Thermal cycling induced reversibility/irreversibility in the ac electrical properties of La0.50Ca0.50MnO3+δ by impedance spectroscopy

In this work, we address the issue of microstructural changes in the electrical properties of charge ordered polycrystalline La0.50Ca0.50MnO3+delta sample during thermal cycling through impedance spectroscopy. The modulations of relaxation time with temperatures and different thermal cycling shows different magnitudes of impedance plane plots. Thermal cycling induces varying mechanical and electrical stresses in the grains and in the grain boundaries, changing the resistance of the grains (R-1) and the grain boundaries (R-2) by changing the boundary potentials and trap state scattering cross sections. At metal-insulator transition temperature (MIT), R-1 increases with polynomial first order and R-2 increases with polynomial second order. The values of R-1 and R-2 show different orders of variations along with reversible and irreversible behaviors around and below MIT. We report a method by virtue of which charge ordering temperature T-CO has been determined with the change in the magnitude of grain boundary resistance by thermal cycling. Localization of charge carriers in the traps with thermal cycling around MIT and delocalization of charge carriers with different frequencies have been discussed. (c) 2009 American Institute of Physics. [doi:10.1063/1.3238301]