Local structural phenomena in pure and Ru-doped0.9PbZn1/3Nb2/3O3−0.1PbTiO3near the morphotropic phase boundary as revealed by Raman spectroscopy

Abstract

The development of polar order in perovskite-type (ABO${}_{3}$) 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 0.9PbZn${}_{1/3}$Nb${}_{2/3}$O${}_{3}$-0.1PbTiO${}_{3}$ (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 $\mathit{Fm}\overline{3}m$) are highly sensitive to the nucleation and development of polar atomic arrangements. The doublet near 50 cm${}^{\ensuremath{-}1}$ observed in the high-temperature $\mathrm{Z\ifmmode \bar{}\else \={}\fi{}}(\mathit{XY})Z$ spectra of PZN-$x$PT ($x$ $=$ 0--0.1) is attributed to two distinct cubic states of Pb ions in regions with a local chemical B-site order of the type $\mathrm{Pb}({\mathrm{B}}_{2/3}^{2+}{\mathrm{B}}_{1/3}^{5+}){}_{0.5}{{\mathrm{B}}_{0.5}}^{5+}{\mathrm{O}}_{3}$: Pb1 ions surrounded mainly by Nb${}^{5+}$ and Pb2 ions surrounded by both Zn${}^{2+}$ and Nb${}^{5+}$. The singlet in the high-temperature $\mathrm{Z\ifmmode \bar{}\else \={}\fi{}}(\mathit{XY})Z$ 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-$x$PT 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.