Evidence for local monoclinic structure, polarization rotation, and morphotropic phase transitions in (1-x)BiFeO3-xBaTiO3 solid solutions: A high-energy synchrotron x-ray powder diffraction study

We have presented evidence for composition-driven rhombohedral-to-cubic and cubic-to-tetragonal morphotropic phase transitions in (1 - x)BiFeO3-xBaTiO(3) (BF-xBT) solid solutions at x approximate to 0.35 and 0.85, respectively. It is shown that these transitions lead to the appearance of two peaks in the composition variation of dielectric constant, similar to that in well-known piezoelectric ceramics. The existing controversies about the true structure of the so-called cubic phase of BF-xBT are resolved using high-energy synchrotron x-ray powder diffraction data. It is shown that the ferroelectric cubic phase of BF-xBT has got locally monoclinic structure of M-A type in the Cm space group whose polarization vector rotates on its mirror plane on increasing the concentration of BaTiO3 (x). The ferroelectric-to-paraelectric phase transition in pseudocubic monoclinic phase is shown to be of tricritical nature.