Pressure-Induced Li+ Migration and Second-Order Phase Transition in LiNbO3: Yb/Er Nanocrystals Revealed by Variable-Pressure Optical and Impedance Studies

Pressure is a key thermodynamic parameter for modulating the crystal lattice and physical properties of ionic conductors. It is essential to probe the pressure-induced ion migration and the resulting structure variation in ionic conductors; yet, it remains a challenge to date. Herein, we report an in situ study on the ionic conductor of LiNbO3: Yb/Er nanocrystals were characterized with variable-pressure optical and impedance spectroscopy. It is found that LiNbO3: Yb/Er nanocrystals undergo a second-order phase transition near 6 GPa, which is triggered by Li+ migration. The surge of impedance upon elevated pressure may be dominated by the fracture of the symbiotic catenuliform nanocrystals and the rearrangement of the nanocrystals. The diffusion coefficient of Li+ should be combinedly influenced by the dynamic interface among nanocrystals, phase transition, and the newly formed defects. The study not only presents an alternative optical technique to probe the pressure-induced ion migration but also gives a perspective on the influence of crystal morphology on pressure-induced Li+ ion diffusion in ionic conductors.