Dielectric tunability of La${}_2$Ni${}_{1-x}$Fe${}_x$MnO${}_6$ double perovskites for energy storage applications

The dielectric characteristics of double perovskite La${}_2$Ni${}_{1-x}$Fe${}_x$MnO${}_6$; (x = 0, 0.2, 0.5, 0.8 & 1.0) developed via sol-gel route, were studied through analysis of B-site cation substitution. La${}_2$Ni${}_{0.8}$Fe${}_{0.2}$MnO${}_6$ attained a higher dielectric constant than pure La${}_2$NiMnO${}_6$ owing to comparatively higher B-site cationic disorder. The crystallite size and strain were determined using Williamson-Hall analysis, and strain in samples decreased with increasing Fe substitution. High-resolution transmission electron microscopy (HRTEM) authenticated the crystalline nature and selected area electron diffraction patterns (SAED) confirmed the purity of the sample. X-ray photoelectron spectroscopy (XPS) confirmed the co-occurrence of different oxidation states. Dielectric constant and tangent loss of overall composition decreased with frequency, while AC conductivity increased. AC conductivity decreased upon Fe-substitution and was strongly affected by the degree of B-site cationic disorder. Cole-cole plots and combined variation of Z${}^{\prime \prime }$ & M${}^{\prime \prime }$ with frequency suggested non-Debye relaxation in developed compositions. The tuned dielectric parameters suggest the possible applicability of developed double perovskite materials for fabricating energy storage devices.