First principle study of electronic, optoelectronic, and thermoelectric properties of zintl phase alloys BaAg2X2 (X = S, Se, Te) for renewable energy

Abstract

Novel Zintl phases exhibiting promising thermoelectric properties have garnered considerable traction, largely attributed to the accuracy of computational estimates. In the present investigation, the density functional theory-based WIEN2k code is employed to analyze the structural, optoelectronic, and transport behavior of the BaAg₂X₂ (X = S, Se, Te) Zintl phase. All these compositions belong to the stable trigonal phase with nominal expansion in the unit cell with the replacement of S with Se and Te. A negative value of enthalpy of formation of −2.30, −2.0, and −1.80 for BaAg₂S₂, BaAg₂Se₂, and BaAg₂Te₂, respectively, assures their thermodynamic stability. These compositions demonstrate dynamic stability, as evidenced by the nonexistence of negative (-ve) frequency values in their phonon spectra. Increasing the size of chalcogens enhances the spin-orbit coupling and reduces the bandgap value from 2.10 to 1.55 eV. The examination of optical response suggests that studied compositions display high absorption and low energy loss in the visible range, rendering them suitable for optoelectronic devices. The temperature-dependent transport behavior is computed using BoltzTrap code, and the RT value of power factor is recorded as 0.89 × 10¹¹, 0.65 × 10^11, and 0.54 × 10¹¹ Wm^{− 1}K^{− 2} for BaAg₂X₂ (X = S, Se, Te). A high power factor value at elevated temperatures indicates the promising efficacy of studied compositions in thermoelectric device applications.