Borate melt structure: Temperature‐dependent B–O bond lengths and coordination numbers from high‐energy X‐ray diffraction

Borate melts containing <20 mol% Na₂O have been studied using high‐energy synchrotron X‐ray diffraction. Temperature dependencies of the mean B–O bond lengths are shown to vary strongly with soda content, by comparison to previous measurements on liquid B₂O₃ and Na₂B₄O₇. Whereas in liquid B₂O₃ linear thermal expansion of the BØ₃ units is observed, with coefficient α_BO = 3.7(2) × 10^?6 K^?1, this expansion is apparently slightly suppressed in melts containing <20 mol% Na₂O, and is dramatically reversed at the diborate composition. These effects are interpreted in terms of changes in the mean B–O coordination number, where the reaction BØ₄^? + BØ₃ ? BØ₃ + BØ₂O^? shifts to the right with increasing temperature. The empirical bond‐valence relationship is used to convert measured bond lengths, r_BO, to coordination numbers, n_BO, including a correction for the expected thermal expansion. This method is more accurate and precise than direct determination of n_BO from peak areas in the radial distribution functions. Gradients of Δn_BO/ΔT = ?3.4(3) × 10^?4 K^?1 close to the diborate composition, and Δn_BO/ΔT = ?0.3(1) × 10^?4 K^?1 for a 13(3) mol% Na₂O melt are observed, in reasonable agreement with Raman spectroscopic observations and thermodynamic modeling, with some quantitative differences. These observations go toward explaining isothermal viscosity maxima and changes in fragility across the sodium borate system.