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

• The limit of Bi2W1−xMexO6−y solid solutions is at x = 0.1, 0.15 for Me = Nb, Ta.
• Ta and Nb substitutions for W suppress the reconstructive phase transition.
• Bi2W0.9Nb0.1O6−y samples belong to Aurivillius-type structure up to their melting.
• Nb and Ta doping shifts ferroelectric transition to low temperatures up to 200 °C.
• The highest conductivity reaches 10−1 S/cm at 800 °C (x = 0.05, 0.1; Me = Nb, Ta).

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.

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