Unraveling cation distribution and defect roles in substituted ferrite performance: An atomistic DFT study

Abstract

Nanoscale spinel-structured ferrites substituted with multivalent cations have garnered significant attention due to their tunable functional properties, essential for high-frequency electronics, energy conversion, and electromagnetic shielding applications. In this study, we investigate MnZn-ferrites with nominal composition Mn0.5Zn0.5Fe2O4, and oxygen-vacancy (Ov) systems Mn0.5Zn0.5Fe2O4−x substituted with tetravalent (Si4+), trivalent (Co3+), and divalent (Ca2+, Mg2+, and Sn2+) ions. Through a comprehensive first-principles approach, we systematically examine how cation substitution at tetrahedral and octahedral sites influences structural stability, electronic structure, magnetic anisotropy, and transport properties. Density Functional Theory (DFT), coupled with the Boltzmann transport theory, was employed to evaluate the thermoelectric transport behavior of pristine and doped MnZn-ferrites. Formation-energy calculations indicate that substitutions with Ca2+, Si4+, and Mg2+ enhance the thermodynamic stability of the lattice, while Co3+ and Sn2+ induce a slight destabilization. Electronic-structure analysis confirms that all substituted variants retain finite bandgaps, preserving their semiconducting nature. Magnetic anisotropy energy (MAE) analyses reveal that site-specific substitution leads to a narrower MAE distribution in octahedral tetrahedral systems, signifying uniform magnetic behavior and reduced coercivity-desirable traits for soft magnetic materials. In contrast, oxygen-vacancy (Ov) systems exhibit broader MAE distributions, indicative of increased magnetic disorder and potential for higher eddy-current losses under high-frequency operation. At 300 K, thermoelectric analysis shows that doping at both sites decreases electrical conductivity (σ) while enhancing the Seebeck coefficient (S). This observed inverse σ-S relationship is attributed to enhanced carrier scattering and modifications in the electronic density of states near the Fermi level due to defect and dopant interactions. Overall, these results elucidate the critical role of multivalent cation substitution, crystallographic site occupancy, and defect engineering in tailoring the multifunctional performance of MnZn-ferrites for advanced magnetic, electronic, and energy-related applications.