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We report ab-initio calculations of the stability, lattice dynamics,electronic and thermoelectric properties of cubic La3-yX4 (X=S,Se,Te) materialsin view of analyzing their potential for thermoelectric applications. Thelanthanum motions are strongly coupled to the tellurium motions in thetelluride, whereas the motions of both types of atoms are decoupled in thesulfides. Nevertheless, this has no impact on their thermal properties becauseexperimentally all compounds have low thermal conductivity. We believe thatthis is due to Umklapp scattering of the acoustical modes, notably by the lowenergy optical modes at about 7-8 meV found in all three chalcogenides, as incage compounds such as skutterudites or clathrates, even though there are nocages in the cubic Th3P4 structure. We find that the energy bandgap increasesfrom the telluride to the sulfide in good agreement with the experiments.However, due to their similar band structure, we find that all three compoundshave almost identical thermoelectric properties. Our results agreequalitatively with the experiments, especially in the case of the telluride forwhich a great amount of data exists. All our results indicate that the sulfideshave strong potential for thermoelectricity and could replace the tellurides ifthe charge carrier concentration is optimized. Finally, we predict also alarger maximum ZT for the p-type doped materials than for the n-type dopedones, even though compounds with p-doping have still to be synthesized. Thusour results indicate the possibility to make high temperature performingthermo-generators based only on La3X4 compounds.

Physical properties of the thermoelectric cubic lanthanum chalcogenides La3-yX4 (X=S,Se,Te) from first-principles