On the factors affecting the high temperature insulator-metal transition in rare-earth manganites

The measurement of resistivity across a wide temperature range - from 15 to 1473 K - in rare-earth manganite series of compounds reveals a very interesting feature : normally observed insulating pattern beyond Tc (Curie Point) gives way to a reentrant metallic pattern around a characteristic temperature T*. The transport activation barrier Ea collapses to zero around T*. T* is found to be dependent on the carrier concentration or the concentration of the Jahn-Teller-active Mn(3+) ions as well as on the average A-site radius <rA> for a fixed carrier concentration. These factors govern the effective lattice distortion and hence lead to the variation in the conduction bandwidth. Our data cover a wide range - from T*>>Tc for smaller bandwidth to T* tending towards Tc for larger bandwidth. These results seem to provide evidence of the onset of lattice distortion at high temperature (around T*) and its variation. Since lattice distortion governs the magnetic, transport and other important behaviors significantly, our data assume importance as they offer a new measure of the effective distortion and its tunability.