Synthesis, Structural Characterization, and Properties of Perovskites Belonging to the xBiMnO3−(1 − x) PbTiO3 System

The x BiMnO 3 −(1 − x ) PbTiO 3 solid solution has been synthesized by an accurate, nonclassical, wet-chemistry method. Results show the existence of two structurally different perovskites, depending on the x value: tetragonal for 0≤ x <0.36; pseudocubic for 0.44< x ≤0.8; and a morphotropic phase boundary (MPB) between both symmetries for 0.36≤ x ≤0.44. When x ≥ 0.76, unidentified diffraction lines appear together with those of the perovskite. The evolution of the unit-cell parameters has been studied by X-ray diffraction techniques. The stability of different symmetries can be correlated with the Goldschmidt’s tolerance factors ( t ) for perovskites, with the MPB only existing for a critical t ≈ 1.00 value. Materials with x ≤ 0.4 exhibit the ferroelectric transition above room temperature; their Curie temperatures have been determined by thermal analysis, dielectric versus temperature measurements, and dynamical−mechanical analysis. This temperature decreases as the bismuth and manganese contents increase, being as low as Tc = 446 K for x = 0.4. Elastic properties also suggest the existence of a second low temperature transition to a nonferroelastic phase, in which temperature increases with x . Trends of the two transitions point to the existence of a critical composition above which a ferroelectric transition does not occur, in coincidence with the limit of the tetragonal phase and the end of the MPB region. Moreover, magnetic interactions have also been detected at low temperature in x ≥ 0.2 samples.