Two-dimensional Sc2Cl2S2: A promising n-type thermoelectric material with significant anisotropy

Abstract

Two-dimensional (2D) FeOCl materials have garnered significant attention as promising candidates for next-generation thermoelectric (TE) devices. However, achieving dimensionless figure of merit (ZT) in various FeOCl compounds continues to be a major challenge. This paper investigates the TE properties of FeOCl-type materials composed of Sc, Cl, and S, leveraging their structural characteristics and elemental properties. Specifically, the TE performance of 2D FeOCl Sc₂Cl₂S₂ is studied by density function theory and Boltzmann transport theory. The thermodynamic and kinetic stability of 2D Sc₂Cl₂S₂ is confirmed by ab initio molecular dynamics (AIMD) simulations and phonon dispersion curves, revealing a low lattice thermal conductivity (κ_l) of 0.384 W/mK (0.457 W/mK) along the x (y) axis at 700 K. In addition to meeting the requirements of dynamic and thermodynamic stability, it is necessary to further verify the mechanical stability to indicate the possibility of the stable existence of the structure. To this end, we calculated the mechanical matrix through the perturbation method. The calculation results show that the value of (C₁₁C₂₂-C₁₂C₁₂) is greater than zero, indicating that its mechanical structure is very stable. Finally, the Young's moduli in the x - direction and y - direction were calculated to be 106.28 N/m and 83.26 N/m respectively. Compared with other materials, its stability is good. Furthermore, 2D Sc₂Cl₂S₂ exhibits pronounced anisotropy. Based on n-type doping, the optimal power factor (PF) in x-axis and y-axis is 9.39 mW/mK² and 21.88 mW/mK², respectively, demonstrating a 57 % difference. Combining the high PF value and low κ_l, the highest ZT value at 700 K is found to be 4.21 in the y-axis and 2.55 in the x-axis, with a high anisotropy ratio of 1.65. This study provides a valuable reference for understanding and screening the thermoelectric properties of FeOCl-type materials, and advances the application potential of these materials.