Dynamic stability, half-metallicity, and optical properties of Fe2CrX (X = Si, Ge) full-heusler alloys: Competition between L21 and XA ordering

We performed spin-polarized density functional theory (DFT) studies on the structural, mechanical, dynamic, thermodynamic, electronic, magnetic, and optical properties of Fe2CrSi and Fe2CrGe full-Heusler alloys. Using GGA-PBE and mBJ-GGA, we found that Fe2CrSi is stable in the L2(1)-type structure, while Fe2CrGe prefers the XA-type structure. Phonon calculations confirm the dynamic stability, while elastic constants indicate mechanical stability for Fe2CrGe in both structures. However, Fe2CrSi in L2(1) does not satisfy the Born mechanical stability criteria due to a negative (C-11- C-12) value, though its dynamic stability and negative formation energy suggest its potential experimental realizability. Electronic structure analysis shows half-metallic behavior with GGA-PBE and half-semiconducting gaps of 0.47 eV (Fe2CrSi) and 0.60 eV (Fe2CrGe) using mBJ-GGA. Fermi Surface analysis reveals distinct topologies and carrier distributions influencing electronic transport properties. Fe2CrSi exhibits a more symmetric and interconnected Fermi Surface, favoring high carrier mobility, whereas Fe2CrGe displays a fragmented topology, suggesting more localized states. Optical properties highlight Fe2CrGe's superior absorption across the visible spectrum, making it suitable for photovoltaics, while Fe2CrSi exhibits strong UV absorption, promising for UV-sensitive devices. Additionally, Fe2CrGe shows higher optical conductivity, indicating potential in light-harvesting and optoelectronic applications. These findings highlight Fe(2)CrZ alloys as promising candidates for spintronics and optoelectronic applications.