Isothermal kinetic of phase transformation and mixed electrical conductivity in Bi3NbO7

Bismuth niobate (Bi3NbO7) exists under two crystallographic modifications, a tetragonal (type-III) phase between 800 and 900 deg. C, and a pseudocubic (type-II) phase above and below this thermal range. The quenching at room temperature of pseudocubic type-II phase made it possible to carry out a detailed study of the transformation kinetics of this metastable type-II phase to the stable type-III phase, using isothermal in situ X-ray diffraction. The obtained Avrami exponent and activation energy for the transition are around 2.5 and 3.25 eV, respectively. The value of the Avrami exponent is consistent with a three-dimensional diffusion-controlled transformation with constant nucleation rate. Investigations of electrical properties using AC impedance spectroscopy and Wagner polarization method show that the tetragonal phase exhibits higher ionic and electronic conductivities than those of the pseudocubic form. Such a deviation is likely to originate from different distributions of cations/electronic-lone-pairs and oxygen vacancies. - Graphical abstract: The metastable type-II form of Bi3NbO7, whose phase transformation kinetics to type-III form is studied in isothermal conditions, is shown to have a larger volume and a lower anionic (and electronic) conductivity than the type-III form of this fluorite-type bismuth niobate