Structure of glasses and melts in the Na2O-B2O3 system from high-temperature Raman spectroscopic data: II. Superstructural units in melts

Abstract

The specific features and possible mechanisms of transformation of the intermediate-range order structure in the Na2O-B2O3 system are investigated as a function of the composition and temperature by analyzing the integrated intensities of the characteristic bands at 808, 770–780, and 750–760 cm−1 (due to the vibrations of boroxol, triborate, and di-triborate rings, respectively) in the Raman spectra of glasses and melts in this system. It is demonstrated that an increase in the Na2O concentration leads to sequential transformations of boroxol rings into triborate groups and then into di-triborate groups. An increase in the temperature results in a decrease in the fraction of main structural units joined into superstructural units. The concentration of different borate rings can change depending on the temperature due to both the more random distribution of main units in the melt structure as compared to the glass structure and the formation of nonbridging bonds within superstructural units. Moreover, different superstructural units can transform into each other.