In situ observation of dehydration-induced phase transformation from Na ₂Nb ₂O ₆-H ₂O to NaNbO ₃

We have monitored the phase transformation from a Sandia octahedral molecular sieve Na ₂Nb ₂O ₆-H ₂O to a piezoelectric NaNbO ₃ nanowire through in situ X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements at high temperatures. After dehydration at 288 °C, the Na ₂Nb ₂O ₆-H ₂O becomes significantly destabilized and transforms into NaNbO ₃ with the increase of time. The phase transformation time is exponentially proportional to the inverse of temperature, for example, ∼10 ⁵ s at 300 °C and ∼10 ¹ s at 500 °C, and follows an Arrhenius equation with the activation energy of 2.0 eV. Real time TEM investigation directly reveals that the phase transformation occurs through a thermally excited atomic rearrangement due to the small difference of Gibbs free energy between two phases. This work may provide a clue of kinetic control for the development of high piezoelectric lead-free alkaline niobates and a deep insight for the crystallization of oxide nanostructures during a hydrothermal process.