties. However, the thermal transport properties of NbOCl₂ remain largely unexplored. Here, we investigate the lattice thermal conductivity of bulk NbOCl₂ and its underlying physical principles by the three-phonon interaction and the phonon frequency anharmonic renormalization with quartic anharmonicity. Our results reveal that NbOCl₂ exhibits highly anisotropic and low lattice thermal conductivities, which are 3.41, 1.81, and 0.22 Wm⁻¹ K⁻¹ along the x, y and z axes at 300 K, respectively. Notably, further analyses imply that the optical phonons predominantly contribute to the thermal conductivity along the x-direction ( κ_x ) . More than 68% of the κ_x in NbOCl₂ are contributed by its optical phonons. In contrast, the lattice thermal conductivities along the y- and z-directions are primarily governed by the acoustic phonons. Moreover, the low κ_x and κ_y are correlated with the avoided-crossing behaviors between acoustic and optical phonon branches. The low κ_z is attributed to the weak interlayer interactions. This study provides insight into the underlying microscopic mechanism of optical-phonon-dominated thermal transport and low lattice thermal conductivities with strong anisotropies in NbOCl₂. Our work may offer novel avenues for integrating NbOCl₂ into flexible or nano-electronic devices requiring thermal control.