Phase structure of the ceramic samples of the BiScO${}_3$-PbTiO${}_3$-PbMg${}_{{}^1{/}_3}$Nb${}_{{}^2{/}_3}$O${}_3$ system near the morphotropic phase boundary studied by the Rietveld method

An X-ray diffraction study of ceramic samples of the BiScO${}_3$-PbTiO${}_3$-PbMg${}_{{}^1{/}_3}$Nb${}_{{}^2{/}_3}$O${}_3$ system with compositions close to the morphotropic phase boundary was carried out at room temperature. The existence of wide areas of solid solutions has been established. The symmetry of the PbTiO${}_3$-based solid solutions is tetragonal (space group P4mm). The symmetry of the PbMg${}_{{}^1{/}_2}$Nb${}_{{}^2{/}_3}$O${}_3$-based solid solutions is cubic (space group Pm$\overline{3}$m). Near the BiScO${}_3$ side, the solid solutions are rhombohedral (space group R3m). During the morphotropic phase transition from the cubic solid solutions to the tetragonal ones, additional phases appear. If a tetragonal phase prevails ((1 - 2x)BiScO${}_3$-xPbTiO${}_3$-xPbMg${}_{{}^1{/}_3}$Nb${}_{{}^2{/}_3}$O${}_3$ x = 0.46; (1 - 2x)BiScO${}_3$-1.1xPbTiO${}_3$-0.9xPbMg${}_{{}^1{/}_3}$Nb${}_{{}^2{/}_3}$O${}_3$ x = 0.45 and 0.42), a satisfactory model is a model with a minority cubic phase (space group Pm$\overline{3}$m). If a cubic phase prevails ((1 - 2x)BiScO${}_3$-1.1xPbTiO${}_3$-0.9xPbMg${}_{{}^1{/}_3}$Nb${}_{{}^2{/}_3}$O${}_3$ x = 0.39 and 0.36; (1 - 2x)BiScO${}_3$-0.8xPbTiO${}_3$-1.2xPbMg${}_{{}^1{/}_3}$Nb${}_{{}^2{/}_3}$O${}_3$ x = 0.4), a model with a minority monoclinic phase (space group Cm) or with two minority phases: tetragonal (space group P4mm) and monoclinic (space group Cm) is satisfactory.