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Glasses and amorphous materials

In a sodium silicate glass of 30 mol% Na₂O-70 mol% SiO₂, assuming each Na₂O creates two non-bridging oxygens, which is closest to the number of non-bridging oxygens per SiO₄ tetrahedron (NBO/T)?

  1. AAbout 0.43
  2. BAbout 0.60
  3. CAbout 0.86
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Answer: C. About 0.86

One mole of Na₂O creates two moles of non-bridging oxygens, so NBO/T = 2×30/70 ≈ 0.86. On average, 0.86 corner oxygens per tetrahedron are not linked to a neighboring tetrahedron, lowering the degree of network connectivity. About 0.43 is the value obtained by counting one non-bridging oxygen per Na₂O, and about 0.60 is the value obtained by dividing by the total 100 mol instead of the number of tetrahedra (the amount of SiO₂). NBO/T = 0 means a fully three-dimensional network as in silica glass, 2 means SiO₄ linked in chains or rings, and 4 means isolated tetrahedra. NBO/T is often used as a guide to viscosity and other properties when comparing the compositions of glasses and slags.

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