Physical insights into the ultralow lattice thermal conductivity and high thermoelectric performance of bulk LiMTe2 (M = Al, Ga)

Abstract

Ternary chalcogenides are well known in the world of thermoelectric research, but most of them show noticeable efficiency for either their n-type or p-type cases. Herein, combining the results of first-principles calculations and the solution of the Boltzmann transport equation, we have shown that both n-type and p-type ternary chalcogenides LiAlTe₂ and LiGaTe₂ can be prominent thermoelectric materials. Low phonon group velocities, high lattice anharmonicity due to the presence of weak anharmonic bonds and bonding heterogeneity, high anharmonic scattering rates, and high phase space for the three-phonon scattering processes lead to ultra-low lattice thermal conductivities (κ_l,xx, κ_l,yy, κ_l,zz) of (0.177, 0.170, 0.160) and (0.076, 0.073, 0.069) W m⁻¹ K⁻¹ for LiAlTe₂ and LiGaTe₂, respectively, at 800 K. Both n-type and p-type systems show high values of Seebeck coefficients due to the high density of state effective masses of the charge carriers arising from the large and sharp increase in density of states at the band edges, optimum values of electrical thermal conductivity due to moderate values of charge carrier mobility and the presence of multi-valleys at the band edges, and low values of electrical thermal conductivity. The calculated values of the figure of merits ((ZT)_xx, (ZT)_yy, (ZT)_zz) based on the evaluated electrical and thermal transport properties reach up to (3.49, 3.55, 3.54) and (1.71, 1.84, 2.02) for n-type and p-type LiAlTe₂, respectively, and (5.20, 5.29, 4.11) and (3.07, 3.11, 2.41) for n-type and p-type LiGaTe₂, respectively, at 800 K and charge carrier concentration 5 × 10¹⁹ cm⁻³.