Divalent ion doping in CaFe₂O₄: A strategy for enhancing electrical conductivity in energy storage materials

Abstract

This study investigates the microstructural and electrical properties of CaFe₂O₄ concentrating on cations' distribution in its spinel structure regarding tetrahedral and octahedral sites. This is achieved by substituting Ca²⁺ with divalent metal ions like Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺. The transition metal-doped CaFe₂O₄ (Ca-ferrite) and other samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy, dielectric measurements, and electrical resistivity analysis. The creation of a single-phase orthorhombic structure (space group Pnam, No. 62) devoid of impurity phases was validated by XRD patterns, with 64–27 nm crystallite sizes, depending on the dopant (from Co to Zn). SEM micrographs revealed inhomogeneous, agglomerated grains with sizes varying between 95 nm and 35 nm. EDX analysis verified the expected elemental composition, free of impurities. Dielectric characteristics were assessed between 20 Hz and 1 MHz in frequency, adhering to the Maxwell-Wagner polarization model. A notable decrease in DC electrical resistivity was observed, from 4.9 × 10⁷ Ω-cm in undoped CaFe₂O₄ to 3.2 × 10⁶ Ω-cm in Zn-doped samples. This reduction in resistivity is attributed to substituting Ca²⁺ with transition metal ions of smaller ionic radii, which reduces the hopping length and enhances electron mobility, thereby improving electrical conductivity. These findings suggest that CaFe₂O₄, particularly when doped with conductive elements like Cu and Zn, holds significant potential for application in energy storage devices.