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

Abstract

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 LiNbO 3 : Yb/Er nanocrystals were characterized with variable-pressure optical and impedance spectroscopy. It is found that LiNbO 3 : 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 pressureinduced ion migration but also gives a perspective on the influence of crystal morphology on pressure-induced Li + ion diffusion in ionic conductors.