On the Polar Structural Fragments in Glasses from Dielectric Spectroscopic Data

Abstract

The dielectric spectra of vitreous, glass-ceramic, and single-crystal Pb5Ge3O11 are measured in the frequency ranges 3–33 and ∼ 30–1000 cm–1. The inference is made that the Pb5Ge3O11 glass can represent a polar medium in which the origin of dipoles is associated with the spatial inhomogeneity and the possible existence of crystal-like structural nanofragments that had time to form during cooling of the melt and are responsible for the medium-range order in the glass. This assumption is confirmed by the fact that low-frequency (at 30 cm–1) values of e"" for the glass and the crystal along the polar axis are equal to each other and that the permittivity e""s of the glass (20.7) at submillimeter frequencies is intermediate between the permittivities e" of the crystal along the polar (34.3) and nonpolar (19.5) axes. Such a ratio between the permittivities e"s of a crystal and a glass of the same composition is revealed for the first time owing to the high accuracy in measurements of the spectrum in the submillimeter region and strong anisotropy of the Pb5Ge3O11 crystal. At the same time, the close values of the permittivities e"s for the glass and the crystal along the nonpolar axis indicate that polar atomic groupings in the glass are disordered and their structure on the nanoscale only resembles the structural motif of the crystal. The presence of microcrystals in the Pb5Ge3O11 glass-ceramics leads to a noticeable increase in the permittivity e" (∼ 25). The similarity between the IR spectra of all the Pb5Ge3O11 crystalline modifications studied and the spectrum of a glass of the same composition (with due regard for the diffuseness of the spectrum of the glass) suggests that the glass and its crystalline analog have similar short- and medium-range order structures.