Transition from hopping conduction to band conduction in LaFexNi1-xO3

Electrical transport and the nature of carriers in the homovalent substitution LaFexNi1-xO3 were investigated as a function of component, in the temperature range 10K-300K, by dielectric properties, de and ac conductivities and thermopowers. At x>0.6, the positive thermopowers indicate the major carriers of holes which are introduced by the superexchange rule. At x = 0.9 and 0.8, a hopping process of small polarons of holes contributes predominantly to the electrical transport at temperatures higher than about 170 K, which is confirmed by both the dielectric measurements and the Arrhenius relations between sigma T and 1/T. As the temperature falls below 170 K, the non-Arrhenius conductivities appear, which is consistent with the fact that the small-polaron hopping becomes usually non-Arrhenius as the temperature is reduced. At x = 0.7 and 0.6, the frequency dependences of the dielectric loss tangent indicate reorientation of polarons deeply bound to traps at temperatures below about 70 K. As x decreases below 0.6 , the conductivities become high, while the magnitude of the activation energy required for the conduction decreases, and finally a metal-insulator (MI) transition takes place at x approximate to 0.35. Simultaneously, the thermopowers change from positive to negative, which indicates a transition of the major carriers from holes to electrons and the breakdown of the superexchange rule at the Ni-rich composition in which no dielectric relaxation was observed. The MI transition is discussed in terms of the closing of the charge-transfer (CT) gap between O 2p orbitals and Ni e(g) bands. In the composition with x = 0.5 or 0.4, a thermally activated excitation of electrons from the O 2p band to the Ni 3d upper band is expected because of the narrow CT gap. These results provide evidence on the transition from hopping conduction to band conduction with decreasing x in LaFexNi1-xO3.