Conductivity behaviour of a cubic/tetragonal phase stabilized nanocrystalline La2O3–ZrO2 Original Research Article

La2O3 doped nanocrystalline zirconia (ZrO2) was prepared by chemical co-precipitation method for the 3, 5, 8, 10, 15, 20 and 30 mol.% concentrations of La2O3. Structural studies were performed using X-ray diffraction (XRD). All the as-synthesized samples were found to be in monoclinic phase. As-synthesized samples were given heat treatment at higher temperatures for tetragonal/cubic structural phase stabilization. Sintering the samples at temperature 1173 K stabilized the tetragonal and cubic phases. A slight shift in the 100% peak of the cubic phase was observed towards the low diffraction angle indicating the substitution of the bigger La3+ ion into the ZrO2 lattice. Grain sizes were found to lie between 10 and 13 nm. Electrical conductivity studies were performed on the cubic phase stabilized La2O3–ZrO2 by complex impedance spectroscopy. The conductivity increases up to the dopant concentration 10 mol.% and then decreases with further increase in La2O3 concentration. Initial increase in conductivity is correlated to the stabilization of the cubic phase and the subsequent decrease in the conductivity with the dopant content is interpreted on the basis of the oxygen-ion movement model. Electrical conductivity has contributions from grain and grain boundary regions. But the grain boundary conductivity is slightly higher than the corresponding grain conductivity. Higher grain boundary conductivity shows higher diffusion coefficient for the atoms on the surface of the ZrO2 grains. The possible mechanism of the oxygen ion conduction in the La2O3 stabilized zirconia (LSZ) is reported. The Barton, Nakajima and Namikawa (BNN) relation has been applied to the conductivity data and found that the d.c. and a.c. conductions have been correlated to each other by the same mechanism.