Tunable mechanical, opto-electronic, and thermoelectric properties of \( Sr_2NbXO_6( X = Al, \) In, and Tl) double perovskites as a sustainable energy application: insights from first-principles calculations

Using density functional theory (DFT), we investigated the mechanical, optoelectronic, and thermoelectric properties of the double perovskites \(Sr_{2}NbXO_{6} (X = Al, In, \text {and}~Tl)\). Calculations of the formation energy, cohesion energy, tolerance factor (\(\tau \)), as well as the analysis of the Born–Huang approximation stability criteria, confirm the thermodynamic and mechanical stability of these compounds. Electronic properties show that the compounds \(Sr_{2}NbInO_{6}\) and \(Sr_{2}NbTlO_{6}\) have an indirect band gap with values of 3.43 eV and 2.60 eV,  respectively. In contrast, \(Sr_{2}NbAlO_{6}\) has a direct band gap of 3.69 eV. An analysis of the optical properties, including the real and imaginary parts of the dielectric function \((\varepsilon _1, \varepsilon _2)\), the absorption coefficient \(\alpha (\omega )\), the refractive index \(n(\omega )\), the optical conductivity \(\sigma (\omega )\), and the reflectivity \(R(\omega )\), was also performed. The results reveal significant absorption of incident light in the ultraviolet (UV) region, indicating that these oxides have strong potential for use in UV optical sensors as well as in various other UV-based optoelectronic devices. Furthermore, the BoltzTraP2 code reveals promising values for the Seebeck coefficient, electrical and thermal conductivity, power factor, and figure of merit (ZT). These findings suggest that \(Sr_2NbXO_6\) compounds are suitable candidates for sustainable energy technologies, particularly in thermoelectric and UV sensor devices.