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Microscopic origin of polarity in quasiamorphous Ba Ti O 3

2005; American Physical Society; Volume: 71; Issue: 2 Linguagem: Inglês

10.1103/physrevb.71.024116

1550-235X

Anatoly I. Frenkel, Yishay Feldman, V. Lyahovitskaya, Ellen Wachtel, Igor Lubomirsky,

Microwave Dielectric Ceramics Synthesis

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.