Phase transitions and electrical properties of Bi2W1-xNbxO6-y and Bi2W1-xTaxO6-y

Polycrystalline samples of Bi2W1-xMexO6-y (Me = Nb, Ta) solid solutions have been prepared by solid-state reactions, and the influence of Nb and Ta substitutions for W on the polymorphism and electrical properties of Bi2WO6 has been studied. The limit of the solid solutions is at x = 0.1 for Me = Nb and at x = 0.15 for Me = Ta. The distinctive features of the polymorphism of the Nb- and Ta-doped materials have been identified. According to differential scanning calorimetry data, tantalum and niobium substitutions for tungsten increase the temperature of the high-temperature, orthorhombic-to-monoclinic reconstructive phase transition and suppress the transition starting at x = 0.05 for Me = Nb and x = 0.10 for Me = Ta. As a result, the Bi2W1-xNbxO6-y samples have an orthorhombic Aurivillius-type structure up to their melting point. The Bi2W1-xTaxO6-y solid solutions at high temperatures consist of a mixture of an orthorhombic and a monoclinic phase. Nb and Ta doping shifts the ferroelectric phase transition to lower temperatures by more than 200 degrees C, thus markedly extending the stability range of the nonpolar orthorhombic paraelectric phase, which exists in a temperature range as narrow as 930-960 degrees C in the case of undoped Bi2WO6. The increase in oxygen vacancy concentration due to heterovalent substitutions of Nb5+ and Ta5+ for W6+ leads to an increase in conductivity by two orders of magnitude relative to the unsubstituted compound. (C) 2013 Elsevier B. V. All rights reserved.