Engineering (Ge/Sn)-Mn halide double perovskites for spintronics, optoelectronics, and energy conversion

In this study, we perform a comprehensive first-principles investigation of the structural, electronic, magnetic, and optical properties of Mn-based halide double perovskites Cs₂(Ge,Sn)MnX₆ (X = F, Cl, Br, I) using the WIEN2k code. Structural stability is assessed through the Goldschmidt tolerance factor and phonon dispersion, confirming the robustness of their cubic phases. The electronic properties and magnetic properties, including the Curie temperature, total magnetic moment, and polarization, confirm the ferromagnetic semiconducting nature with varying band gaps under the generalized gradient approximation(GGA), the GGA plus Hubbard U correction(GGA+U), and the modified Becke Johnson potential (mBJ) approximations, and demonstrate half-metallic ferromagnetic behavior with 100% spin polarization and a high Curie temperature under the Heyd Scuseria Ernzerhof hybrid functional(HSE) approximation. Furthermore, the presence of halogens enhances the optical properties of these materials, making them highly suitable for high-efficiency energy conversion technologies and optoelectronic devices. Our findings reveal a strong correlation between structure and properties, highlighting the tunability of these materials for a wide range of applications.