An oxygen in a glass network bonded to only one network-forming cation and not linked to a neighboring polyhedron. It increases when modifying oxides are added.
Japanese name: 非架橋酸素
Non-bridging oxygen appears in 2 practice questions across 1 topic, including Glasses and amorphous materials.
In the network of silicate glasses, an oxygen at a corner of an SiO₄ tetrahedron that is bonded to two Si atoms and links neighboring tetrahedra is a bridging oxygen (BO), and one bonded to only one Si, with its remaining negative charge neutralized by a modifier cation, is a non-bridging oxygen (NBO). If the number of bridging oxygens among the four corners of a tetrahedron is n and the tetrahedron is written Qⁿ, fused silica is all Q⁴, and as non-bridging oxygens increase it changes to Q³ and Q². Many Q³ units resemble sheet-like linkage and many Q² units chain-like linkage, so this serves as an index of network connectivity.
In binary Na₂O-SiO₂ glass, one Na₂O breaks one bridging oxygen and creates two non-bridging oxygens, so the number of non-bridging oxygens per tetrahedron, NBO/T, can be estimated as 2x/(1 − x), where x is the mole fraction of Na₂O. For example, for Na₂O·2SiO₂ (about 33 mol% Na₂O), NBO/T = 1, and on average every tetrahedron would be Q³. In real glasses, however, disproportionation occurs to some extent, with Q⁴ and Q² mixed to give an average of Q³, so the distribution of Qⁿ is not determined by composition alone. NBO/T is used as an average value.
When Al₂O₃ is added to a glass containing Na₂O, Al³⁺ has its charge compensated by Na⁺ and enters the network as AlO₄ tetrahedra, turning non-bridging oxygens back into bridges. When the molar amounts of Na₂O and Al₂O₃ are equal, non-bridging oxygens almost disappear on paper and the network becomes well connected. This makes it possible to obtain glasses that contain alkali yet have high viscosity and good chemical durability, and aluminosilicate glasses are used for cover glass for chemical strengthening and other uses. When Al₂O₃ exceeds Na₂O, there are not enough cations to compensate the charge, and the structure becomes more complex.
The amount of non-bridging oxygen and the distribution of Qⁿ are evaluated by separating the peaks with different chemical shifts for each Qⁿ in ²⁹Si solid-state NMR, or by examining the bonding states of oxygen with Raman spectroscopy or X-ray photoelectron spectroscopy (XPS). Non-bridging oxygens are also the sites where leaching of glass into water and exchange of alkali ions occur. In ironmaking and smelting slags, too, NBO/T is used as a quantity expressing how broken the network of molten silicates is, and serves as a guide to viscosity, the capacity to take up sulfur and other properties. In geoscience it is also used to estimate the viscosity of magma.
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.
The network becomes less connected, so viscosity and the glass transition temperature fall and thermal expansion grows. Also, the modifier cations next to non-bridging oxygens readily exchange with hydrogen ions in water, so chemical durability falls. Forming becomes easier, while heat resistance and durability decline.
A non-bridging oxygen carries one extra negative charge, and modifier cations such as Na⁺ or Ca²⁺ nearby neutralize it. Monovalent Na⁺ compensates the charge of one non-bridging oxygen and divalent Ca²⁺ that of two. The number of non-bridging oxygens can therefore be estimated from the amount and valence of the modifier cations.
With small amounts of alkali, instead of non-bridging oxygens forming, boron changes from three-coordinated BO₃ to four-coordinated BO₄, which actually strengthens the network. Beyond a certain amount of alkali, non-bridging oxygens begin to form, so the change in properties bends partway through the composition range.
One mole of Na₂O creates two moles of non-bridging oxygens, so NBO/T = 2×30/70 ≈ 0.86.
In a sodium silicate glass of 30 mol% Na₂O-70 mol% SiO₂, as…Each Na₂O added breaks one Si-O-Si bridging oxygen and creates two non-bridging oxygens bonded to only one Si, while the Na⁺ ions occupy interstices in the network and balance the charge.
How do the structure and properties of SiO₂ glass change wh…Glass transition temperature Coefficient of linear thermal expansion Amorphous silicon Chemically strengthened glass Metallic glass Network former Network modifier Reduced glass transition temperature Tempered glass