Microscopic origin of polarity in quasiamorphous BaTiO3

The recent observation of pyroelectricity in quasiamorphous thin films of BaTiO3 introduced a previously unreported type of polar ionic solid where the appearance of a macroscopic dipole moment is not accompanied by long-range crystal-like order. This poses a question regarding the mechanism of polarity in noncrystalline ionic systems and the nature of their local dipoles. By combining x-ray diffraction and x-ray-absorption fine-structure spectroscopy techniques we have identified the local dipoles as stable but distorted TiO6 octahedra. The magnitude of the off-center displacement of the Ti ion and the concomitant dipole moment in both quasiamorphous (polar) and amorphous (nonpolar) BaTiO3 were found to be nearly twice as large as those in bulk BaTiO3. We propose that the mechanism of macroscopic polarity in quasiamorphous BaTiO3 is in a weak orientational ordering of the TiO6 bonding units. In this view, one may expect that other amorphous ionic oxides containing stable local bonding units, for example NbO6, TiO6, or VO6, may also form noncrystalline polar phases.