Abstract text (incl. figure legends and references)

The EU directive for the Restriction of Hazardous Substances (RoHS) has driven researchers to look for alternatives in order to replace lead-containing components in electronic devices, due to growing concerns over health and environmental risks. The lead-free (1-x)Na_1/2Bi_1/2TiO₃-xBaTiO₃ (NBT-BT) system has gained considerable attention, especially for high-power and high-frequency applications, such as ultrasonic transducers.

The relaxor ferroelectric solid solution exhibits enhanced piezoelectric properties at its morphotropic phase boundary (MPB) near x = 0.06. Relaxors are characterized by the presence of nanometer-sized polar regions (PNRs/nanodomains), which allow for a high electromechanical response and high permittivity values. In the relaxor-ferroelectric transition of NBT-BT ceramics, the electromechanical properties are highly linked to a complex domain structure, which strongly varies with composition.

The local structure and phase assemblage of NBT-BT ceramics with 6, 9 and 12 mol % BaTiO3 (BT) was studied by using different imaging techniques and electron diffraction in transmission electron microscopy (TEM). Deploying dark-field (DF) imaging of superlattice reflections, the relaxor nanostructure is visualized, consisting of densely packed tetragonal P4bm nanodomains and a minor fraction of rhombohedral R3c nanodomains. The 5-30 nm sized P4bm regions display an elongated shape and are oriented along the {001}_pc planes. Mapping the polar displacement in high-resolution high-angle annular dark field (HAADF) images reveals a heterogeneous local structure with short-range fluctuations of a few nanometres in size (PNRs). The nanoscale structure correlates to the frequency dispersion of the temperature-dependent permittivity, typical for relaxors. With increasing BT fraction, long-range ferroelectric lamellar domains of tetragonal P4mm symmetry emerge. Within these ferroelectric domains, the nanodomain contrast considerably declines, which is associated with the disappearance of octahedral tilting and a transition to the non-tilted P4mm structure, reflected in a more ferroelectric nature of BT-rich compositions. Nevertheless, a reduced population of both P4bm and R3c nanodomains remains embedded within the lamellar ferroelectric domains, indicating a complex hierarchical domain arrangement. The findings emphasize the important role of a direct microscopic analysis in order to elucidate on the nanostructure-property correlation in relaxor ferroelectric solid solutions.