Effect of local elastic strain on the structure of Pb-based relaxors: A comparative study of pure and Ba- and Bi-doped PbSc0.5Nb0.5O3

The temperature evolution of the nanoscale structure of PbSc0.5Nb0.5O3 (PSN) and (Pb,A('))Sc0.5Nb0.5O3, (A(')=Ba,Bi) is analyzed by applying synchrotron single-crystal and high-resolution powder x-ray diffraction as well as polarized Raman spectroscopy. The study compares the effect of incorporation of two-valent cations with isotropic electron structure (Ba2+) and three-valent cations with stereochemically active lone pairs (Bi3+) on the structure of Pb-based perovskite-type relaxor ferroelectrics. The results reveal that the violation of the host system of cations with lone-pair electrons (Pb2+), i.e., the reduction in ferroic species with coherent off-centered Pb atoms, is the major factor for the suppression of long-range ferroelectric ordering at low temperatures. The local charge imbalance associated with A site chemical disorder has negligible impact on the development of ferroelectric order if the second type of A-positioned cations also forms lone-pair electrons. The substitution of Bi3+ for Pb2+ even enhances the cubic-to-ferroelectric transformation processes in PSN and results in a structural state consisting of abundant ferroelectric domains, which are large enough to be identified by polarized Raman spectroscopy as crystalline, but with an average size close to the intrinsic detection limit of synchrotron single-crystal x-ray diffraction.