Effects of oxygen content and heating rate on phase transition behavior in Bi2(V0.95Ti0.05)O5.475−x

The phase transition behavior of oxide-ion conductor Bi2(V0.95Ti0.05)O5.475−x, which has various thermal histories and sample forms, has been studied by means of differential scanning calorimetry. Thermogravimetric analysis revealed that the oxygen content per compositional formula varied with the applied thermal treatment, although no significant structural difference was observed by X-ray diffraction (XRD) analysis. The phase transition behavior from αf to βf and from βf to γf, observed at a heating rate of 10 K min−1, are markedly affected by the sample preparation. For example, the endothermic peak of the transition from αf to βf appeared at around 400 °C for quenched powder and at around 320 °C for powder cooled at 0.5 K min−1. The trend of the transition temperatures can be qualitatively explained in terms of oxygen content, i.e., Bi2(V0.95Ti0.05)O5.475−x with less oxygen content exhibits the transition from αf to βf at a higher temperature and the transition from βf to γf at a lower temperature. We confirmed the two types of transition behavior from αf to βf depending on heating rate of DSC and high-temperature X-ray diffraction (HT-XRD) analysis. At rapid heating rates of 10 and 40 K min−1, αf transformed to βf directly. Meanwhile, at a slow heating rate of 2 K min−1, the βf precipitated from αf because slow heating allowed the diffusion of Ti and oxygen vacancies induced by the aliovalent doping. This suggests that the small differences of atomic arrangement also affect the phase transition behavior because additional elements have preferable crystallographic sites in αf and βf.

► Phase transition behavior of oxide-ion conductor Bi2(V0.95Ti0.05)O5.475−x, which has various thermal histories and physical forms.
► At the same heating rate of 10 K min−1, Bi2(V0.95Ti0.05)O5.475−x with less oxygen content exhibits transition from αf to βf at a higher temperature and the transition from βf to γf at a lower temperature.
► αf directly transformed to βf at fast heating rates. At a slower heating rate of 2 K min−1, βf precipitated from αf due to the sufficient diffusion of Ti and oxygen vacancies.