Evaluating A2SrGeI6 (A = K and Rb) Lead-Free double Perovskites: Structural, Elastic, and optoelectronic insights for clean energy

In this study, we employ for the first time a full-potential linearized augmented plane wave approach to analyze the structural, elastic, electronic, and optical characteristics of lead-free halide double perovskites A2SrGeI6 (A = K and Rb) using the Perdew-Burke-Ernzerhof generalized gradient approximation and the Tran-Blaha modified Becke-Johnson potential (PBE-GGA + Tb-mBJ). The analysis of tolerance factors, negative formation energies, ab initio molecular dynamics simulations, and elastic constants confirm the structural, thermodynamic, and mechanical stability of the compound. The Poisson and Pugh ratios reveal that Rb2SrGeI6 is ductile, while K2SrGeI6 demonstrates flexibility. Both materials exhibit high melting temperatures, ensuring the stability under elevated s. Moreover, the study estimates the bandgap values of Rb2SrGeI6 and K2SrGeI6 to be 2.70 eV and 2.69 eV, respectively, suggesting that these materials exhibit the characteristics of indirect bandgap semiconductors with promising optical properties such as the dielectric function, refractive index, reflectivity, and absorption coefficient. The results demonstrate that both materials are ideal candidates for optoelectronic and photovoltaic applications, with K2SrGeI6 being particularly suitable for flexible solar cells.