Metastable monoclinic and orthorhombic phases and electric field induced irreversible phase transformation at room temperature in the lead-free classical ferroelectricBaTiO3

Abstract

For decades it has been a well-known fact that among the few ferroelectric compounds in the perovskite family, namely, ${\mathrm{BaTiO}}_{3}$, ${\mathrm{KNbO}}_{3}$, ${\mathrm{PbTiO}}_{3}$, and ${\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}{\mathrm{TiO}}_{3}$, the dielectric and piezoelectric properties of ${\mathrm{BaTiO}}_{3}$ are considerably higher than the others in polycrystalline form at room temperature. Further, similar to ferroelectric alloys exhibiting morphotropic phase boundary, single crystals of ${\mathrm{BaTiO}}_{3}$ exhibit anomalously large piezoelectric response when poled away from the direction of spontaneous polarization at room temperature. These anomalous features in ${\mathrm{BaTiO}}_{3}$ remained unexplained so far from the structural standpoint. In this work, we have used high-resolution synchrotron x-ray powder diffraction, atomic resolution aberration-corrected transmission electron microscopy, in conjunction with a powder poling technique, to reveal that at 300 K (i) the equilibrium state of ${\mathrm{BaTiO}}_{3}$ is characterized by coexistence of metastable monoclinic Pm and orthorhombic (Amm2) phases along with the tetragonal phase, and (ii) strong electric field switches the polarization direction from the [001] direction towards the [101] direction. These results suggest that ${\mathrm{BaTiO}}_{3}$ at room temperature is within an instability regime, and that this instability is the fundamental factor responsible for the anomalous dielectric and piezoelectric properties of ${\mathrm{BaTiO}}_{3}$ as compared to the other homologous ferroelectric perovskite compounds at room temperature. Pure ${\mathrm{BaTiO}}_{3}$ at room temperature is therefore more akin to lead-based ferroelectric alloys close to the morphotropic phase boundary where polarization rotation and field induced ferroelectric-ferroelectric phase transformations play a fundamental role in influencing the dielectric and piezoelectric behavior.