Abstract
Hard-mode Raman spectroscopy was applied to analyze the temperature-induced transformation processes in perovskite-type $({\mathrm{ABO}}_{3})$ single crystals of $(1\text{\ensuremath{-}}x){\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_{3}\text{\ensuremath{-}}x{\mathrm{BaTiO}}_{3}$ (NBT-$x\mathrm{BT})$ in a wide temperature range between 100 and 1010 K and a composition range of $x=0--0.074$ across the morphotropic phase boundary (MPB). The results show abundant uncoupled ferroic structural distortions even at 1010 K and coexistence of two types of mesoscopic-scale ferroic order at lower temperatures. Octahedral ${\mathrm{BO}}_{6}$ tilting is typical of pure NBT, while the incorporation of A-site ${\mathrm{Ba}}^{2+}$ suppresses the tilting and promotes the off centering of ${\mathrm{BO}}_{6}$ octahedra. The temperature evolution of the phonon modes clearly reveals the two macroscopically observed critical temperatures ${T}_{m}$ and ${T}_{d}$ as well as, in the case of $x\ensuremath{\ne}0$, two characteristic temperatures ${T}^{\ensuremath{'}}$ and ${T}^{\ensuremath{'}\ensuremath{'}}$ preceding the ${T}_{m}$ and ${T}_{d}$, respectively, which are attributed to mesoscopic-scale antiferroelectric and ferroelectric coupling processes within the A-site-cation subsystem. At $x\phantom{\rule{4pt}{0ex}}l\phantom{\rule{4pt}{0ex}}{x}_{\mathrm{MPB}}$ the two sublattices, off-centered A-site cations and off-centered B-site cations, remain incoherent in the entire temperature range. Only when the amount of Ba reaches ${x}_{\mathrm{MPB}}$ the two subsystems couple dynamically, which can explain the enhancement of properties at the MPB. The overall ferroic distortion, however, has a minimum at the MPB, probably because at ${x}_{\mathrm{MPB}}$ the ferroelectric coupling between B-site ${\mathrm{Ti}}^{4+}$ cations is reduced to a greater extent than the enhancement of ferroelectric coupling between A-site ${\mathrm{Bi}}^{3+}$ cations.
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