Ultralow Lattice Thermal Conductivity and Distinctive Electronic Features Lead Giant Thermoelectricity in Layered MSe...

We present a systematic comparative study of the structural, mechanical, vibrational, electronic, and thermoelectric (TE) properties of layered III-VI semiconductors GaSe, InSe, and TlSe using first-principles density functional theory and phonon transport calculations. Among the three, GaSe exhibits the highest thermodynamic and mechanical stability, whereas InSe displays ductile behavior, and TlSe remains mechanically softer, with lower elastic moduli. This reduced mechanical stiffness in TlSe plays a crucial role in its thermal transport properties, particularly contributing to a significantly lower lattice thermal conductivity. The Seebeck coefficient is relatively higher for GaSe and InSe due to the presence of unconventional band convergence, which enhances the density of states near the Fermi level and thereby improves the thermopower. In contrast, the absence of such convergence in TlSe results in a lower Seebeck coefficient. However, TlSe exhibits superior electrical conductivity, which, together with its ultralow lattice thermal conductivity, compensates for the reduced Seebeck response. TlSe demonstrates the highest TE efficiency, driven primarily by its ultralow lattice thermal conductivity resulting from pronounced phonon softening, flat acoustic phonon branches, and dominant out-of-plane vibrational modes, which enhance phonon-phonon scattering. While the mid-frequency optical phonon group velocities remain comparable across the systems, the differences in $\kappa {}_l$ are primarily governed by variations in acoustic and low- and high-frequency optical phonon velocities. The TE figure of merit (ZT) calculations further support the superior performance of TlSe, with a maximum ZT value of 2.89 at 600 K, exceeding that of GaSe (1.68) and InSe (1.41) across the temperature range studied. Bader charge analysis, charge density distribution, and Debye temperature calculations further support the superior TE efficiency of TlSe among the studied compounds. These results highlight TlSe as a promising candidate for TE applications compared to GaSe and InSe, where its ultralow thermal conductivity and optimized phonon dynamics effectively compensate for its relatively lower Seebeck coefficient.