IMPEDANCE MEASUREMENTS ON SINGLE-CRYSTALS OF THE OXYGEN-ION CONDUCTOR BICUVOX

Single crystals of nominal composition Bi2V0.9Cu0.1O5.35 were obtained by growth from the melt. The crystal structure was examined by X-ray diffraction. The impedance of the crystals was measured with Pt electrodes in the frequency range 1.47 x 10(-3) -10(7) Hz at constant temperatures between 370 and 990 K in a fully automated set-up. The conductivity, determined by least-squares analysis of the impedance spectra, was larger by more than a factor of 100 in the direction parallel to the layers of the intergrowth structure than in the perpendicular direction. The activation energies were also different. These results confirm a previous conclusion that the ionic conduction occurs primarily inside the layers of the intergrowth structure. A time dependence of conductivity was observed in a limited temperature range. During both heating and cooling, at temperatures between 650 and 705 K the conductivity decreased with time for several hours. On heating the conductivity increased with time between 760 and 830 K. Only at temperatures above this range were the values of conductivity independent of the thermal history of the crystal.