Multi-scale structural response of the lead-free perovskite-type ferroelectric solid solution (1-x)Na0.5Bi0.5O3-xBaTiO3 to high pressures

Abstract

The goal of projects MI 1127/12-1 and BO 2550/10-1 was to analyze in detail the pres-sure-induced structural changes in environmentally friendly perovskite-type (ABO3) (1-x)Na0.5Bi0.5TiO3-xBaTiO3 (NBT-xBT) ferroelectric solid solution across the morphotropic phase boundary (MPB, a compositional region where the symmetry of the ferroelectric phase chang-es, enhancing material properties), to elucidate the effect of cation substitution on the nanoscale structure at ambient conditions and thus, on material properties. NBT-xBT single crystals with representative compositions x = 0.013 < xMPB, x = 0.048 ~ xMPB, and x = 0.074 > xMPB were subjected to in situ high-pressure polarized Raman spectroscopy and x-ray diffraction (in-house high-precision and with synchrotron radiation), to follow the pressure evolution of the structure on the short-, intermediate-, and long-range scale. Raman spectroscopy at elevated pressures was applied also to pure NBT, as the previously published data were incomplete. The main achievements can be summarized as follows: (i). Independently of the structural difference at ambient conditions and similar to pure NBT, all three NBT-xBT compounds (x = 0.013, 0.048 and 0.074) undergo a reversible pressure-induced phase transition to an orthorhombic Pnma phase, which is comprised of antipolar A-cation off-centred shifts along the a axis (corresponding to the pseudo-cubic [110] direction), mixed BO6 tilts of type a-b+a- (the orthorhombic b axis corre-sponding to the pseudocubic [001] direction), and uncorrelated, reduced in magnitude (but persisting at least up to 10 GPa) polar shifts of B-site cations. (ii). The pressure-induced phase transition is preceded by stepwise structural changes on the mesoscopic scale, consisting of reduction of the B-cation polar shifts, dynamic de-coupling of adjacent A-site and B-site local dipoles, increased orientation disorder of the B-site dipoles, reorientation of A-site dipoles, and enhancement of the correlation length of coherent BO6 tilts. The increase in the amount of A-site Ba2+ cations shifts these short- and intermediate-range atomic rearrangements to higher pressures and consequently, the phase-transition pressure pc also increases with the Ba content (pc ~ 3.5, 4.3 and 4.9 GPa for x = 0.013, 0.048, and 0.074, respectively). (iii). The presence of Ba2+ in the NBT matrix induces random local elastic stresses and hinters the pressure-induced reduction of cationic shifts for all studied compounds. Nevertheless, the comparison of the bulk moduli reveals that the MPB compound is softer than pure NBT. (iv). The pressure evolution of the structure at different length scales reveals that depending on the level of doping, during the crystal growth Ba is incorporated into the structure in a different way. For x = 0.013 < xMPB Ba2+ cations are accommodated into the tetrago-nal nanoregions containing short-range correlated in-phase BO6 tilts, which leads to elastic strains in the dominant pseudorhombohedral matrix containing intermediate-range correlated antiphase tilts. For x = 0.048 ~ xMPB the dominant tilt pattern is still antiphase, but the correlated length of the in-phase tilts increases and the Ba2+ cations are accommodated in both the tetragonal nanoregions and pseudorhomhedral matrix, without inducing macroscopic elastic strain. For x = 0.074 > xMPB Ba2+ cations are also accommodated into both the tetragonal and pseudorhombohedral fraction, but the former dominates. (v). At ambient pressure NBT-0.048BT and NBT-0.074BT exhibit a modulated structure due to self-organized extended defects with mixed BO6 tilts and antipolar shifts of A-site cations, resembling Pnma-type layers. The structural modulation is resistant to low levels of external pressure (below pc).