First-principles study of the lattice thermal conductivity in NbOCl₂

Abstract Recently, a layered ferroelectric semiconductor, NbOCl 2 , 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 NbOCl 2 remain largely unexplored. Here, we investigate the lattice thermal conductivity of bulk NbOCl 2 and its underlying physical principles by the three-phonon interaction and the phonon frequency anharmonic renormalization with quartic anharmonicity. Our results reveal that NbOCl 2 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>κ</mml:mi> <mml:mi>x</mml:mi> </mml:msub> </mml:mrow> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> . More than 68% of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>κ</mml:mi> <mml:mi>x</mml:mi> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> in NbOCl 2 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>κ</mml:mi> <mml:mi>x</mml:mi> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>κ</mml:mi> <mml:mi>y</mml:mi> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> are correlated with the avoided-crossing behaviors between acoustic and optical phonon branches. The low <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>κ</mml:mi> <mml:mi>z</mml:mi> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> 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 NbOCl 2 . Our work may offer novel avenues for integrating NbOCl 2 into flexible or nano-electronic devices requiring thermal control.