Investigation of order-disorder transition in BICUVOX single crystals

Single crystals of nominal composition Bi2V0.9Cu0.1O5.35 were obtained by g rowth from the melt. DTA curves exhibit effects due to phase transitions, o ne endothermic effect during heating and one exothermic during cooling. Ele ctron diffraction experiments combined with in-situ heating were done by TE M. Evidence was found that superlattice spots present on diffraction patter ns in the low temperature range vanish for temperatures higher than 720 K. During cooling the superlattice spots reappear reversibly. The impedance of the crystals was measured in the direction parallel to the layers of the i ntergrowth structure. An Arrhenius-type dependence of conductivity with two activation energies different for the low and high temperature regions was observed. The transition temperatures observed in DTA coincide with those observed in electron diffraction experiments and correspond to the slope ch anges in the Arrhenius plots. A phase transition, involving an order-disord er process of oxygen ions, is considered responsible for this phenomenon. ( C) 1998 Published by Elsevier Science B.V. All rights reserved.