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Borate melt structure: Temperature‐dependent B–O bond lengths and coordination numbers from high‐energy X‐ray diffraction
Author(s) -
Alderman Oliver L. G.,
Benmore Chris J.,
Lin Alex,
Tamalonis Anthony,
Weber J. K. Richard
Publication year - 2018
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/jace.15529
Subject(s) - coordination number , boron , bond length , chemistry , thermal expansion , isothermal process , valence (chemistry) , crystallography , raman spectroscopy , analytical chemistry (journal) , bond energy , thermodynamics , molecule , crystal structure , ion , physics , organic chemistry , chromatography , optics
Borate melts containing <20 mol% Na 2 O 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 B 2 O 3 and Na 2 B 4 O 7 . Whereas in liquid B 2 O 3 linear thermal expansion of the BØ 3 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% Na 2 O, 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 BØ 4 − + BØ 3 ⇌ BØ 3 + BØ 2 O − 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 Δ n BO /Δ T = −3.4(3) × 10 −4 K −1 close to the diborate composition, and Δ n BO /Δ T = −0.3(1) × 10 −4 K −1 for a 13(3) mol% Na 2 O 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.