Multiscale identification of local tetragonal distortion in NaNbO3-BaTiO3 weak relaxor ferroelectrics by Raman, synchrotron x-ray diffraction, and absorption spectra

Locally persistent tetragonal distortion in NaNbO3-BaTiO3 relaxor ferroelectrics was identified at multiple length scales by means of in-situ synchrotron x-ray diffraction, absorption fine structure, and Raman spectra although their average symmetry seemingly evolved from a tetragonal to a cubic-like phase with the increasing BaTiO3 content. Theoretical calculations of Nb-O peaks in the R range of 1–2 Å provided a clear evidence that Nb5+ in an NbO6 octahedron should be spontaneously shifted along [001]. The competition between B-site Nb5+ ([001]) and Ti4+ ([111]) spontaneous off-center displacement directions would contribute to the disruption of long-range polar ordering. Raman spectra suggested that A-site Ba2+ and Na+ cations should be located at the center of the cubes in the absence of the polar contribution unlike Pb2+/Bi3+ with lone-pairs. A relatively low field-induced strain in NN-BT relaxors was thus ascribed to the small spontaneous polarization magnitude and the absence of structural phase transition during electric loading.