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

Area of Science:

• Materials Science
• Condensed Matter Physics
• Inorganic Chemistry

Background:

• Boron trioxide (B2O3) exists as a glass composed of hexagonal [B3O6] boroxol rings.
• Understanding the high-temperature liquid structure of B2O3 is crucial for materials applications.
• Previous studies suggested a structural transition in molten B2O3, but direct evidence remained limited.

Purpose of the Study:

• To investigate the structural evolution of glassy and molten B2O3 at high temperatures.
• To determine the structural changes occurring in B2O3 from the glassy state to the liquid state.
• To correlate diffraction data with spectroscopic evidence for structural transitions.

Main Methods:

• High energy x-ray diffraction was employed to measure structure factors of B2O3 up to 1710 K.
• First-principles and polarizable ion model molecular dynamics simulations were conducted.
• Raman spectroscopy data, particularly the boroxol breathing mode at 808 cm−1, was used for comparison.

Main Results:

• Diffraction data showed systematic changes consistent with the dissolution of hexagonal [B3O6] rings with increasing temperature.
• At high temperatures (>1500 K), the liquid structure of B2O3 is better described as a random network of [BO3] triangular units.
• The study confirmed a continuous structural transition in molten B2O3, aligning with Raman spectroscopy findings.
• The mean thermal expansion of the B-O bond was measured as 3.7(2)×10−6 K−1.

Conclusions:

• X-ray diffraction is sensitive to the presence of small rings in molten B2O3.
• A continuous structural transition occurs in molten B2O3, driven by the dissolution of boroxol rings.
• The high-temperature liquid structure is dominated by [BO3] units in a random network.
• The B-O bond expansion contributes significantly to the bulk expansion of B2O3 at elevated temperatures.