  For decades it has been a well-known fact that among the few ferroelectric
compounds in the perovskite family namely BaTiO3, KNbO3, PbTiO3 Na1/2Bi1/2TiO3
the dielectric and piezoelectric properties of BaTiO3 is considerably higher
than the others in polycrystalline form at room temperature. Further, similar
to ferroelectric alloys exhibiting morphotropic phase boundary, single crystals
of BaTiO3 exhibits anomalously large piezoelectric response when poled away
from the direction of spontaneous polarization at room temperature. These
anomalous features in BaTiO3 remained unexplained so far from the structural
stand point. In this work we have used high resolution synchrotron X-ray powder
diffraction, atomic resolution aberration corrected transmission electron
microscopy, in conjunction with a novel powder poling technique, to reveal that
(i) the equilibrium state of BaTiO3 is characterized by coexistence of a subtle
monoclinic (Mc) phase and tetragonal phase, and (ii) strong electric field
induces an orthorhombic phase at 300 K. These results suggest that BaTiO3 at
room temperature is within an instability regime, and that this instability is
therefore the fundamental factor responsible for the anomalous dielectric and
piezoelectric properties of BaTiO3 as compared to the other homologous
ferroelectric perovskite compounds. The results demonstrate that pure BaTiO3 at
room temperature more akin to lead-based ferroelectric alloys close to the
morphotropic phase boundary where polarization rotation and field induced
ferroelectric-ferroelectric phase transformations play fundamental role in
influencing the piezoelectric behavior.
