The ferroelectric solid solution $\left(1-x\right){\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_3\text{−}{\mathrm{BaTiO}}_3 \left(\mathrm{NBT}\text{−}x\mathrm{BT}\right)$ is a promising material to substitute for the environmentally undesired Pb-based ferroelectrics. The strong enhancement of the dielectric permittivity and piezoelectric coefficients near the morphotropic phase boundary (MPB) in ferroelectric $AB{\mathrm{O}}_3$-type solid solutions is commonly related to the existence of a single or several long-range-ordered phases resulting in a complex nanodomain pattern. Here, $\mathrm{NBT}\text{−}x\mathrm{BT}$ is studied by Raman scattering and complementary synchrotron x-ray total elastic scattering to follow the temperature evolution of the mesoscopic-scale structural transformations for $x$ below, at, and slightly above $x_{\mathrm{MPB}}$. The most remarkable result is that at $x\sim x_{\mathrm{MPB}}$ the phonon mode involving vibrations of both off-centered A-site Bi and B-site Ti experiences strong softening and damping near the triple-point temperature in the phase diagram, in strong contrast to the compounds with $x<x_{\mathrm{MPB}}$ or $x>x_{\mathrm{MPB}}$. The chemically enhanced coupling between the Bi and Ti subsystems at $x_{\mathrm{MPB}}$ is facilitated by the subtle disturbance of the coupling processes within the Bi subsystem induced by Ba at $x_{\mathrm{MPB}}$. These mesoscopic-scale structural and dynamic phenomena seem to be significant for the enhanced response functions at the MPB in $\mathrm{NBT}\text{−}x\mathrm{BT}$.

The development of polar order in perovskite-type (ABO3) solid solutions of relaxors and normal ferroelectrics is studied on the basis of the temperature dependence of polarized Raman spectra of pure and Ru-doped 0.9PbZn1/3Nb2/3O3-0.1PbTiO3 (PZN-0.1PT) single crystals as well as complementary x-ray diffraction analysis. It is shown that the Raman peaks related to Pb-localized vibrations existing solely in a double-perovskite structure (prototype symmetry Fm3\ifmmode\bar\else\textasciimacron\fi{}m) are highly sensitive to the nucleation and development of polar atomic arrangements. The doublet near 50 cm-1 observed in the high-temperature Z\ifmmode \bar{}\else \={}\fi{}(XY)Z spectra of PZN-xPT (x = 0\char21{}0.1) is attributed to two distinct cubic states of Pb ions in regions with a local chemical B-site order of the type Pb(B2/32+B1/35+)0.5B0.55+O3: Pb1 ions surrounded mainly by Nb5+ and Pb2 ions surrounded by both Zn2+ and Nb5+. The singlet in the high-temperature Z\ifmmode \bar{}\else \={}\fi{}(XY)Z spectra is assigned to off-centered Pb2 ions. The Raman spectra of PZN-0.1PT indicate that off-centered Pb2 ions induce coherent polar shifts of ferroelectrically active B cations, which in turn facilitates the off-centering of Pb1 ions. A low degree of B-site doping with mixed-valence elements like Ru can substantially influence the development of ferroelectric order in PZN-xPT solid solutions due to the different preferred types of local octahedral distortion associated with the different valence states of the doping cations. The incorporation of Ru into the structure of PZN-0.1PT slightly decreases the Burns temperature and shifts the tetragonal-to-monoclinic phase transition to lower temperatures, thus favoring the tetragonal state in a wider temperature range as compared to the undoped compound. An assumption is made that the nanoscale polar order in Pb-based B-site complex perovskite-type relaxors might be of ferrielectric type rather than of ferroelectric type.

The understanding of the atomistic origin of the morphotropic phase boundary (MPB) occurring in composition-temperature phase diagrams of ferroelectric solid solutions is a key topic in material science because materials often exhibit anomalous properties at the MPB. Here we reveal mesoscopic-scale structural correlations for a leading Pb-free ferroelectric system, $\left(1\text{−}x\right){\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_3\text{−}x{\mathrm{BaTiO}}_3$ (NBT-$x\mathrm{BT}$), by examining atomic pair distribution functions and Raman scattering data at ambient conditions. We demonstrate that the amplification of the piezoelectric properties of NBT-$x\mathrm{BT}$ at the MPB are predominantly driven by an easy switchability resulting from a progressive decoupling between strain and polarization as the Ba content increases from zero to the critical MPB composition. It was observed that as Ba content increases towards MPB, competing local correlations, such as A-site chemical order, antiferrodistortive correlations of correlated ${\mathrm{BO}}_6$ tilts, and antipolar Bi shifts, are reduced, which in turn renders favorable conditions for easy switching of local dipoles under external fields. In addition, the evolving characteristics of the atomic dynamics as a function of composition suggest that the local potential functions of the cations are not completely flat at the MPB. Altogether, our results reveal atomistic mechanisms responsible for the observed elevated MPB properties in the case of NBT-$x\mathrm{BT}$ which imply that the so-called MPB of NBT-$x\mathrm{BT}$ should not be categorized as originally introduced for Pb-containing solid solutions.

The structural alteration induced by the substitution of three-valent cations with an isotropic electronic outermost shell for Pb${}^{2+}$ in perovskite-type relaxors was investigated in the solid solutions Pb${}_{1-x}$La${}_x$Sc${}_{(1+x)/2}$Ta${}_{(1-x)/2}$O${}_3$, $x$ $=$0.08 (PST-La) and Pb${}_{1-x}$La${}_x$Sc${}_{(1+x)/2}$Nb${}_{(1-x)/2}$O${}_3$, $x$ $=$0.23 (PSN-La). In order to distinguish the “charge” effects from “strain” effects associated with the incorporation of La${}^{3+}$ in the structure, Sr-containing PbSc${}_{0.5}$Nb${}_{0.5}$O${}_3$ was characterized as well. The structure of the compounds was analyzed by in situ Raman spectroscopy, single-crystal x-ray diffraction, and powder neutron diffraction at different temperatures or pressures. It is shown that the embedding of La${}^{3+}$ strongly affects the ferroic structural species due to strain effects through a disturbance of the system of lone-pair electrons associated with Pb${}^{2+}$ and a decrease in the tolerance factor. La doping suppresses the dynamical coupling between off-centered Pb and $B$-site cations and enhances antiphase $B$O${}_6$ octahedral tilting which, depending on the level of doping, may lead to long-range order of antiphase $B$O${}_6$ tilts at ambient conditions and frustrated antiferroelectric order of Pb ions at low temperatures.