Conduction mechanisms in pure and doped polycrystalline orthorhombic manganites

In this work we present magnetic and transport measurements as a function o f temperature of polycrystalline pure and doped RMnO3 (R=La, Nd and Ho) sam ples. We have performed AC transport measurement (in the range from DC to 2 MHz) in pellets of La0.7Ca0.3MnO3 annealed at several temperatures, which present different connectivity between grains keeping the same mean grain s ize (about 1 mum). A large increase of resistance is observed at temperatur es below the ferromagnetic order temperature where a metallic regime is exp ected. The behavior with temperature and frequency of the resistance of low temperature annealed pellets can be explained taking into account that two kinds of conduction paths are present in the sample: low resistive metalli c ones, where the carriers percolate through well connected grains and high ly resistive channels for high intergrain boundaries that behave as capacit ors. In order to obtain the conduction mechanisms for non-doped RMnO3 sampl es, we have measured the resistance in a large temperature range up to 1400 K. We present a discussion about the deviation of the measured resistance from any kind of polaronic conduction at high and low temperature. The diff erences in the activation energy with the rare earth ion are also discussed . (C) 2001 Elsevier Science B.V. All rights reserved.