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

Abstract

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 L2₁ −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 L2₁ does not satisfy the Born mechanical stability criteria due to a negative (C₁₁ − C₁₂) 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 Fe₂CrZ alloys as promising candidates for spintronics and optoelectronic applications.