Liquid B₂O₃ up to 1700 K: x-ray diffraction and boroxol ring dissolution

Using high energy x-ray diffraction, the structure factors of glassy and molten B₂O₃ were measured with high signal-to-noise, up to a temperature of T  =  1710(20) K. The observed systematic changes with T are shown to be consistent with the dissolution of hexagonal [B₃O₆] boroxol rings, which are abundant in the glass, whilst the high-T (⪆1500 K) liquid can be more closely described as a random network structure based on [BO₃] triangular building blocks. We therefore argue that diffraction data are in fact qualitatively sensitive to the presence of small rings, and support the existence of a continuous structural transition in molten B₂O₃, for which the temperature evolution of the 808 cm⁻¹ Raman scattering band (boroxol breathing mode) has long stood as the most emphatic evidence. Our conclusions are supported by both first-principles and polarizable ion model molecular dynamics simulations which are capable of giving good account of the experimental data, so long as steps are taken to ensure a ring fraction similar to that expected from Raman spectroscopy. The mean thermal expansion of the B-O bond has been measured directly to be α _BO  =  3.7(2)  ×  10⁻⁶ K⁻¹, which accounts for a few percent of the bulk expansion just above the glass transition temperature, but accounts for greater than one third of the bulk expansion at temperatures in excess of 1673 K. (paper)