Conduction mechanisms in pure and doped polycrystalline orthorhombic manganites

In this work we present magnetic and transport measurements as a function of temperature of polycrystalline pure and doped RMnO 3 (R=La, Nd and Ho) samples. We have performed AC transport measurement (in the range from DC to 2 MHz) in pellets of La 0.7 Ca 0.3 MnO 3 annealed at several temperatures, which present different connectivity between grains keeping the same mean grain size (about 1 μm). A large increase of resistance is observed at temperatures below the ferromagnetic order temperature where a metallic regime is expected. 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 metallic ones, where the carriers percolate through well connected grains and highly resistive channels for high intergrain boundaries that behave as capacitors. In order to obtain the conduction mechanisms for non-doped RMnO 3 samples, 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 differences in the activation energy with the rare earth ion are also discussed.