Realization of half-metallic ferromagnetism and physical Property analysis in double perovskites A2FeHfO6(A=Mg, Ca, Ba, Ra) and A2FeZrO6(A =Mg, Ca, Ba): A first-principles study

In this first-principles study, we methodically investigated the electronics, magneto-optical and mechanical characteristics of the compounds $A_2{\text{FeHfO}}_6\left(A=\text{Mg},\text{Ca},\text{Ba},\text{Ra}\right)$ and $A_2{\text{FeZrO}}_6\left(A=\text{Mg},\text{Ca},\text{Ba}\right)$ using GGA-PBE, GGA + U, PBE-sol and WC approximations. All compounds exhibit a stable cubic phase, supported by tolerance factor analysis. Birch-Murnaghan equation fitting confirms that the ferromagnetic state is energetically more favorable than the non-magnetic state. At the equilibrium lattice constant, all the compounds are half-metallic and exhibit an approximately integral magnetic moment. Electronic structure calculations reveal direct band gaps and half-metallic behavior in the studied double perovskites. GGA + U corrections enhance band gap accuracy, highlighting the role of electron correlation in these materials. It has been observed that the Fe-d and O-p states make significant contributions to the density of states at the Fermi level. All compounds are stable according to mechanical stability criteria; four are ductile, three are brittle, and two exhibit isotropic behavior. Hf-based and Mg-based materials exhibit superior stiffness and ductility, while Ba- and Ra-based compounds show brittleness and anisotropic behavior. Phonon dispersion and thermodynamic calculations confirm dynamic stability in Ba and Ra-based double perovskites, while Mg and Ca-based compounds exhibit soft modes linked to structural instabilities. Heat capacity and entropy trends correlate with A-site atomic radii, supporting thermal performance predictions across temperature ranges. The optical spectra of these compounds show minimal energy loss and excellent absorption and conductivity, indicating their potential application in UV-based optoelectronic devices. The electronic, optical, and magnetic properties we studied suggest that these materials hold significance for spintronic and optical applications.