Multi-scale structural response of the lead-free perovskite-type ferroelectric solid solution (1-x)Na0.5Bi0.5TiO3-xBaTiO3 to high pressures

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Complex perovskite-type (ABO3) ferroelectrics are among the key functional materials due to their ability to convert various types of external force fields into electrical signals. Currently, the most important industrial ferroelectric solid solution is PbZr1-xTixO3 (PZT), exhibiting a composition-driven phase transition, called a morphotropic phase boundary (MPB), at which the dielectric, piezoelectric, pyroelectric, and optoelectric properties are enhanced. Lead is however noxious and hence, environmentally friendly replacements of PZT are necessary to be found. The solid solution (1-x)Na0.5Bi0.5TiO3 -xBaTiO3 (NBT-xBT) is very promising as a Pb-free alternative because, similar to PZT, it has a nearly temperature-independent MPB. However, in spite of the presence of macroscopic polarization, the average structure of NBT-xBT appears to be pseudocubic at MPB, indicating abundant nanoscale structural inhomogeneities and incipient ferroic clustering. Therefore comprehensive in situ structural analyses under different external stimuli (temperature, elastic stress/pressure, electric field) that can boost the ferroic order are required, in order to disclose the intrinsic structural features governing the material properties in alkali-bismuth-based systems. There are a number of studies at different temperatures and electric fields but surprisingly, high-pressure structural analyses of NBT-xBT are scarce and mostly limited to the end members. The current project aims to fill this gap of knowledge. We are going to study the compositional dependence of the pressure-induced structural transformations in NBT-xBT single crystals across the MPB (x = 0, 0.013, 0.048, 0.053, 0.074) at different length scales by Raman spectroscopy, synchrotron X-ray diffuse scattering, and high-precision in-house X-ray diffraction up to ~21 GPa, using the diamond-anvil-cell technique. The comparison between the pressure evolution of the phonons as well as of X-ray diffuse scattering and the equation of state will make it possible to identify the atomistic origin of elastic instabilities and flattening of the local potentials, supporting the property enhancements. The volume compressibility as well as the phonon Grüneisen parameters as a function of composition will be determined. Furthermore, the effect of anisotropic stress versus that of hydrostatic pressure on the local structure and dynamics of NBT-xBT single crystals across the MPB is going to be analyzed on the basis of Raman scattering under non-hydrostatic conditions up to ~5.6 GPa. The expected results should provide a deeper insight into the structure of NBT-xBT and better reveal the intrinsic nanoscale structural difference between Pb-based and Pb-free systems, which in turn can help designing NBT-based materials with properties comparable to those of Pb-rich materials.