Composition-dependent structural, physical, optical, and electrical properties of Ba_0.5Ca_0.5Eu_xFe_12−xO₁₉ hexaferrites for prospective applications

Abstract

The Ba_0.5Ca_0.5Eu_xFe_12−xO₁₉ (x = 0, 0.5, 1.0, 1.5, and 2.0) series of samples with crystallite size 29–38 nm were prepared using the co-precipitation synthesis technique. The X-ray diffraction pattern analysis confirms the hexagonal structure of the Ba_0.5Ca_0.5Eu_xFe_12−xO₁₉ samples. The value of lattice parameters of the Ba_0.5Ca_0.5Fe_12−xO₁₉: xEu samples increased as compared to the pure Ba_0.5Ca_0.5Fe₁₂O₁₉ sample. This increase is related to cation substitution i.e., smaller cations (Fe³⁺ ions) are replaced by larger cations (Eu³⁺ ions). The crystallite size of the Ba_0.5Ca_0.5Fe_12−xO₁₉: xEu samples decreases continuously with increasing Eu³⁺ ions substitution. The shifting of octahedral and tetrahedral clusters was observed in the FTIR spectra due to substitution of Eu³⁺ ions. The photoluminescence (PL) spectra of the Ba_0.5Ca_0.5Eu_xFe_12−xO₁₉ samples were observed at 365 nm which corresponds to ⁵D₄ – ⁷F₀ transition of Eu³⁺ ions. The PL spectra shifted toward the higher wave length side for x ≥ 1.0 due to reabsorption. The dielectric dispersion in the Ba_0.5Ca_0.5Eu_xFe_12−xO₁₉ samples can be understood by Maxwell–Wagner type of interfacial polarization based on Koop’s theory. The increased hopping rate at octahedral sites due to the substitution of Eu³⁺ ions is responsible for a higher dielectric constant. The activation energy of the Ba_0.5Ca_0.5Eu_xFe_12−xO₁₉ samples was estimated from the temperature dependence of dc conductivity plots. The frequency dependency of ac conductivity of Ba_0.5Ca_0.5Eu_xFe_12−xO₁₉ samples can be described on the basis of Jonscher’s power law and correlated barrier hopping (CBH) model. Below 380 K, the CBH mechanism describes the conduction mechanism, and above 380 K, the conduction mechanism can be understood by small polaron tunneling without overlapping (NSPT) model in the Ba_0.5Ca_0.5Fe₁₂O₁₉ compound. In the Eu-substituted Ba_0.5Ca_0.5Fe₁₂O₁₉ compound, only CBH mechanism dominates in the conduction process for all the temperatures. The Ba_0.5Ca_0.5Eu_xFe_12−xO₁₉ samples exhibit non-Debye dielectric behavior as confirmed by complex impedance spectroscopy analysis. By substituting Eu³⁺ ions, dielectric constant increases drastically and dielectric loss decreases as compared to pure sample. This indicates that the Ba_0.5Ca_0.5Eu_xFe_12−xO₁₉ samples can be potentially applied for microwave absorbers.