title: Microstructural characterization of ferroelectric (Na₁⸝₂Bi₁⸝₂)TiO₃-BaTiO₃ ceramics via transmission electron microscopy
Creators: name: Fetzer, Ann-Katrin
Publisher: name: UNSPECIFIED
Year: 2023

description: Lead-free ferroelectrics increasingly gain significance as replacement for lead-containing ceramic materials in electronic devices. A main focus lies in modifying their functional properties in order to meet industry demands. Quenching of the lead-free relaxor ferroelectric solid solution (1−x)(Na₁⸝₂Bi₁⸝₂)TiO₃-xBaTiO₃ (NBT-BT) was developed as a procedure for increasing the material’s low depolarization temperature (Td), which strongly limits the application range. Quenching also increases the lattice distortion and affects the temperature-dependent permittivity response. However, there is a lack of clear understanding about the interrelation of such characteristics and the local microstructure. This work investigates the compositional range of (1−x)NBT-xBT ceramics (0.03 ≤ x ≤ 0.12), primarily using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). It sheds light on the relaxor ferroelectric micro- and nanostructural features and examines the microstructure-structure-property correlation upon quenching. Di- and piezoelectric properties of NBT-BT ceramics vary profoundly, dependent on the composition. Close to the morphotropic phase boundary (MPB) of x ≈ 0.06, the relaxor behavior reflects in the presence of short-range polar structures (nanodomains/polar nanoregions) and the coexistence of rhombohedral and tetragonal symmetries. The stabilization of a more ferroelectric character upon quenching is accompanied by a shift in the phase assemblage towards the formation of abundant lamellar tetragonal (P4mm) and rhombohedral (R3c) domains, coexisting with the relaxor P4bm phase. Quenching thus promotes a spontaneous onset of ferroelectric order in unpoled relaxor compositions, where the increased non-cubic lattice distortions manifest in an enforced formation of lamellar ferroelastic domains in order to accommodate the corresponding strain. In-situ TEM analyses illustrate the temperature-dependent evolution of the domain and phase assemblage and investigate on the stability of the quenched structural features with increasing temperature. The depolarization of the material (Td) is strongly linked to the disintegration of the ferroelectric domain structure. The lamellar P4mm domains in the quenched MPB composition exhibit a partial stability up to elevated temperatures (∼300 °C), which benefits a delayed depolarization and relates to the overall increase in Td. The reversibility of the temperature-dependent structural transitions is furthermore investigated. TEM dark-field imaging using superlattice reflections visualizes the relaxor nanostructure of P4bm and R3c nanodomains. High-resolution STEM investigations display a heterogeneous cation displacement with nanometer-sized polar fluctuations, which are denoted as polar nanoregions (PNRs) and relate to the relaxor behavior. In BT-rich compositions, a complex hierarchical domain configuration is revealed, where a reduced population of P4bm and R3c nanodomains prevails embedded within the lamellar P4mm domain structure. The findings demonstrate that TEM constitutes a viable tool for unravelling the complex nanostructure of multi-phase relaxor ferroelectric systems.