Exploration of Promising Ferromagnetism and Unravelling Ultralow Lattice Thermal Conductivity in Lead-Free Double Perovskite Halides for Semiconductor Spintronics and Green Technologies

In this report, an extensive computational study has been conducted via density functional theory based on the full-potential linearized augmented plane wave (FP-LAPW) on Pb-free FM halide semiconductors Na₂GeVCl₆ and Na₂GeVBr₆ to realize them for advanced spintronic and sustainable energy applications. Initially, the computational procedure was established to calculate their system energy by performing the structural optimization upon the state-of-the-art Brich Murnaghan equation of state which encapsulates the least amount of stabilization energy in the ferromagnetic (FM) in contrast to their competing non-magnetic (NM) phase. Meanwhile, these alloys have been accessed in terms of mechanical stability by evaluating three stiffness constants (C_ij,s) describing ductile nature. More likely, the electronic structure of these systems has been verified with the help of Perdew-Burke-Ernzerhof Generalized gradient approximation (PBE-GGA) followed by Spin–orbit coupling (SOC) and along the Tran-Blaha modified Becke-Johnson (TB-mBJ). Furthermore, their electronic structures trigger a net integer magnetic moment of 3μ_B mostly arising from triply degenerate V-atom having a d³ configuration. Subsequently, the thermoelectric and thermodynamical parameters has been keenly studied under BoltzTraP and Gibbs2 Packages respectively. Meanwhile, the ultra-low suppressed value lattice thermal conductivity (κ_L) for Na₂GeVCl₆ (0.17 W/mK) and Na₂GeVBr₆ (0.14 W/mK) at room temperature is quite noticeable. In nutshell, our designed Pb-free FM halide semiconductors with diminished thermal conductivity and decent value of figure of merit (ZT) equal to unity (1) would turn their supportive stand in green energy harvesting technologies.