Structural, morphological and dielectric analyses of La_1 − xSr_xFeO₃ solid solutions

Abstract

Dielectric and conduction properties were investigated on La_1 − xSr_xFeO₃ (0 ≤ x ≤ 0.5) ceramics synthesized by solid-state reaction method. X-ray powder diffraction pattern of all prepared samples and their Rietveld refinement revealed that all samples crystallize in orthorhombic structure with Pnma space group. Sr substitution induced the volume unit cell to decrease and the crystallite size to vary in a non-monotonic manner. Scanning electron microscopy images revealed also Sr-dependent morphology of the studied ceramics. Fourier transform infrared (FTIR) spectra evidenced structural distortion effects on ν_Fe–O–Fe, σ_Fe–O–Fe and ν_La–O vibrations and showed the presence of the carbonate ion impurity for the compositions x = 0.2, 0.3, 0.4 and 0.5. Ac conductivity analysis was accomplished according to equation: $\sigma_{ac} \left( \omega \right) = \frac{{\sigma_{s} }}{{1 + \tau^{2} \omega^{2} }} + \frac{{\sigma_{\infty } \tau^{2} \omega^{2} }}{{1 + \tau^{2} \omega^{2} }} + A \omega^{n}$ for the parent compound and its doped one La_0.9Sr_0.1FeO₃. However, this analysis was realized by using Jonscher law: $\sigma_{ac} \left( \omega \right) = \sigma_{dc} + A \omega^{n}$ for the compositions x = 0.2, 0.3, 0.4 and 0.5. It revealed that the hopping process occurred through long distance for 0 ≤ x ≤ 0.3 compositions, whereas for x = 0.4 and 0.5 compositions the hopping occurred between neighboring sites. Complex impedance analysis performed on these ceramics by means of different electrical equivalent circuits owing to their different morphologies allowed probing grain boundary effect on ac conductivity.