Insights on the various structural, optical and dielectric characteristics of La1-xCaxFeO3 perovskite-type oxides synthesized through solution-combustion technique

Abstract

Perovskite-type oxides La1-xCaxFeO3 (x = 0.0–1.0) were synthesized using combustion technique. The samples have been investigated using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, ultraviolet–visible diffuse reflectance spectroscopy (DRS), X-ray powder diffraction (XRPD) using Rietveld refinement, thermogravimetric analysis (TGA) and dielectric spectroscopy. The SEM images showed spherical-shaped agglomerates having a non-uniform distribution of size. The EDX results suggested that the experimental chemical composition was in good accordance with the nominal values. FT-IR revealed multiple peaks in the range of 580–510 cm−1 due to asymmetric stretching of the octahedron. The reflectance spectra showed characteristic d→ d transitions arising from the Fe 3d and the highest optical band gap energy (Eg) of 2.57 eV was exhibited by LaFeO3, while the lowest Eg (1.41 eV) was shown by La1-xCaxFeO3 with x = 0.6. The results suggest that these samples are suitable for photocatalytic applications. From phase evaluation of the diffraction patterns, it was found that five distinct phases exist in the series. A phase transformation from orthorhombic (Pnma) to cubic (Pm3 ¯ m) was found when x was increased from 0.4 to 0.6. TGA showed that LaFeO3 was the most stable with a residual mass of ~ 97 wt% at 900 ∘C compared to CaFeO3 (~ 70 wt% at 900 ∘C). The evaluation of frequency-dependent (20 Hz–20 MHz) dielectric properties agreed with the Maxwell–Wagner two-layer model. After Ca2+ substitution, tan δ at 20 Hz significantly reduced from ~ 103 for x = 0 to ~ 100 for x = 1.0. The high polarization observed was due to e− hopping between Fe2+↔ Fe3+. The electrical models of the samples show non-Debye type relaxation behavior, while the ac conductivity enhanced with increasing frequency which was in accordance with Koop’s phenomenological theory.