  We report ab-initio calculations of the stability, lattice dynamics,
electronic and thermoelectric properties of cubic La3-yX4 (X=S,Se,Te) materials
in view of analyzing their potential for thermoelectric applications. The
lanthanum motions are strongly coupled to the tellurium motions in the
telluride, whereas the motions of both types of atoms are decoupled in the
sulfides. Nevertheless, this has no impact on their thermal properties because
experimentally all compounds have low thermal conductivity. We believe that
this is due to Umklapp scattering of the acoustical modes, notably by the low
energy optical modes at about 7-8 meV found in all three chalcogenides, as in
cage compounds such as skutterudites or clathrates, even though there are no
cages in the cubic Th3P4 structure. We find that the energy bandgap increases
from the telluride to the sulfide in good agreement with the experiments.
However, due to their similar band structure, we find that all three compounds
have almost identical thermoelectric properties. Our results agree
qualitatively with the experiments, especially in the case of the telluride for
which a great amount of data exists. All our results indicate that the sulfides
have strong potential for thermoelectricity and could replace the tellurides if
the charge carrier concentration is optimized. Finally, we predict also a
larger maximum ZT for the p-type doped materials than for the n-type doped
ones, even though compounds with p-doping have still to be synthesized. Thus
our results indicate the possibility to make high temperature performing
thermo-generators based only on La3X4 compounds.
