Control of spin on ferromagnetism and thermoelectric properties of K2GeMnX6 (X = Cl, Br, I) halide perovskites: emerging candidates for semiconductor spintronics and thermoelectric applications

Abstract

In this systematic report, ab initio simulations based on density functional theory (DFT) have been performed to examine the structural and elastic stability, electronic profile, and transport properties of K₂GeMnX₆ (X = Cl, Br, I) double halide perovskites. The structural optimization, evaluation of mechanical stability criteria, and assessment of the tolerance factor collectively confirm the stability of the halide perovskites in a cubic structure with Fmm symmetry. The stability of the primary magnetic phase is established through minimization of total crystal energy at the behest of Birch–Murnaghan's equation across diverse magnetic phases. The ferromagnetic state is identified as the fundamental ground state, supported by positive Curie–Weiss constant values of 101 K for K₂GeMnCl₆, 100 K for K₂GeMnBr₆ and 90 K for K₂GeMnI₆. Additionally, the dynamic stability has been assessed through the calculation of phonon band structures utilising density functional perturbation theory (DFPT). The electronic band structures and density of states, obtained from both the generalized gradient approximation (GGA) and the TB-mBJ potential, designate a semiconducting ferromagnetic behavior characterized by a substantial spin-splitting gap, indicating their promising potential for semiconductor spintronics. The investigation into magnetism reveals values of 5μ_B for each compound, primarily originating from the transition metal atom (Mn⁺²). Also, the Curie temperature for each compound has been determined, with values reaching 710 K for K₂GeMnCl₆, 650 K for K₂GeMnBr₆, and 570 K for K₂GeMnI₆, ensuring exceptional stability of the ferromagnetic phase well beyond typical ambient conditions. The analysis of the transport properties of K₂GeMnX₆ (X = Cl, Br, I) double perovskites involved examining both the temperature and chemical potential dependencies of thermoelectric coefficients, specifically focusing on the Seebeck coefficient, electrical conductivity, and figure of merit. The significantly low thermal conductivity values of 2.2 K W mK⁻¹ for K₂GeMnCl₆, 2 K W mK⁻¹ for K₂GeMnBr₆, and 1.95 K W mK⁻¹ for K₂GeMnI₆ highlight their potential for efficient waste heat recovery. Furthermore, with figure of merit (zT) values of 1.01, 1.00, and 0.99 at room temperature for K₂GeMnCl₆, K₂GeMnBr₆, and K₂GeMnI₆ double halide perovskites respectively, these materials exhibit promising potential for both thermoelectric and renewable energy applications. The study also investigates the optical and dielectric properties, unveiling substantial absorption and photoconductivity in the visible and UV regions, thereby endorsing their potential as promising lead-free candidates for optoelectronics and solar cell applications. The comprehensive investigation overall lends support to the potential use of these materials in semiconductor spintronics, thermoelectric technology, optoelectronics, and other emerging technological domains.