Structure, electrical and magnetic properties of La0.67Ca0.33−x K x MnO3 polycrystalline ceramic

Abstract

A systematic investigation of structure, electrical and magnetic properties of polycrystalline ceramics La0.67Ca0.33−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 4.5T4.5; 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.