Unlocking the role of 3d electrons on ferromagnetism and spin-dependent transport properties in K2GeNiX6 (X=Br, I) for spintronics and thermoelectric applications

In our quest for novel materials, we undertake a rigorous ab-initio investigation to unravel the structural stability, electronic profile, and thermoelectric properties of halide double perovskites K2GeNiX6 (X = Br, I). Our exploration commences with a meticulous evaluation of structural and thermodynamic stability, utilizing diverse metrics for comprehensive scrutiny. Subsequently, we optimize the structures using the GGA-PBE potential at the behest of the Birch-Murnaghan equation of state to identify energetically favoured configurations. The ferromagnetic phase emerges as the ground state, supported by positive Curie-Weiss constant values of 120 K for K2GeNiBr6 and 100 K for K2GeNiI6, respectively. To elucidate the precise determination of the electronic structure, we utilized the highly sophisticated TB-mBJ potential, unveiling a ferromagnetic semiconductor nature for both materials. The band structure exhibits a pronounced asymmetry, indicating the presence of spin-magnetic moments. Notably, K2GeNiBr6 and K2GeNiI6 display substantial magnetic moments of 2 μB each, primarily associated with the 3d-transition element Ni2+. Additionally, we determine the Curie temperature, disclosing substantial values of 540 K and 470 K for K2GeNiBr6 and K2GeNiI6, respectively. Our investigation extends to encompass thermodynamic parameters, considering vibrational contributions to internal energy, Helmholtz free energy, and other factors that collectively serve as indicators of thermal stability. Finally, we examine the impact of both chemical potential and temperature variations on key thermoelectric parameters, specifically the Seebeck coefficient, electrical conductivity, and the figure of merit (zT). The findings unveil a significant thermoelectric figure of merit (zT) with values of 1.00 and 0.99 for K2GeNiBr6 and K2GeNiI6, respectively. The overall comprehensive analysis underscores the substantial potential of these tailored materials in applications pertaining to semiconductor spintronics and thermoelectric devices.