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

Abstract

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°C, thus markedly extending the stability range of the nonpolar orthorhombic paraelectric phase, which exists in a temperature range as narrow as 930–960°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.