Structural transformations in (1-x)Na0.5Bi0.5TiO3-xBaTiO3 single crystals studied by Raman spectroscopy

Hard-mode Raman spectroscopy was applied to analyze the temperature-induced transformation processes in perovskite-type (ABO(3)) single crystals of (1-x)Na0.5Bi0.5TiO3-xBaTiO(3) (NBT-xBT) 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 BO6 tilting is typical of pure NBT, while the incorporation of A-site Ba2+ suppresses the tilting and promotes the off centering of BO6 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 x not equal 0, two characteristic temperatures T' and T '' 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 < x(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(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(MPB) the ferroelectric coupling between B-site Ti4+ cations is reduced to a greater extent than the enhancement of ferroelectric coupling between A-site Bi3+ cations.