In Situ Observation of Ion Migration in a Ferroelectric Ionic Conductor Rb-KTP during Thermal Annealing

Ion exchange in Rb-doped KTiOPO₄ has facilitated significant advancements in ferroelectric domain engineering, yet understanding the underlying mechanisms remains in its infancy. We perform time-of-flight secondary ion mass spectrometry analysis on multiple periodically ion-exchanged and periodically poled Rb-doped KTiOPO₄ samples under different temperatures and annealing durations. The results are compared between annealing in air, which involved ex situ annealing before periodic poling, and vacuum annealing conducted in situ after periodic poling. The Rb⁺ diffusion profile after periodic ion exchange forms a tooth-shaped pattern. We show that in situ annealing causes a surface pinning effect on the nonpolar face, limiting Rb⁺ migration along the polar axis at the surface. Once the pinned layer is removed through milling, the underlying Rb⁺ diffusion is distinctively different from the surface. Additionally, the rate of Rb⁺ diffusion during in situ annealing is linear, while the periodic domain structures remain stable after annealing. These results contribute to understanding the ionic diffusion process in a ferroelectric ionic conductor and using ion exchange to tailor the linear and nonlinear properties of KTiOPO₄.