POLARONIC HOPPING CONDUCTION IN REDUCED LITHIUM-NIOBATE SINGLE-CRYSTALS

The d.c. conductivity of lithium niobate single crystals has been measured as a function of temperature for different samples with varying degrees of reduction, to observe the effect of reduction on the d.c. conductivity and its activation energy. At low temperatures, both the conductivity and the activation energy are temperature dependent; however, at high temperatures the activation energy becomes temperature independent. The conductivity and activation energy are found to depend strongly on the degree of reduction and the conductivity exhibits a peak as a function of degree of reduction. These results have been analysed assuming small-polaron hopping conduction between Nb5+ and Nb4+ ion sites, and the conduction is found to be in the non-adiabatic regime. Various physical parameters associated with small-polaron hopping conduction have been evaluated and show systematic changes with the degree of reduction. © 1990 Taylor & Francis Ltd.