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

The measurement of resistivity (rho) across a wide temperature (T) range-fr om 15 to 1473 K-in the rare-earth manganite series of compounds reveals a v ery interesting feature: the normally observed insulating pattern beyond T- c (the Curie point) undergoes a broader transition and eventually gives way to a reentrant metallic pattern around a characteristic temperature T*. Co nsidering a model with coexisting metallic and non-metallic phases beyond T -c, it has been shown that T* marks the temperature at which the metallic v olume fraction v(m) (T) reaches 100% as a result of progressive removal of lattice distortion at higher temperature. T* is found to be dependent on th e carrier concentration as well as on the average A-site radius ((r(A))) fo r a fixed carrier concentration.