STRUCTURE-DEPENDENT IMPEDANCE OF BICUVOX

The impedance of a polycrystalline sample of Bi2V0.9Cu0.1O5.35, sinter ed from crushed single crystals, was measured in air in the frequency range 3.2mHz to 10MHz at constant temperatures up to 980K using an aut omated setup which allowed measurement of high impedances. The measure d spectra were simulated by least-squares fitting of equivalent circui ts. A time dependence of conductivity was found in a limited temperatu re range similar to the earlier observations for single crystals. Prol onged 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 wer e lower, by about a factor 2, than the conductivity of the single crys tal measured in the direction parallel to the layers of the intergrowt h structure. Separation of the total resistance into intragrain and in tergrain components was feasible only in the high conductivity state o f the polycrystalline sample at temperatures below 670K.