On the role of lanthanum substitution defects in reducing lattice thermal conductivity of the AgSbTe₂ (P4/mmm) thermoelectric compound for energy conversion applications

We perform lattice dynamics first-principles calculations for the technologically-important AgSbTe₂ thermoelectric compound. Based on its calculated vibrational density-of-states, we hypothesize that the formation of substitution defects at the Ag-sublattice sites will impede lattice vibrations, thereby reducing the lattice thermal conductivity. Further calculations performed for a La_0.125Ag_0.875SbTe ₂ compound indicate significant reduction of the average sound velocity from 1727 to 1046 m s^-1 due to La-doping. This corresponds to an estimated decrease of lattice thermal conductivity by a factor of 2.7, which is expected to yield a significant improvement to the thermoelectric figure of merit for La_xAg_1-xSbTe ₂-based materials up to values larger than 3.