Boroxol ring dissolution in molten and glassy B2O3 by neutron and x-ray diffraction difference methods

The structure of 11B2O3 boron oxide glass and its liquid have been measured over a wide temperature range by pulsed neutron diffraction, from T = 14 K up to 1500 K. Contrary to prior neutron scattering results in the literature, thermal expansion of the B-O bond is resolved, with a coefficient of αBO = 4.1(3) ppm K-1, in quantitative agreement with the result previously derived by high-energy x-ray diffraction. Exploiting the scattering contrast between neutrons and x rays, difference functions are derived that eliminate contributions due to O-O pairs, revealing, for the first time, the nearest-neighbor B-B peak in the pair distribution function. This peak occurs at rBB = 2.430(1) Å in B2O3 glass, consistent with a mean B-O-B bond angle β ≃ 124° and a large boroxol ring fraction. In the liquid, a much larger rBB ≃ 2.54 Å and β ≃ 134° are indicative of either a much lower ring fraction f, a larger non-ring B-O-B bond angle, βNR, or a combination of both. The latter scenario is supported by comparison to a range of molecular dynamics models with varying boroxol ring fractions.