Single crystals having the nominal composition, Bi2V0.9M0.1O5.35, were studied by means of impedance spectroscopy (0.01Hz to 10MHz) at 300 to 980K. The impedance was measured parallel to, and perpendicular to, the structural layers of the crystals. The conductivity was determined by performing a non-linear least-squares analysis of the impedance spectra. Arrhenius-type plots of the conductivity, measured in the two directions for the Cu and Ni variants, exhibited linear regions at high and low temperatures; with different associated activation energies. In the intermediate region, the conductivity decreased with time during annealing. The decrease was very marked in the Cu variant where, in the low-temperature region, 2 distinct conductivity states were found for each direction. The ratio of the conductivity values for the 2 states was equal to about 55 in the parallel direction, and equal to about 6 in the perpendicular direction. The activation energy was larger in the low-conductivity state. In the high-temperature region, the ionic conductivity did not depend upon the thermal history of the specimen. The activation energy was almost the same for both variants and was close to 0.5 and 0.8eV, respectively, in directions parallel to, and perpendicular to, the layers of the crystal.

Stability of Ionic Conductivity of BIMEVOX Single Crystals. P.Kurek, J.R.Dygas, M.W.Breiter: Molecular Physics Reports, 2000, 27, 53-9