Microscopic Origin of Polarity in Quasi-Amorphous BaTiO3.

The recent observation of pyroelectricity in quasi-amorphous 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 non-crystalline 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 quasi-amorphous (polar) and amorphous (non-polar) 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 quasi-amorphous 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 non-crystalline polar phases.