STRUCTURE-DEPENDENT IMPEDANCE OF BICUVOX

The impedance of a polycrystalline sample of Bi2V0.9Cu0.1O5.35, sintered from crushed single crystals, was measured in air in the frequency range 3.2mHz to 10MHz at constant temperatures up to 980K using an automated setup which allowed measurement of high impedances. The measured spectra were simulated by least-squares fitting of equivalent circuits. A time dependence of conductivity was found in a limited temperature range similar to the earlier observations for single crystals. Prolonged annealing at about 700K resulted in a low conductivity state at low temperatures, while the high conductivity state was recovered upon heating to 830K. The values of conductivity were only independent of the thermal history of the polycrystalline sample above 830K. They were lower, by about a factor 2, than the conductivity of the single crystal measured in the direction parallel to the layers of the intergrowth structure. Separation of the total resistance into intragrain and intergrain components was feasible only in the high conductivity state of the polycrystalline sample at temperatures below 670K.