Silver-Antimony-Telluride: From First-Principles Calculations to Thermoelectric Applications

Silver-antimony-telluride (AgSbTe 2 ) based compounds have emerged as a promis- ing class of materials for thermoelectric (TE) power generation at the mid-temperature range. This Chapter demonstrates utilization of first-principles calculations for predict- ing TE properties of AgSbTe 2 -based compounds and experimental validations. Predictive calculations of the effects of La-doping on vibrational and electronic properties of AgSbTe 2 compounds are performed applying the density functional theory (DFT), and temperature-dependent TE transport coefficients are evaluated applying the Boltzmann transport theory (BTE). Experimentally, model ternary (AgSbTe 2 ) and quaternary (3 at. % La-AgSbTe 2 ) compounds were synthesized, for which TE transport coefficients were measured, indicating that thermal conductivity decreases due to La-alloying. The latter also reduces electrical conductivity and increases Seebeck coefficients. All trends corre- spond with those predicted from first-principles. Thermal stability issues are essential for TE device operation at service conditions, e.g. changes of matrix composition and second-phase precipitation, and are also addressed in this study on both computational and experimental aspects. It is shown that La-alloying affects TE figure-of-merit posi- tively, e.g., improving from 0.35 up to 0.50 at 260 °C. We highlight the universal aspects of this approach that can be applied for other TE compounds. This enables us screening their performance prior to synthesis in laboratory. A significant step in this direction is introduced by our previous investigations of AgSbTe 2 based phase, involving both computational and experimental aspects [57, 58]. Vibrational properties of both AgSbTe 2 -based and La-doped-AgSbTe 2 alloys, including frequency-depen-dent vibrational density of states functions (v-DOS), temperature-dependent heat capacity, sound velocities, and Debye temperatures were evaluated employing lattice dynamics first-principles calculations. It was reported that La-doping reduces average sound velocity and varies v-DOS of AgSbTe 2 -based phase [57, 58]. Quantitatively, lattice thermal conductivity of La 0.125 Ag 0.875 SbTe 2 alloy was calculated to be lower by ca. 14%, than that of AgSbTe 2 -based phase at 300 K [58]. Experimental validations of these effects of La-alloying on reducing lattice thermal conductivity were made, as well [58]. Experimentally, model ternary (AgSbTe 2 ) and quaternary (3 at.% La-AgSbTe 2 ) alloys were synthesized by vacuum melting followed by quenching and hot-pressing. The appropriate conditions enabling formation of AgSbTe 2 -matrix that dissolves La-atoms with no La-rich precipitates were established. DSC tests enable observation of Ag 2 Te+Sb 2 Te 3 → δ-AgSbTe 2 phase transition at 360°C for La-free alloys only, indicating improvement of alloy’s thermal stability due to La-additions. Temperature-dependent thermal conductivity of both alloys indicate reduction in thermal conductivity as a result of La-alloying from 0.92 to 0.71 W m −1 K −1 at 573 K, which corresponds with the trend predicted from first-principles. Measurements of tem-perature-dependent electrical conductivity and Seebeck coefficients indicate that La-doping reduces electrical conductivity and increases Seebeck coefficients, as predicted from first-principles. Eventually, it is shown that La-doping has positive effects on TE figure-of-merit ZT , which is improved, e.g., from 0.35 up to 0.50 at 260°C.