Ultralow thermal conductivity and anharmonic rattling in two-dimensional CrSX (X = Cl, Br, I) monolayers

Abstract

Using first-principles calculations and Boltzmann transport theory, we investigate the thermoelectric properties of the 2D CrSX (X = Cl, Br, I) family. The ultralow lattice thermal conductivity (κ_l) values of CrSX monolayers along the x-axis at 300 K are 0.624, 0.419, and 0.179 W m⁻¹ K⁻¹, respectively, while along the y-axis they are 0.193, 0.158, and 0.093 W m⁻¹ K⁻¹. These low values are attributed to the small phonon relaxation time and flat acoustic phonon dispersions of CrSX monolayers, resulting from the extraordinarily strong anharmonicity. Vibrational pattern analysis confirms that the strong anharmonicity in CrSX monolayers arises from the reciprocal vibrations of the X atoms around equilibrium positions, which can be considered as a generalized effective rattling phenomenon. Due to their low lattice thermal conductivity and favorable electrical transport properties, the CrSX monolayers demonstrate optimal ZT values of 1.33 (5.23), 1.18 (5.84), and 2.13 (5.22) along the x-axis (y-axis) at 300 K, respectively. This work not only highlights the significance of rattling modes in driving phonon anharmonicity but also provides valuable insights for discovering and designing thermoelectric materials with ultralow lattice thermal conductivity and high conversion efficiency.