Structural changes during the crystallization of the Bi4Ge3O12 glasses

Bismuth–germanate glass ceramics with the composition 40% Bi2O3–60% GeO2 (in molar percents) were prepared through controlled crystallization of melt-quenched glass. The Raman and FTIR spectra recorded in the as-made glasses show broad bands at 240, 400, 780 cm− 1 and 400, 745 cm− 1 have been assigned Ge–O bonds which appear right after preparation. X-ray diffraction has shown that the as-made glasses are amorphous, but after annealing above the crystallization temperature at 558 °C, BGO nano-crystallites with a size of about 50 nm precipitate in the glass matrix. The Raman and FTIR spectra reveal sharp peaks associated to the “internal” and “external vibrations” of GeO4 tetrahedral groups inside the BGO nano-crystallites. In the glass ceramic sample the transparency region is shifted at longer wavelengths compared to as-made glass, due to the Rayleigh scattering on the BGO nano-crystallites.

Research Highlights
► Annealing of the bismuth germanate glasses induces the crystallization of BGO nano-crystallites.
► The Raman and FTIR spectra are due to GeO4 groups in the BGO nano-crystallites.
► The formation of stable nuclei is the dominant step in the glass crystallization.