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.

• Received 16 January 2011

DOI:https://doi.org/10.1103/PhysRevB.83.134106