The recent observation of pyroelectricity in quasiamorphous thin films of $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ 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 $\mathrm{Ti}{\mathrm{O}}_6$ octahedra. The magnitude of the off-center displacement of the Ti ion and the concomitant dipole moment in both quasiamorphous (polar) and amorphous (nonpolar) $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ were found to be nearly twice as large as those in bulk $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$. We propose that the mechanism of macroscopic polarity in quasiamorphous $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ is in a weak orientational ordering of the $\mathrm{Ti}{\mathrm{O}}_6$ bonding units. In this view, one may expect that other amorphous ionic oxides containing stable local bonding units, for example $\mathrm{Nb}{\mathrm{O}}_6$, $\mathrm{Ti}{\mathrm{O}}_6$, or $\mathrm{V}{\mathrm{O}}_6$, may also form noncrystalline polar phases.

• Received 10 September 2004
• Corrected 8 February 2005

DOI:https://doi.org/10.1103/PhysRevB.71.024116