Tracking boron coordination change with temperature in a barium borosilicate glass melt by neutron diffraction

Pulsed neutron diffraction measurements on a barium borosilicate glass and its supercooled liquid reveal clear, quantitative evidence for a tetrahedral to trigonal boron transition above the glass transition temperature, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>T</a:mi><a:mi mathvariant="normal">g</a:mi></a:msub></a:math>. In particular the tetrahedral boron fraction, <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>N</c:mi><c:mn>4</c:mn></c:msub></c:math>(<d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mi>T</d:mi></d:mrow></d:math>), is about 50(1)% in a <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mn>28</e:mn><e:mi>BaO</e:mi><e:mo>·</e:mo><e:mn>48</e:mn><e:msub><e:mi mathvariant="normal">B</e:mi><e:mn>2</e:mn></e:msub><e:msub><e:mi mathvariant="normal">O</e:mi><e:mn>3</e:mn></e:msub><e:mo>·</e:mo><e:mn>24</e:mn><e:mi>Si</e:mi><e:msub><e:mi mathvariant="normal">O</e:mi><e:mn>2</e:mn></e:msub></e:mrow></e:math> (BBS352) glass, independent of temperature, but falls continuously above <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mrow><i:msub><i:mi>T</i:mi><i:mi mathvariant="normal">g</i:mi></i:msub><i:mo>=</i:mo><i:mn>901</i:mn></i:mrow></i:math> K to 36(1)% at 1200 K. The data are interpreted in terms of simplified chemical equilibria, from which it is inferred that the boron coordination change contributes between 30% to 40% of the total calorimetric configurational heat capacity at <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"><k:msub><k:mi>T</k:mi><k:mi mathvariant="normal">g</k:mi></k:msub></k:math>, as well as to the entropy and liquid fragility. The thermal expansion coefficient of the B–O bond in BBS352 glass is measured to be <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"><m:mrow><m:msub><m:mi>α</m:mi><m:mi>BO</m:mi></m:msub><m:mo>=</m:mo><m:mn>6.5</m:mn></m:mrow></m:math>(1.7) ppm <n:math xmlns:n="http://www.w3.org/1998/Math/MathML"><n:msup><n:mrow><n:mi mathvariant="normal">K</n:mi></n:mrow><n:mrow><n:mo>−</n:mo><n:mn>1</n:mn></n:mrow></n:msup></n:math>, slightly larger than for pure <p:math xmlns:p="http://www.w3.org/1998/Math/MathML"><p:mrow><p:msub><p:mi mathvariant="normal">B</p:mi><p:mn>2</p:mn></p:msub><p:msub><p:mi mathvariant="normal">O</p:mi><p:mn>3</p:mn></p:msub></p:mrow></p:math> glass and very similar to the known bulk expansion, implying a remarkably simple vibrational thermal expansion mechanism for the solid. Resolution of B–O and Si–O correlations in the real-space pair distribution function is demonstrated for <s:math xmlns:s="http://www.w3.org/1998/Math/MathML"><s:mrow><s:mn>50</s:mn><s:mi>BaO</s:mi><s:mo>·</s:mo><s:mn>25</s:mn><s:msub><s:mi mathvariant="normal">B</s:mi><s:mn>2</s:mn></s:msub><s:msub><s:mi mathvariant="normal">O</s:mi><s:mn>3</s:mn></s:msub><s:mo>·</s:mo><s:mn>25</s:mn><s:mi>Si</s:mi><s:msub><s:mi mathvariant="normal">O</s:mi><s:mn>2</s:mn></s:msub></s:mrow></s:math> (BBS211) glass, and the factors governing if these features can be resolved are discussed.