⁷Li NMR short-range ordering in hardened lithium sodium niobate.

Abstract

Thermal treatments play a crucial role in tuning the functional properties of lithium-based perovskites, which are of interest for applications in energy storage, actuation and sensing. In the present work, the impact of aging and quenching on the local structure and lithium mobility in lithium sodium niobate (LNN) ceramics was investigated using Density Functional Theory (DFT), transmission electron microscopy (TEM) and solid-state Nuclear Magnetic Resonance (NMR) techniques. Our results demonstrate that quenching inhibits the formation of a lithium-rich phase, leading to significant modifications in dipolar and quadrupolar interactions. Aging induces the formation of LiNbO₃ within the perovskite matrix, with no indication of lithium mobility at the boundaries of LiNbO₃ grains. The presence of multiple lithium environments in aged samples was indicated by transmission electron microscopy and confirmed through ⁷Li Magic Angle Spinning (MAS) NMR, revealing distinct structural differences between aged and unaged samples. NMR parameters were also compared with parameters calculated by Density Functional Theory. The findings suggest the formation of LiNbO₃ and an additional lithium poor phase in aged LNN samples. These results indicate that the quenching effect reduces dipolar interactions and considerably sharpens the ⁷Li NMR spectra, while structural modifications induced by thermal processing affect lithium ordering, providing new insights into the phase behavior of LNN ceramics.