DFT-driven insights into (Sr/Ba)2GaBiO6 double perovskites for next-generation optoelectronic and thermoelectric technologies

This study investigates the structural, electronic, elastic, optical, and thermoelectric properties of double perovskite oxides A2GaBiO6 (A = Sr, Ba). It uses Density Functional Theory (DFT) combined with BoltzTrap2 within the Full-Potential Linearized Augmented Plane Wave (FP-LAPW) method. Exchange-correlation potential is described by using the well-organized Modified Becke-Johnson (mBJ) with the combination of Spin-Orbit Coupling (SOC). The structural and thermodynamic stabilities of these compounds are validated through energy–volume analysis and favorable formation and cohesive energy values. The calculated value of elastic constants i.e., C11 = 194.64/192.46 GPa, C12 = 24.13/22.02 GPa, C44=70.98/28.72 GPa for (Sr/Ba)2​GaBiO6​, respectively​. These values confirm mechanical stability and reveal anisotropic mechanical behavior throughout the series. Analysis of the electronic band structure shows direct band gaps at the Γ-point. These band gaps can be tuned by substitution at the A-site, indicating potential applications in semiconducting and optoelectronic devices. The studied compounds showing semiconductor nature with a bandgap of 1.149 eV and 1.148 eV for Sr2GaBiO6 and 0.517 eV and 0.515 eV for Ba2GaBiO6 with mBJ and mBJ + SOC potentials, respectively. Their optical response characterized by absorption spectra, refractive indices, and reflectivity suggests strong activity in the UV-visible range. Sr2​GaBiO6​ exhibits Seebeck coefficients of 235.92 µV/K (300 K) and 211.01 µV/K (1200 K) with maximum ZT = 0.49 (1200 K), while Ba2​GaBiO6​ shows Seebeck coefficients of 412.14 and 250.60 µV/K at 300 and 1200 K, respectively with ZT = 0.643 (1200 K). The lattice thermal conductivity decreases with increasing temperature for each compound. For example, in Sr2GaBiO6, κl reduces from 0.225 W/mK at 300 K to 0.033 W/mK at 1200 K, indicating enhanced phonon scattering at higher temperatures. This performance is attributed to reduced lattice thermal conductivity and strong transport properties as predicted by BoltzTrap2. Overall, A2GaBiO6 perovskites show promising potential for thermoelectric and photovoltaic applications, as indicated by the calculated electronic and optical properties. Further experimental validation is required to confirm their suitability for practical energy and optoelectronic applications.