Abstract
The structures of K 2O–B 2O 3 glasses and melts have been studied by high-temperature Raman spectroscopy. With an increase in the K 2O content and with increasing temperature, the boroxol rings, which only consist of B Ø 3 triangular units, were converted into pentaborate groups which consist of B Ø 4 tetrahedral (Ø=bridging oxygen atom) and B Ø 3 units in the 10 and 20 mol% K 2O–B 2O 3 glasses/melts. In the 30 mol% K 2O–B 2O 3 glass/melt, the Raman spectra indicated a structural change in the borate species from B Ø 4 tetrahedral units to metaborate, B Ø 2O −, triangular units with non-bridging oxygen with increasing temperature. The fraction of four-coordinated boron atoms, N 4, obtained from deconvolution of the Raman bands was gradually reduced above the glass transition temperature and converged to a constant value over 1400 K. This phenomenon could be explained by a thermodynamic process.
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