Abstract
In situ temperature-dependent Raman-scattering experiments under an external dc electric field E have been performed on (1−x)Na0.5Bi0.5TiO3-xBaTiO3 single crystals with x ranging from 0.013 to 0.074 in order to gain further insights into the atomistic mechanism of polar coupling in lead-free perovskite-type (ABO3) ferroelectric solid solutions near the morphotropic phase boundary (MPB). The polarized Raman spectra collected between 770 and 100 K under an E applied along the pseudocubic [100] direction were analyzed in terms of hard-mode spectroscopy. The results reveal enhanced coupling between the polar displacements of the A-site Bi3+ and B-site Ti4+ cations for x=0.048∼xMPB when a moderate E[100] field is applied, a local-scale phenomenon that might be responsible for the known drift of the MPB under external stimuli. In addition, for all studied compounds, the characteristic temperatures TA−A′ and TA−A′′, preceding the macroscopic Tm and Td and related to mesoscopic-scale antiferroelectric and ferroelectric coupling processes within the A-site-cation subsystem in a zero field, merge into one even under a weak electric field, suggesting that the structural state between the paraelectric and ferroelectric zones in the x-T diagram is weak ferrielectric.
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