Microscopic origin of polarity in quasiamorphous BaTiO ₃

The recent observation of pyroelectricity in quasiamorphous thin films of BaTiO ₃ 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 TiO ₆ octahedra. The magnitude of the off-center displacement of the Ti ion and the concomitant dipole moment in both quasiamorphous (polar) and amorphous (nonpolar) BaTiO ₃ were found to be nearly twice as large as those in bulk BaTiO ₃. We propose that the mechanism of macroscopic polarity in quasiamorphous BaTiO ₃ is in a weak orientational ordering of the TiO ₆ bonding units. In this view, one may expect that other amorphous ionic oxides containing stable local bonding units, for example NbO ₆, TiO ₆, or VO ₆, may also form noncrystalline polar phases.