In Situ Observation of Dehydration-Induced Phase Transformation from Na2Nb2O6-H2O to NaNbO3

We have monitored the phase transformation from a Sandia octahedral molecular sieve Na2Nb2O6-H2O to a piezoelectric NaNbO3 nanowire through in situ X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements at high temperatures. After dehydration at 288 degrees C, the Na2Nb2O6-H2O becomes significantly destabilized and transforms into NaNbO3 with the increase of time. The phase transformation time is exponentially proportional to the inverse of temperature, for example, similar to 10(5) is at 300 degrees C and similar to 10(1) at 500 degrees 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.