Alternating-Current Transport Properties in Nd_0.7Sr_0.3MnO₃ Ceramic with Secondary Phases

Nd_0.7Sr_0.3MnO₃ ceramics with secondary phases were prepared by ball-milling and post heat- treatment at 1623 K for 3, 7 and 13 h, respectively. The results from x-ray diffraction and energy dispersed spectroscopy show that some secondary phases are introduced and grow gradually with sintering time. These secondary phases have significant effects on the ac transport. For all the samples, the real part of impedance (Z_r ) decreases with increasing frequency and the Z_r peak moves to a higher temperature. Interestingly, for a given frequency the Z_r peak decreases with sintering time. However, for samples B and C which were sintered for a longer time than sample A, an additional Z_r peak appears at a higher temperature and gradually increases with sintering time. The reposition of trapped charges in phase/grain boundaries or secondary phases is supposed to be responsible for the unusual relaxation process.