Low temperature - Dielectric, impedance, and conductivity - Study for lanthanum nickelate-manganate ceramics

Abstract

A solid-state reaction was applied to obtain lanthanum nickelate manganate ceramics. This double perovskite was sintered at TS = 1350 °C. An occurrence of monoclinic P21/n major phase of La2NiMnO6 at room temperature was confirmed by X-ray diffraction analysis. The minor phase of cubic Fm-3m of the MnNi6O8 phase was detected. Scanning electron microscopy (SEM) images exhibited agglomerated irregular grains of sizes ranging from ∼1 to 5 μm, separated by voids. The energy dispersion X-ray spectroscopy (EDS) method confirmed the occurrence of La, Mn, Ni, and O elements and the absence of other atoms. The Nyquist plots exhibited overlapped semi-circular-like arcs, attributed to grain and grain boundary effects at low temperature range. The imaginary impedance peak, which shifted when temperature and frequency increased, was attributed to the relaxation process. The activation energy Ea,τ = 0.10 eV and characteristic relaxation time τ0 = 1.49 ps for the relaxation process was estimated. The Bode plot helped to discern different mechanisms involved in the impedance observed in the low-temperature range. The conductivity scaling behavior confirmed the hopping mechanism of the small polarons. The estimated dc electrical conductivity activation energy, Edc, varied from 0.13 eV to 0.17 eV, depending on the temperature range. Variable range hopping of the small polaron model consistent with the structural disorder was appropriate in the low-temperature range. The dielectric loss response was explained in the framework of the conductivity effect. The non-coincidence of the peak positions of impedance Z″ and modulus M″ spectra confirmed the short-range motion of charge carriers. The manifestation of the magneto-dielectric effect was deduced.