Pressure and Doping Effects on the Structural Stability of Thermoelectric BaAg2Te2

Ternary chalcogenides have attracted great attention for their potential applications in thermoelectric devices. Here, we investigate the pressure and doping effects on the structural stability of BaAg2Te2using first-principles calculations. Imaginary frequencies are observed in the calculated phonon dispersions of the reportedPnmastructure, indicating thatPnmaBaAg2Te2is lattice dynamically unstable at 0 K. Although the imaginary phonon frequencies are small, we find that hydrostatic pressure cannot effectively stabilize the structure. Based on the soft mode at Γ point, a new monoclinic phase with a space group ofP21/cis proposed. Fromab initiomolecular dynamics simulations, theP21/cphase is predicted to transform to thePnmaphase at a low temperature below 100 K. Electron/hole doping effects on the lattice dynamical stability of thePnmaphase are also studied. It is found that hole doping is superior to electron doping in stabilizing thePnmaphase. Further study on the electrical transport properties of thePnmaphase reveals a higher performance alongbaxis than that along the other two directions. This work paves an avenue to better understand the structural stability and electrical transport properties of thermoelectric BaAg2Te2.