Structural relaxation of ferroelectric phase in hard sodium lithium niobate solid solutions studied by Solid-State NMR

Hard sodium lithium niobate (LNN) solid solutions offer a promising environmentally benign alternative to lead-based ferroelectric materials for electronic devices. A major challenge in their practical application is that their ferroelectric phase relaxes over a timescale ranging from weeks to years to an orthorhombic phase, limiting their long-term performance. In order to understand the structural changes in the relaxation process, we utilized 1D and 2D ²³Na solid-state Nuclear Magnetic Resonance (ssNMR) to investigate short-range structural changes in high mechanical quality factor Li_xNa_1-xNbO₃ solid-solutions exposed to ambient conditions over 24 months. NMR results reveal a system with phase-changes as a function of aging time and temperature. The samples exhibit a multiphase structure compromised of crystalline R and Q orthorhombic domains, along with two types of amorphous regions. A significant amount of ferroelectric phase persists in the ceramics after 24 months of exposure to ambient conditions. A structural model based on short-range order of sodium was suggested and agrees well with the lattice parameter of the freshly prepared samples.