Outstanding Thermoelectric Performance of MCu₃X₄ (M = V, Nb, Ta; X = S, Se, Te) with Unaffected Band Degeneracy under Pressure

Few authors reported high TE performance in MCu3X4, reaching the figure of merit (ZT) above 2 at 1000 K, from first-principles calculations neglecting electron–phonon scattering, spin-orbit coupling effect (SOC), and energy-dependent carrier lifetime. Here, thermoelectric transport properties of MCu3X4 are reinvestigated through considering these parameters, and significant discrepancies are found. The ZT of p-type TaCu3Te4 can reach ∼3 at 1000 K among these compounds due to its low lattice thermal conductivity (κl) (0.38 W m–1 K–1). The presence of heavier elements Ta and Te, high anharmonicity, low group velocity, and Debye temperature are responsible for the low lattice thermal conductivity of TaCu3Te4. Interestingly, the value of κl is reduced to 0.17 W m–1 K–1 through 1 GPa pressure, which causes phonon softening and strengthens the acoustic and optical phonon interactions, while the power factor is slightly improved due to bandgap reduction and unaffected band degeneracy, leading to an extraordinary ZT = 5.368 at 1000 K. Although the substitution of Se causes a slight reduction of κl to ∼0.3 W m–1 K–1, the power factor is reduced significantly due to the dramatic reduction of band degeneracy and DOS near the Fermi level, which leads to lower the Seebeck coefficient largely and increase electrical conductivity slightly.