Structure, electrical and magnetic properties of La_0.67Ca_0.33−x K _x MnO₃ polycrystalline ceramic

Abstract

A systematic investigation of structure, electrical and magnetic properties of polycrystalline ceramics La_0.67Ca_0.33−x K _x MnO₃ (x = 0.05, 0.10, 0.15, 0.20, 0.25) samples, prepared by sol–gel method had been undertaken. As K content increases the crystal structures were transformed from orthorhombic to rhombohedral structure identified by X-ray diffraction, and the effect of increasing K ion is to increment the Mn–O–Mn bond angle. The surface morphology was investigated by scanning electron microscope, which indicates that grain size decreasing with increasing of K⁺. Temperature dependence of resistivity (ρ − T) was measured by standard four-probe method. The insulator–metal transition temperature (T _P ) shifted to higher temperature and the temperature coefficient of resistivity decreased sharply with the substitution K⁺ for Ca²⁺ ion. The temperature dependence of magnetization (M–T) shown that Curie temperature (T _C ) was increasing with the increase of K content, which can be explained by enhancement of double–exchange interaction. The data of resistivity on low-temperature (T < T _P ) had been fitted with the relation ρ(T) = ρ ₀ + ρ ₂T² + ρ _4.5T^4.5; the high-temperature (T > T _P ) resistivity data were explained using small-polaron hopping and variable-range hopping models. Resistivity data in whole temperature range (100–320 K) could be fitted by percolation model. Polaron activation energy E _a was found to decrease with the content K⁺ increasing, which suggested that K doping increase bond angle Mn–O–Mn, thereby the effective band gap was decreased and the double exchange coupling was increased of, this is the reason for the decrease of resistivity.