Ab initio study of mechanical and thermal properties of GeTe-based and PbSe-based high-entropy chalcogenides

GeTe-based and PbSe-based high-entropy compounds have outstanding thermoelectric (TE) performance and crucial applications in mid and high temperatures. Recently, the optimization of TE performance of high-entropy compounds has been focused on reducing thermal conductivity by strengthening the phonon scattering process to improve TE performance. We report a first-principles investigation on nine GeTe-based high-entropy chalcogenide solid solutions constituted of eight metallic elements (Ag, Pb, Sb, Bi, Cu, Cd, Mn, and Sn) and 13 PbSe-based high-entropy chalcogenide solid solutions: Pb_0.99-ySb_0.012Sn_ySe_1-2xTe_xS_x (x = 0.1, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, and y = 0) and Pb_0.99-ySb_0.012Sn_ySe_1-2xTe_xS_x (y = 0.05, 0.1, 0.15, 0.2, 0.25 and x = 0.25). We have investigated the mechanical properties focusing on Debye temperature (Θ_D), thermal conductivity (κ), Grüneisen parameter (γ_α), dominant phonon wavelength (λ_dom), and melting temperature (T_m). We find that the lattice thermal conductivity is significantly reduced when GeTe is alloyed into the following compositions: Ge_0.75Sb_0.13Pb_0.12Te, Ge_0.61Ag_0.11Sb_0.13Pb_0.12Bi_0.01Te, and Ge_0.61Ag_0.11Sb_0.13Pb_0.12Mn_0.05Bi_0.01Te. This reduction is due to the mass increase and strain fluctuations. The results also show that Ge_0.61Ag_0.11Sb_0.13Pb_0.12Bi_0.01Te solid solution has the lowest Young's modulus (30.362 GPa), bulk and shear moduli (18.626 and 12.359 GPa), average sound velocity (1653.128 m/sec), Debye temperature (151.689 K), lattice thermal conductivity (0.574 W.m^-1.K^-1), dominant phonon wavelength (0.692 Å), and melting temperature (535.91 K). Moreover, Ge_0.61Ag_0.11Sb_0.13Pb_0.12Bi_0.01Te has the highest Grüneisen parameter with a reduced and temperature-independent lattice thermal conductivity. The positive correlation between Θ_D and κ is revealed. Alloying of PbSe-based high-entropy by Sb, Sn, Te, and S atoms at the Se and Pb sites resulted in much higher shear strains resulted in the reduction of phonon velocity, a reduced Θ_D, and a lower lattice thermal conductivity.